Bytes missing error modbus poll

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Note:
This article was updated on 2023-02-03 and previously published under WIKI_Q210794

What is Modbus Poll Bytes Missing Error error?

The Modbus Poll Bytes Missing Error error is the Hexadecimal format of the error caused. This is common error code format used by windows and other windows compatible software and driver vendors.

This code is used by the vendor to identify the error caused. This Modbus Poll Bytes Missing Error error code has a numeric error number and a technical description. In some cases the error may have more parameters in Modbus Poll Bytes Missing Error format .This additional hexadecimal code are the address of the memory locations where the instruction(s) was loaded at the time of the error.

What causes Modbus Poll Bytes Missing Error error?

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Modicon M580 — Modbus TCP через MOXA 3660 в Deif — Modbus RTU

Modicon M580 — Modbus TCP через MOXA 3660 в Deif — Modbus RTU

Сообщение swaips-87 » 06 фев 2021, 02:57

Modicon M580 — Modbus TCP через MOXA 3660 в Deif — Modbus RTU

Сообщение ogorsv » 06 фев 2021, 15:48

Я бы начал с проверки «физики» (80 % моих проблем были именно там:-)):
1. Правильный кабель, топология, коннекторы, монтаж
2. Терминаторы и подтягивающие резисторы (в руководстве написано о них)
3. Может (а вдруг?) влияют помехи извне
4. Какой режим МОХА вы используете: Прозрачный или Агент?
5. Зачастую проблемы подобного рода возникали из-за разных таймаутов ведущего и ведомого. Представьте, что мастер не получает ответ за указанное время — он делает повтор посылки. А слэйву просто нужно было время на обработку и отправку ответа
6. В инструкции есть такой параметр, как «Inter-frame delay» — по дефолту в нуле.
7. А как у вас организована в М580 посылка команд? Через ФБ или DIO-scanner? Не слишком ли высока скорость опроса? Обрабатываете ли вы успешный/неуспешный ответ?
Сбрасываете ли команду по успешному ответу/неуспешному/статусу установки/таймауту или единица продолжает висеть?

По DEIF @Jackson, наверно, может подсказать — были ли похожие проблемы и как их решали

По проблемам обмена через шлюз был ещё FAQ нашего Центра поддержки клиентов
https://www.se.com/ru/ru/faqs/index?pag . cale=ru_RU

Modicon M580 — Modbus TCP через MOXA 3660 в Deif — Modbus RTU

Сообщение swaips-87 » 09 фев 2021, 06:43

Modicon M580 — Modbus TCP через MOXA 3660 в Deif — Modbus RTU

Сообщение Sergey_P » 09 фев 2021, 14:55

Мне кажется, это слишком много.
В документе «MODBUS over serial line specification and implementation guide V1.02» на странице 13 сказано:
In RTU mode, message frames are separated by a silent interval of at least 3.5 character times

На скорости 9600 это примерно 0.3 мс.

Думаю, Inter-frame delay в MOXA предназначена для вставки задержек между пакетами, чтобы медленные слейвы успевали переключаться на прием после отправки ответа на запрос. Тормоза у вас появились скорее всего именно из-за того, что вы тут задали 300 мс. Попробуйте задать 10 мс. Все должно работать.

Modicon M580 — Modbus TCP через MOXA 3660 в Deif — Modbus RTU

Сообщение swaips-87 » 10 фев 2021, 05:52

Источник

Modbus errors

I’m trying to connect two climate devices that’s using modbus and jbus communication, with a KNX system using the gateway: IBOX-KNX-MBRTU-A of manufacture Intesis software.

The climate devices are:
— ventilation-unit with a modbus interface and,
— chiller with a jbus interface.

For monitoring the modbus messages, I use the software tool: modbus poll.
The monitoring of the jbus messages doesn’t show any errors. Only during monitoring the modbus messages the following errors are shown:

— Bytes missing error
— Read error: Framing error
— CRC error

What mean these errors and how can i solve these problems?

Hopefully someone can help me.

Lynn August Linse

Framing errors is a bad/unusual thing, something you should never see in a correctly designed system — look it up on the Internet. What it usually means:

#1) a baud rate mismatch — I don’t think that’s your problem since the Modbus device won’t answer if the master sends ‘garbage’ which is what incorrect baud rate data looks like.

#2) improper RS-485 grounding, which means bits/bytes are being misinterpreted because of no ground wire and a ground potential approaching the over-voltage levels.

#3) lack of line bias on RS-485, which means when the master stops sending & before the slave starts responding, the RS-485 lines float which means you likely might see some garbage 0xFF or 0xFE bytes at the start of the response.

#4) just plain too much line-noise. You might need to double check your shielding.

#5) a slave which starts replying BEFORE the master is ready to receive, therefore part or all of the first byte is lost . leading to a possible framing error during the first received byte and also missing data, which leads to CRC error.

Источник

Modbus Poll user manual

1. Modbus Poll

Modbus Poll is an easy to use Modbus master simulator developed for many purposes. Among others:

Designers of Modbus slave devices for quick and easy testing of protocol interface

Automation engineers that need to test Modbus devices or networks on site

Service engineers that want to read out and/or change specific service data from a device

Change Modbus registers in a slave device

Log data from Modbus devices

Troubleshooting and compliance testing

1.1. System requirements for Modbus Poll

Processor; 1 GHz or faster recommended
1 GB RAM
5 MB of available hard drive space
1024 x 768 display resolution

All Windows versions from Windows 7 to Windows 11 are supported.
Modbus Poll version 7 runs on Windows XP.

1.1.1. Silent install

Silent install require no user intervention and have no user interface. The user doesn’t see any dialog and isn’t asked any questions.
Use the command line /S switch.

1.2. End User License Agreement

You should carefully read the following terms and conditions before using Modbus Poll. Unless you have a different license agreement signed by Witte Software, your use of this software indicates your acceptance of this license agreement and warranty. If you do not accept these terms you must cease using this software immediately.

Copyright.
Modbus Poll («The Software») is copyright 2002-2023 by Witte Software, all rights reserved.

Evaluation and Registration.
This is not free software. You are hereby licensed to use the Software for evaluation purposes without charge for a period of 30 days.

If you use the Software after the 30 day evaluation period a registration fee is required.

Unregistered use of the Software after the 30-day evaluation period is in violation of U.S. and international copyright laws.

One registered copy of the Software may either be used by a single person who uses the software personally on one or more computers, or installed on a single computer used by multiple people, but not both.

For information on order and pricing, please visit https://www.modbustools.com/order.html

Modbus Poll licenses are perpetual. Once you buy a license to a specific major version, and as long as you abide by the license agreement, you can use that version forever with no additional cost.

Distribution.
Provided that you do not include your License Key you are hereby licensed to make copies of the Software; give exact copies of the original to anyone; and distribute the Software in its unmodified form via electronic means. You are specifically prohibited from charging for any such copies.

LIMITED WARRANTY.
THE SOFTWARE IS PROVIDED AS IS AND WITTE SOFTWARE DISCLAIMS ALL WARRANTIES RELATING TO THIS SOFTWARE, WHETHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.

LIMITATION ON DAMAGES.
NEITHER WITTE SOFTWARE NOR ANYONE INVOLVED IN THE CREATION, PRODUCTION, OR DELIVERY OF THIS SOFTWARE SHALL BE LIABLE FOR ANY INDIRECT, CONSEQUENTIAL, OR INCIDENTAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE SUCH SOFTWARE EVEN IF WITTE SOFTWARE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES OR CLAIMS. IN NO EVENT SHALL WITTE SOFTWARE’S LIABILITY FOR ANY DAMAGES EXCEED THE PRICE PAID FOR THE LICENSE TO USE THE SOFTWARE, REGARDLESS OF THE FORM OF CLAIM. THE PERSON USING THE SOFTWARE BEARS ALL RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE.

2. Modbus Poll Features

2.1. Connections

Modbus Poll read/write data from devices using:

Modbus RTU or ASCII on RS232 or RS485 networks. (USB/RS232/485 Converter)

Modbus Over TCP/IP. (Modbus RTU/ASCII encapsulated in a TCP packet)

Modbus over UDP/IP. (Modbus RTU/ASCII encapsulated in a UDP packet)

2.2. Supported Modbus Functions

02: Read discrete inputs

03: Read holding registers

04: Read input registers

05: Write single coil

06: Write single register

15: Write multiple coils

16: Write multiple registers

17: Report server ID

22: Mask write register

23: Read/Write registers

43/14: Read device identification

2.3. Data logging

Log data to a text file

Log data direct into Excel

2.4. Display formats

Each cell can be individual formatted.

Signed 16-bit register

Unsigned 16-bit register

32-bit signed integer with any word/byte order

32-bit unsigned integer with any word/byte order

64-bit signed integer with any word/byte order

64-bit unsigned integer with any word/byte order

32-bit float with any word/byte order

64-bit double float with any word/byte order

2.5. Miscellaneous features

OLE/Automation for interfacing with Excel VBA, Python etc.

Monitoring of data traffic

Print and print preview

Real time charting

3. Overview

Modbus Poll uses a multiple document interface. That means several windows can be opened. Each one with different data contents from different slave devices at the same time.

This picture shows two open windows. One reading 10 Holding registers from slave id 1 and another reading 10 Holding registers from slave id 2.

3.1. Help from anywhere

Press F1 and get context sensitive help on a topic associated with the current selected item.

SHIFT + F1 invokes a special «help mode» in which the cursor turns into a help cursor (arrow + question mark). The user can then select a visible object in the user interface, such as a menu item, toolbar button, or window. This opens help on a topic that describes the selected item.

3.2. Name cells

Here you can type any text for designation of the value cells. You can also copy/paste text from Excel cells.

3.3. Value cells

Show the data values of the Modbus registers. If you double click a value cell a dialog box lets you write a new value to the slave device. Typing a number in a value cell shows the dialog as well. It is possible to select the used Modbus function used to write the value.

The check box «Close dialog on Response ok» is used to automatically close the dialog box when a value is successfully sent. This is convenient when a lot of values are to be changed. In that way it is fast to select a new cell and then type a new value again.

3.4. Change font

Select the cells to be changed and then right click.

Select the cells to be changed and then menu → display → font.

3.5. Conditional colors

Conditional colors help you visually show values in specific ranges.

Default color: This color is used if none of the conditional colors evaluates to true.

Rule 1: This color selection is used if the expression evaluates to true. Rule 1 has precedence over rule 2.

Rule 2: This color selection is used if the expression evaluates to true.

greater than or equal to

less than or equal to

The «and» operator cannot be used when the data type is of float or 32 bit long type. The condition value is entered as a hex number if «and» is selected. It evaluates to true if any of the bits in both the cell and the condition value is 1.

3.5.1. Color example

Green color if the cell value is between 398 and 402

3.6. Scaling

Scaling helps you scale raw values to human readable values. Scaling works only for signed and unsigned 16/32 bit integers.

A line passing through the two points (X1,Y1) and (X2,Y2)

$Slope = m = (Y2 — Y1) / (X2 — X1)$

$Y = m * (X — X1) + Y1$

Number of digits after the decimal point.

Must be enabled to scale the value from the Modbus server/slave. Scaling is automatically disabled if other than a 16/32 bit integer display format is selected.

3.7. Real time charting

The chart can plot 12 series in real time with up to 100000 points in each series.

3.8. Address Scan

Scan an address range for a list of all valid addresses in a device.

3.9. Open a new window

To open another window you have 3 options:

Select new in the file menu

Press on the toolbar

4. Connection dialog

To open the connection dialog you have 2 options:

Select connect from the connection menu

4.1. Connection

There are 5 different connection types:

Serial:
Modbus over serial line. RS232 or RS485. A USB serial converter can be used.

Modbus TCP/IP:
Select TCP/IP if you want to communicate with a MODBUS TCP/IP network. In this case, slave ID is the same as the Unit ID used in MODBUS TCP/IP.
The port number is default 502.
If the connection fails then try if you can ping your device at the command prompt. If the ping command fails then Modbus Poll fails too.

Modbus UDP/IP:
Select UDP/IP if you want to communicate with a MODBUS UDP/IP network. This is the same as Modbus TCP/IP but the connection less UDP protocol is used instead.

Modbus RTU/ASCII over TCP/IP:
This is a RTU or ASCII message sent over a TCP/IP network instead of serial lines.

Modbus RTU/ASCII over UDP/IP:
This is a RTU or ASCII message sent over a UDP/IP network instead of serial lines.

Connection type 3-5 is not standard Modbus as specified by www.modbus.org but they are added for convenience.

Depending on your selection some other settings will be grayed.

4.2. Serial Settings

Use these parameters to set serial port settings. They are only available if the connection type is «Serial Port«.

Use this option to select RTU or ASCII mode. Default RTU.

Specifies the length of time that Modbus Poll should wait for a response from a slave device before giving up. Default is 1000ms.

Min delay between polls

This setting ensures a minimum delay until the next request is transmitted no matter the scan rate. The resolution of this setting is approximately 15ms. It’s possible on some computers to obtain better resolution but not all.

If you set this value lower than 20ms the 3.5 char time gap between response and a new request can’t be guaranteed. This is because the Windows scheduler switches tasks every 10 — 20ms.

If you Poll several slaves in a serial RS485 network you should NOT set the value lower than 20ms. This is to ensure the 3.5 char time gap.

In a TCP/IP network less than 20ms is ok.

Serial connection to only one slave device less than 20ms is ok.

4.3. Remote Server

Remote server settings are only available when using an Ethernet connection.

Servers IP address. Default is localhost 127.0.0.1

Server port number. Default 502

Max time to use to establish a connection. Default 1000

4.4. Advanced settings

RTS Toggle specifies that the RTS line will be high if bytes are available for transmission. After all buffered bytes have been sent, the RTS line will be low.
You can use this to switch direction if you have a 232/485 converter without an automatic direction switch.

The use of RTS controlled RS232/RS485 converters should be avoided if possible. It is difficult to determine the exact time when to switch off the transmitter with non real-time operating systems like Windows and Linux. If it is switched off too early characters might still sit in the FIFO or the transmit register of the UART and these characters will be lost. Hence the slave will not recognize the message. On the other hand if it is switched off too late then the slave’s message is corrupted and the master will not recognize the message.

DSR specifies whether the DSR (data-set-ready) signal is monitored for output flow control. If this member is TRUE and DSR is turned off, output is suspended until DSR is sent again.

CTS specifies whether the CTS (clear-to-send) signal is monitored for output flow control. If this checkbox is enabled and CTS is turned off, output is suspended until CTS is sent again.

DTR specifies whether the DTR will be enabled or disabled whenever the port is opened.

If your device or RS232/RS485 converter echoes the chars just sent.

5. Read/Write definition

Use this command to define the data to be monitored for the active window.

To open the Read/Write Definition dialog you have 3 options:

Select «Read/Write Definition» from the Setup menu

Press on the toolbar

5.1. Slave ID

Range 1 to 255. (MODBUS protocol specifications say 247). The value 0 is also accepted to communicate directly to a MODBUS/TCP or MODBUS/UDPВ device.

5.2. Function code

You can select 1 of 8 function codes.

5.2.1. Read functions

The data returned by the read functions are displayed on the grid window.

01: Read coils (0x)

02: Read discrete inputs (1x)

03: Read holding registers (4x)

04: Read input registers (3x)

5.2.2. Write functions

The write functions write the data displayed on the grid window.

05: Write single coil (Writes to Coil status)

06: Write single register (Writes to Holding registers)

15: Write multiple coils (Writes to Coils)

16: Write multiple registers (Writes to Holding registers)

5.3. Address

Addresses in the Modbus protocol are confusing! Some protocol specifications use the protocol/message address and others use device addressing.

5.3.1. Protocol/message address

Some protocol specifications use the protocol/message address counting from 0 to 65535 along with a function code. This is also what the new Modbus specifications use. This is the address inside the message sent on the wire.

Modbus Poll use protocol/message address counting from 0 to 65535.

5.3.2. Device address

Some protocol specifications use device address/registers. Registers counts from 1. The first digit describes the function to be used. That means the device address 40101 is identified by address 100. The «4» means Holding registers and 4x registers counts from 1. And even more confusing: 4x means function code 03 and 3x means function code 04!

5.3.3. 5 digits vs. 6 digits addressing

The address format 4x counts from 40001 to 49999. The next address is not 50000. In the old days 9999 addresses was enough. There are cases where 9999 is not enough. Then a zero is added. 40101 becomes 400101 and so on. This is called 6 digits addressing or extended addressing.

This is not a problem with Modbus Poll. 410001 become 10000. The «4» is thrown away and the rest 10001 is decremented by 1 as we count from 0 instead of 1.

5.3.4. Address examples

These examples show how to set up Modbus Poll if a specification uses device addresses.

Read Holding Registers

You want to read 20 registers from device address 40011 from slave ID 2 every 1000ms. From the «4» we know this is function 03 «Read Holding Registers».

Function = «03 Read Holding Registers (4x)»

Address = 10 (11 minus 1)

Read Discrete Inputs

You want to read 1000 coils from address 110201 from slave ID 5 every 500ms. From the «1» we know this is function 02 «Read Discrete Inputs»

Function = «02 Read Discrete Inputs (1x)»

Address = 10200 (10201 — 1)

5.4. Scanrate

The scan rate can be set from 0 to 3600000ms. Note that setting the scan rate lower than the transaction time does not make sense. If a serial connection at 9600baud is used and 125 registers are requested the transaction time is roughly 8 + 2 + 250 + 2 = 262ms + the gap (>3.5 char time) between the request and the response. In this case setting the scan rate at e.g. 100ms does not make sense as the transaction time is at least 262ms + delay in the slave (gap) + min time between polls. (Set in the connection dialog F3).

5.5. Read/Write Disabled

The «Read/Write Disabled» checkbox can be used to temporary enable or disable the communication for this window. A text (Disabled) is then shown along with the Tx and Error counters.

If «Read/Write» is disabled you can make single requests with the «Read/Write once» button or press F6.

«Read/Write once» button

5.5.1. Disable on error

Disable Read/Write in case of error.

5.6. Hide name columns

Hide all name columns. This is convenient to make more space if they are not used.

5.7. Address in cell

If enabled, the address is also shown in the value cell like: 2000 = 00000

5.8. PLC Addresses (Base 1)

This option will show the addresses as device addresses.

5.9. Enron/Daniel Mode

Enron or Enron/Daniels Modbus is Standard Modbus with a few «Vendor Extensions». The exact impact of these extensions is context dependent, but most common Modbus commands work as expected. There are some custom vendor-defined functions available — but few users expect or use them. The largest impact has to do with how 32-bit data values are read/written.

Enron-Modbus defines two special 4x holding register ranges:

4×5001 to 4×5999 are assumed 32-bit long integers (4-bytes per register).

4×7001 to 4×7999 are assumed 32-bit floating points (4-bytes per register).

Dealing with 32-bit values in Modbus is NOT unique to Enron-MB. However, Enron-MB takes the debatable step of returning 4-bytes per register instead of the 2-bytes implied by the term «holding register» in the Modbus specification. This means a poll of registers 4×5001 and 4×5002 in Enron-Modbus returns 8-bytes or two 32-bit integers, whereas Standard Modbus would only return 4-bytes or one 32-bit integer treated as two 16-bit integers. In addition, polling register 4×5010 in Enron-MB returns the tenth 32-bit long integer, whereas Standard Modbus would consider this 1/2 of the fifth 32-bit long integer in this range.

5.10. Rows

Specify the number of rows in the grid you prefer.

6. Real time Charting

Use this command to plot up to 12 data series in a chart in real time.
The real time chart is high speed and capable of drawing a new line as fast as new data is received.

All chart settings are stored with workspace. Save/Open Workspace

To open the Real time charting dialog you have 2 options:

Select «Real time Charting» from the Display menu

The X-Axis displays the number of seconds since the chart was started.

When the points reach the end of the chart there are 3 options:

Stop at end: The charting stops.

Restart at end: The charting starts all over again.

Continue: It continues until it reaches the max number of points or stop is pressed.

6.1. Settings

By default all 12 series are linked to the left Y-Axis. Check the «Right Y-Axis» check box if you want to link a series to the right Y-Axis.

The offset is useful to align data points on the same Y-Axis. For example, data points that are either 0 or 1 can be offset so they are not drawn on top of each other.

6.2. Zoom function

Zooming in on the chart can be useful if you want to see more details. The zoom is controlled with the left mouse button. To zoom a specific part of the chart, simply left-click on the chart (this will be the upper-left corner of the zoomed rectangle) and drag to the bottom-right. A rectangle will appear. As soon as you release the mouse button, the axes will automatically adjust themselves to the region you have selected.

If you left-click on the chart (like for starting a zoom) but if you move to the top-left corner instead, all the modifications done with the zoom and pan features will be canceled (the chart will be in the state it was before the manipulations with the pan and zoom).

6.3. Pan function

To pan the control, right-click somewhere on the control and move the mouse. The point under the mouse will follow the movement of the mouse.

6.4. Link data to the chart series

The chart doesn’t know which data to use unless you link a Modbus data cell to one of the 12 series. To do so select a value cell and from Menu→Display select «Link to Chart».

6.5. Export series

Save series data to disk or copy to clipboard. Paste the data direct in Excel for further processing.
The file is given a .csv extension despite the use of a non-comma field separator.

Delimiters: Select the character that separates values in your text file. Use tab delimiter when copy/paste to Excel.

Number of points

Max point value

Min point value

Average point value

7. Address Scan

Scan an address range for a list of all valid addresses in a device. Addresses are read one by one and the read result is shown in a list.

Scanning all 65535 addresses takes some time depending on connection type, server device etc.

7.1. Export Address Scan

Save Address Scan data to disk or copy to clipboard. Paste the data direct in Excel for further processing.
The file is given a .csv extension despite the use of a non-comma field separator.

Delimiters: Select the character that separates values in your text file. Use tab delimiter when copy/paste to Excel.

8. Display formats

Mark the cells to be formatted. Select one of the 28 display formats from the display menu.

8.1. Native Modbus registers

The 16-bit Modbus registers can be displayed in 4 different modes.

8.2. 32-bit signed integer

This combines 2 16-bit Modbus registers. It can be displayed in 4 different word/byte orders.

Signed integer Big-endian

Signed integer Little-endian

Signed integer Big-endian byte swap

Signed integer Little-endian byte swap

Byte Order: AB CD (Big-endian)
The decimal number 123456789 or in hexadecimal 07 5B CD 15
Order as they come over the wire in a Modbus message: 07 5B CD 15

8.3. 32-bit unsigned integer

This combines 2 16-bit Modbus registers. It can be displayed in 4 different word/byte orders.

Unsigned integer Big-endian

Unsigned integer Little-endian

Unsigned integer Big-endian byte swap

Unsigned integer Little-endian byte swap

Byte Order: AB CD (Big-endian)
The decimal number 123456789 or in hexadecimal 07 5B CD 15
Order as they come over the wire in a Modbus message: 07 5B CD 15

8.4. 64-bit signed integer

This combines 4 16-bit Modbus registers. It can be displayed in 4 different word/byte orders.

Signed integer Big-endian

Signed integer Little-endian

Signed integer Big-endian byte swap

Signed integer Little-endian byte swap

Byte Order: AB CD EF GH (Big-endian)
The decimal number -1,234,567,890,123,456,789 or in hexadecimal EE DD EF 0B 82 16 7E EB
Order as they come over the wire in a Modbus message: EE DD EF 0B 82 16 7E EB

8.5. 64-bit unsigned integer

This combines 4 16-bit Modbus registers. It can be displayed in 4 different word/byte orders.

Unsigned integer Big-endian

Unsigned integer Little-endian

Unsigned integer Big-endian byte swap

Unsigned integer Little-endian byte swap

Byte Order: AB CD EF GH (Big-endian)
The decimal number 1,234,567,890,123,456,789 or in hexadecimal 11 22 10 F4 7D E9 81 15
Order as they come over the wire in a Modbus message: 11 22 10 F4 7D E9 81 15

8.6. 32-bit floating

This combines 2 16-bit Modbus registers. It can be displayed in 4 different word/byte orders.

Float Big-endian byte swap

Float Little-endian byte swap

Byte Order: AB CD (Big-endian)
The floating point number 123456.00 or in hexadecimal 47 F1 20 00
Order as they come over the wire in a Modbus message: 47 F1 20 00

8.7. 64-bit double

This combines 4 16-bit Modbus registers. It can be displayed in 4 different word/byte orders.

Double Big-endian byte swap

Double Little-endian byte swap

Byte Order: AB CD EF GH (Big-endian)
The floating point number 123456789.00 or in hexadecimal 41 9D 6F 34 54 00 00 00
Order as they come over the wire in a Modbus message: 41 9D 6F 34 54 00 00 00

9. Save/Open Workspace

If you open many related Modbus windows it is convenient to save a snapshot of the current layout of all open and arranged Modbus Windows in one workspace.

A workspace (*mbw) is a file that contains display information and file names of all open windows. Not the actual contents. To do this, go to File→ Save Workspace.

When you open a workspace file, Modbus Poll opens all Modbus Windows and displays them in the layout that you saved.

10. Export to csv

Export names and values to a Comma, Semicolon or Tab Separated Values File.

Comma Separated Values file (*.csv)
«Temperature»,»19.7″

Semicolon Separated Values file (*.csv)
«Temperature»;»19.7″

Tab Separated Values file (*.txt)
Temperature 19.7

Depending on your system, comma or period is used to separate decimals.

11. Export to Modbus Slave

Export names, values and formatting to a Modbus Slave file. *.mbs

Modbus Slave version 7.4.0 or newer is required to open the file.

12. Test center

The purpose of this test dialog is to help MODBUS slave device developers to test the device with any string of their own composition.
The list box displays the transmitted data as well as the received data.
You can have several test strings in the pull down list box. When you have entered a string then press the «Add to List» button then the string is added to the list.
The selected string is sent when the «Send» button is pressed.

Rest test strings from a file.

Store the test strings to a file.

Clear the test list.

Add the current test string to the list.

Add a CRC or LRC to the end of the input string.

When using the test center you may want to disable communication from other windows. Check the «Read/Write disable» check box in «Read/Write Definition» dialog. Setup→Read/Write Definition.

12.1. ASCII Example

String in the combo box:

The transmitted string if LRC is added

A CR LF pair is also added.

12.2. TCP/IP Example

Read 10 holding registers.

The first 6 bytes are the TCP/IP header.

12.3. Test center string file

With a text editor such as notepad or similar you can prepare strings to be used in the test.
The first line in the file must be the string «TestCenter». This is how Modbus Poll knows that the file is the correct format. Press «Open list» to open the prepared text file.

12.3.1. Content of a string list

12.4. Copy

Use the Copy button to copy selected Tx/Rx strings to the clipboard.
The SHIFT and CTRL keys can be used together with the mouse to select and deselect strings, select groups of strings, and select non-adjacent strings.

Leave this window open while doing other commands.

13. Modbus Data logging

You can log data to either a text file or direct to Microsoft Excel.

13.1. Text file

Select Log from the setup menu or use shortcut keys: Alt+L

Each Modbus Window logs to its individual text file.

When you want to stop the data logging then select the logging off command on the setup menu.

13.1.1. Log Rate

Write a logline for all Modbus requests. Log frequency as scan rate.

Specify the log rate in seconds. Independent of scan rate.

If the scan rate is e.g. 10000ms it makes no sense to set a 1 sec log rate as data is logged only when new data is ready.

13.1.2. Delimiters

As delimiter you can use one of following options:

Means that the values are organized in columns.

Values separated by a comma.

Values separated by a tab.

13.1.3. Log if data changed only

Specify that a new log line is written only if any data is changed since last log.

13.1.4. Log Errors

Specify that errors such a timeout etc. are logged.

13.1.5. Log Date

Specify that the current date is added to the log time.

13.1.6. Use «T» as delimiter

Specify that the time and date is delimited by the letter «T» as specified in ISO 8601.

13.1.7. Log ms

Specify that milliseconds are added to the log time.

13.1.8. Log address

Specify that the Modbus Address is added to the log.

13.1.9. Start Log when ok is pressed

Specify that logging is started when the ok button is pressed. Otherwise the log setup is just stored when *mbp file is saved.

13.1.10. Start Log when *mbp is opened

Specify that logging is automatically started when a *.mbp file is opened.

13.1.11. Flush to file immediately

This ensures that log lines are not cashed in the file system but physically written immediately.

13.1.12. Append

Specify that logs are appended to the selected file. Otherwise a new file is created.

13.1.13. New log file at midnight

Close the current log file and start a new file at midnight. A time stamp is added to your filename.

Insert header: Information is inserted in the top of the log file.

Name cells in top row: Insert names.

Poll definition: Insert ID, Function etc.

Name: Insert a name of your log.

Example of a text file with fixed width:

You can import the data in an Excel spreadsheet.

13.2. Microsoft Excel

This feature requires that Microsoft Excel is installed. Excel 2003 log is limited to 65535 logs as this is the max number of rows in an Excel sheet. Excel 2007 or newer is limited to 1,048,576 rows. Each Modbus Window logs to its individual Excel sheet.

Select Excel Log from the setup menu or use shortcut keys: Alt+X

Do not touch the Excel sheet while logging as this will interrupt the logging.

13.2.1. Log Rate

Each read: Write a logline for all Modbus requests. Log frequency as scan rate.

Select: Specify the log rate in seconds. Log is independent of scan rate.
Remark: If the scan rate is e.g. 10000ms it makes no sense to set a 1 sec log rate as data is logged only when new data is ready.

Stop after: Specify the number of log lines. Note that Excel 2003 is limited to 65,536 rows and Excel 2007 1,048,576 rows.

Insert header: Information is inserted in the top most 3 lines in the Excel sheet.

Name cells in top row: Insert names in row 3.

Poll definition: Insert ID, Function etc. in row 2.

Name: Insert a log name in row 1.

Excel log with header information.

14. Communication traffic

Select the menu Display→Communication to show the traffic on the serial line or Ethernet cable. Use the stop button to temporary stop the update for inspection.

Use the copy button to copy the selected line to the clipboard.

This window shows only data sent and received by Modbus Poll. You can’t use it as a data sniffer.

Leave this window open while doing other commands.

15. OLE/Automation

Automation (formerly known as OLE Automation) makes it possible for one application to manipulate objects implemented in another application.

An Automation client is an application that can manipulate exposed objects belonging to another application. This is also called an Automation controller.

An Automation server is an application that exposes programmable objects to other applications. Modbus Poll is an automation server.

That means you can use any program that supports VBA (Visual Basic for Applications) such as Visual Basic, Excel etc. to interpret and show the modbus data according to your specific requirements.

15.1. Excel example

You should display the Developer tab or run in developer mode when you want to write macros.

15.1.1. Excel 2007

Click the Microsoft office button and then click Excel options.

Click popular and then select the show Developers tab in the ribbon check box.
Note the ribbon is part of the Microsoft fluent user interface.

15.1.2. Excel 2010, 2016

Click on the file tab.

Click options. Excel Options window will open.

On the left pane click Customize Ribbon.

On the right pane, under Main Tabs, check the Developer check box.

Click OK. The Developer tab should now show in the ribbon (right most tab).

15.1.3. Excel sample code

This example opens two windows. One reading registers and another reading Coils.
Modbus Poll is hidden but you can show it by uncommenting the «ShowWindow» line. This will show one of the windows.

An example is also included with the Modbus Poll installation.
Start → All Programs → Modbus Poll → Excel Example

15.2. Python example

This Python example opens a window and set all possible data formats.

15.3. Connection Functions/Properties

The following properties and functions do the same as you setup in the connection dialog (F3).

15.3.1. Connection

Connection selects the desired connection. A serial port or one of the Ethernet connections can be selected.

Property Connection as Integer

0 = Serial port
1 = Modbus TCP/IP
2 = Modbus UDP/IP
3 = Modbus ASCII/RTU over TCP/IP
4 = Modbus ASCII/RTU over UDP/IP

15.3.2. BaudRate

Applicable only for Connection = 0

Property BaudRate as Long

300
600
1200
2400
4800
9600 (Default)
14400
19200
38400
56000
57600
115200
128000
153600
230400
256000
460800
921600

15.3.3. DataBits

Applicable only for Connection = 0

Property DataBits as Integer

15.3.4. Parity

Applicable only for Connection = 0

Property Parity as Integer

0 = None
1 = Odd
2 = Even (Default)

15.3.5. StopBits

Applicable only for Connection = 0

Property StopBits as Integer

15.3.6. SerialPort

Applicable only for Connection = 0

Property SerialPort as Integer

Default value = 1

15.3.7. Mode

Applicable only for Connection = 0

Property Mode as Integer

0 = RTU Mode
1 = ASCII Mode

15.3.8. RemoveEcho

Applicable only for Connection = 0
If your device or RS232/RS485 converter echoes the chars just sent.

Property RemoveEcho as Integer

0 (Default)
1 (Remove echoes)

15.3.9. ResponseTimeout

The ResponseTimeout specifies the length of time in ms that Modbus Poll should wait for a response from a slave device before giving up.

Property ResponseTimeout as Integer

Default value = 1000

15.3.10. DelayBetweenPolls

Property DelayBetweenPolls as Integer

Default value = 20

15.3.11. ServerPort

Applicable only for Connection = 1…4

Property ServerPort as Long

Default value = 502

15.3.12. ConnectTimeout

The ConnectTimeout specifies the length of time that Modbus Poll should wait for a TCP/IP connection to succeed.

Applicable only for Connection = 1…4

Property ConnectTimeout as Integer

Default value = 1000ms

15.3.13. IPVersion

Applicable only for Connection = 1…4

Property IPVersion as Integer

4 = IP Version 4 (Default)
6 = IP Version 6

15.3.14. OpenConnection

Opens the connection selected with the Connection property.

Function OpenConnection() As Integer

This function has no parameters.

For error 3-5: Please check if you have the latest serial port driver.

Serial Port not available

Serial port. Not possible to get current settings from the port driver.

Serial port. Serial port driver did not accept port settings.

Serial port. Serial port driver did not accept timeout settings.

TCP/UDP Connection failed. WSA start up

TCP/UDP Connection failed. Connect error

TCP/UDP Connection failed. Timeout

TCP/UDP Connection failed. IOCTL

TCP/UDP Connection failed. Socket error

TCP/UDP Connection failed. Address information

Connection already open

15.3.15. CloseConnection

Function CloseConnection() As Integer

This function has no parameters.

Zero if success. Nonzero value if failed.

15.3.16. ShowCommunicationTraffic

Shows the communication traffic window.

Function ShowCommunicationTraffic()

This function has no parameters.

15.3.17. CloseCommunicationTraffic

Closes the communication traffic window if shown.

Function CloseCommunicationTraffic()

This function has no parameters.

15.4. Read Functions

The following functions do the same as you setup in the read/write definition dialog (F8). Read functions are associated with a Modbus Poll document. (The window with data)

Error	Description

You must create a Read before you can use properties to get data.

15.4.1. ReadCoils

Modbus function code 01

Function ReadCoils(SlaveID As Integer, Address As Long, Quantity As Integer, ScanRate As Long) As Integer

SlaveID: The slave address 1 to 255
Address: The data address (Base 0)
Quantity: The number of data. 1 to 2000
ScanRate: 0 to 3600000ms

True if success. False if not success

15.4.2. ReadDiscreteInputs

Modbus function code 02

Function ReadDiscreteInputs(SlaveID As Integer, Address As Long, Quantity As Integer, ScanRate As Long) As Integer

SlaveID: The slave address 1 to 255
Address: The data address (Base 0)
Quantity: The number of data. 1 to 2000
ScanRate: 0 to 3600000ms

True if success. False if not success

15.4.3. ReadHoldingRegisters

Modbus function code 03

Function ReadHoldingRegisters(SlaveID As Integer, Address As Long, Quantity As Integer, ScanRate As Long) As Integer

SlaveID: The slave address 1 to 255
Address: The data address (Base 0)
Quantity: The number of data. 1 to 125
ScanRate: 0 to 3600000ms

True if success. False if not success

15.4.4. ReadInputRegisters

Modbus function code 04

Function ReadInputRegisters(SlaveID As Integer, Address As Long, Quantity As Integer, ScanRate As Long) As Integer

SlaveID: The slave address 1 to 255
Address: The data address (Base 0)
Quantity: The number of data. 1 to 125
ScanRate: 0 to 3600000ms

True if success. False if not success

15.5. Automation Write Functions

The write functions write the values stored in the array filled by the properties. The below Write function do not create a data window. To create a data window use the Win functions e.g. WriteMultipleRegistersWin.

15.5.1. WriteSingleCoil

Modbus function code 05.
Writes the first coil stored in the write array.

Function WriteSingleCoil(SlaveID As Integer, Address As Long) As Integer

SlaveID: The slave address 0 to 255
Address: The data address (Base 0)

True if the write array is ready and the data is sent. False if the array is empty or error in the parameters.
The controlling application is responsible for verifying the write operation by reading back the value written.

15.5.2. WriteSingleRegister

Modbus function code 06.
Writes the first register stored in the write array.

Function WriteSingleRegister (SlaveID As Integer, Address As Long) As Integer

SlaveID: The slave address 0 to 255
Address: The data address (Base 0)

True if the write array is ready and the data is sent. False if the array is empty or error in the parameters.
The controlling application is responsible for verifying the write operation by reading back the value written.

15.5.3. WriteMultipleCoils

Modbus function code 15.
Write the coils stored in the write array.

Function WriteMultipleCoils(SlaveID As Integer, Address As Long, Quantity As Integer) As Integer

True if the write array is ready and the data is sent. False if the array is empty or error in the parameters.
The controlling application is responsible for verifying the write operation by reading back the values written.

SlaveID: The slave address 0 to 255
Address: The data address (Base 0)
Quantity The number of data. 1 to 1968

15.5.4. WriteMultipleRegisters

Modbus function code 16.
Write the registers stored in the write array.

Function WriteMultipleRegisters(SlaveID As Integer, Address As Long, Quantity As Integer) As Integer

SlaveID: The slave address 0 to 255
Address: The data address (Base 0)
Quantity: The number of data. 1 to 123

True if the write array is ready and the data is sent. False if the array is empty or error in the parameters.
The controlling application is responsible for verifying the write operation by reading back the value written.

15.5.5. Python example

Python example how to create a window that read 10 registers from address 0 (40001) and then write 5 registers.

15.6. Various Functions

Various functions are associated with a Modbus Poll document. (The window with data)

15.6.1. ShowWindow

As default Modbus document windows are hidden. The ShowWindow function makes Modbus Poll visible and shows the document with data content.

Function ShowWindow()

This function has no parameters.

15.6.2. GetTxCount

Retreives the number of requests.

Function GetTxCount() As Long

This function has no parameters.

The number of requests.

15.6.3. GetRxCount

Retreives the number of response.

Function GetRxCount() As Long

This function has no parameters.

The number of response.

15.6.4. GetName

Retreives the name of a value.

Function GetName(Index As Integer) As String

Index: Index 0 corresponds to the first Modbus address.

15.6.5. SetName

Changes the name of a value. Function SetName(Index As Integer, Name As String)

Index: Index 0 corresponds to the first Modbus address.
Name: The name of the value cell.

15.6.6. FormatAll

Format all value cells with the selected format.

Function FormatAll(Format As Integer)

Format: The format of the value cell.

15.6.7. GetFormat

Retreives the display format of the Modbus value.

Function GetFormat(Index As Integer) As Integer

Index: Index 0 corresponds to the first Modbus address.

Float little-endian byte swap

Double little-endian byte swap

32-bit Signed little-endian byte swap

32-bit Signed big-endian

Float big-endian byte swap

Double big-endian byte swap

32-bit Signed little-endian

32-bit Signed big-endian byte swap

32-bit Unsigned big-endian

32-bit Unsigned little-endian byte swap

32-bit Unsigned big-endian byte swap

32-bit Unsigned little-endian

64-bit Signed big-endian

64-bit Signed little-endian byte swap

64-bit Signed big-endian byte swap

64-bit Signed little-endian

64-bit Unsigned big-endian

64-bit Unsigned little-endian byte swap

64-bit Unsigned big-endian byte swap

64-bit Unsigned little-endian

ID	Format

This setting is only for display. You still need to use byteOrder to get the correct endianness when using Get/Set value functions.

15.6.8. SetFormat

Change the display format of the Modbus values. See Format values above.

Function SetFormat(Index As Integer, Format As Integer)

Index: Index 0 corresponds to the first Modbus address.
Format: The format of the value cell.

15.6.9. ResizeWindow

Resize an opened window to fit the grid.

Function ResizeWindow()

This function has no parameters.

15.6.10. ResizeAllColumns

Resize all columns to fit the values inside the cells.

Function ResizeAllColumns()

This function has no parameters.

15.6.11. Rows

Specify the number of rows in the grid.

Function Rows(NumberRows)

NumberRows: Number of rows in the grid.

10 Rows (Default)

Fit to quantity

15.6.12. ReadResult

Use this property to check if communication established with Read is running successful.

Property ReadResult As Integer

This function has no parameters.

RESPONSE ERROR (The response was not the expected slave id, function or address)

PORT NOT OPEN ERROR

INSUFFICIENT BYTES RECEIVED

BYTE COUNT ERROR

TRANSACTION ID ERROR

ILLEGAL DATA ADDRESS

ILLEGAL DATA VALUE

SERVER DEVICE FAILURE

SERVER DEVICE BUSY

GATEWAY PATH UNAVAILABLE

GATEWAY TARGET DEVICE FAILED TO RESPOND

15.6.13. WriteResult

Use this function to check if a write was successful.
The value is DATA_UNINITIALIZED until the result from the slave is available. See ReadResult for a list of possible values.

Property WriteResult As Integer

Return a write result as an integer.

15.7. Automation data properties

The below properties are used to set or get values in the internal write/read arrays in Modbus Poll. The Index used is not a Modbus Address. The Index always counts from 0 no matter of the address used. The data properties are associated with a Modbus Poll document. (The window with data)

There are 2 version of each data properties:

One with no postfix such as SRegisters which is used to set or get a value from the internal write/read array.

One with Win as postfix such as SRegistersWin which is used to set or get a value direct from the data window. This is used when the data window is used for a Write function e.g. WriteMultipleRegistersWin.

Example 1:

Example 2 with floating point values:
Write 3 floating point values.

Example 3:
Create a window that writes 3 registers.

15.7.1. Coils, CoilsWin

Property Coils(Index As Integer) As Integer

Sets a coil in the write array structure or return a coil from the read array.

Coils(Index) [=newvalue]

15.7.2. SRegisters, SRegistersWin

Property SRegisters(Index As Integer) As Integer

Sets a register in the write array structure or return a register from the read array.

SRegisters(Index) [=newvalue]

15.7.3. URegisters, URegistersWin

Property URegisters(Index As Integer) As Long

Sets a register in the write array structure or return a register from the read array.

URegisters(Index) [=newvalue]

15.7.4. Ints_32, Ints_32Win

Property Ints_32(Index As Integer) As Double

Sets a 32-bit integer in the write array structure or return an integer from the read array.

Ints_32(Index) [=newvalue]

15.7.5. UInts_32, UInts_32Win

Property UInts_32(Index As Integer) As Double

Sets a 32-bit unsigned integer in the write array structure or return an unsigned integer from the read array.

UInts_32(Index) [=newvalue]

15.7.6. Ints_64, Ints_64Win

Property Ints_64(Index As Integer) As Double

Sets a 64-bit integer in the write array structure or return an integer from the read array.

Ints_64(Index) [=newvalue]

15.7.7. UInts_64, UInts_64Win

Property UInts_64(Index As Integer) As Double

Sets a 64-bit unsigned integer in the write array structure or return an unsigned integer from the read array.

UInts_64(Index) [=newvalue]

15.7.8. Floats, FloatsWin

Property Floats(Index As Integer) As Single

Sets a float in the write array structure or returns a float from the read array.

Floats(Index) [=newvalue]

15.7.9. Doubles, DoublesWin

Property Doubles(Index As Integer) As Double

Sets a double in the write array structure or return a double from the read array.

Doubles(Index) [=newvalue]

15.7.10. ByteOrder

Property ByteOrder As Integer

Sets the byte order used by Ints_32, UInts_32, Ints_64, UInts_64, Floats and Doubles properties.
The Win versions do not use this Property ByteOrder.

ID	Description

Little-endian byte swap

Big-endian byte swap

Example for Ints_32:
Byte Order: Big-endian
The decimal number 123456789 or in hexadecimal 07 5B CD 15
Order as they come over the wire in a Modbus message: 07 5B CD 15

ByteOrder [=newvalue]

15.8. Write Functions (Create a data window)

The following functions do the same as you set up in the read/write definition dialog (F8).
The functions creates a data window and the data content in the data windows is written according to the scan rate.

15.8.1. WriteSingleCoilWin

Modbus function code 05.

Function WriteSingleCoilWin(SlaveID As Integer, Address As Long, ScanRate As Long) As Integer

SlaveID: The slave address 1 to 255
Address: The data address (Base 0)
ScanRate: 0 to 3600000ms

True if success. False if not success

15.8.2. WriteSingleRegisterWin

Modbus function code 06.

Function WriteSingleRegisterWin(SlaveID As Integer, Address As Long, ScanRate As Long) As Integer

SlaveID: The slave address 1 to 255
Address: The data address (Base 0)
ScanRate: 0 to 3600000ms

True if success. False if not success

15.8.3. WriteMultipleCoilsWin

Modbus function code 15.

Function WriteMultipleCoilsWin(SlaveID As Integer, Address As Long, Quantity As Integer, ScanRate As Long) As Integer

SlaveID: The slave address 1 to 255
Address: The data address (Base 0)
Quantity: The number of data. 1 to 1968
ScanRate: 0 to 3600000ms

True if success. False if not success

15.8.4. WriteMultipleRegistersWin

Modbus function code 16.

Function WriteMultipleRegistersWin(SlaveID As Integer, Address As Long, Quantity As Integer, ScanRate As Long) As Integer

SlaveID: The slave address 1 to 255
Address: The data address (Base 0)
Quantity: The number of data. 1 to 123
ScanRate: 0 to 3600000ms

True if success. False if not success

16. Exception and error messages

Modbus Exceptions and error messages are displayed in red text in the 2 nd line in each window.

16.1. Modbus Exception Codes

Modbus exceptions are errors returned from the slave device.

The function code received in the query is not an allowable action for the server (or slave). This may be because the function code is only applicable to newer devices, and was not implemented in the unit selected. It could also indicate that the server (or slave) is in the wrong state to process a request of this type, for example because it is not configured and is being asked to return register values.

Illegal Data Address

The data address received in the query is not an allowable address for the server. More specifically, the combination of reference number and transfer length is invalid. For a controller with 100 registers, the PDU addresses the first register as 0, and the last one as 99. If a request is submitted with a starting register address of 96 and a quantity of registers of 4, then this request will successfully operate (address-wise at least) on registers 96, 97, 98, 99. If a request is submitted with a starting register address of 96 and a quantity of registers of 5, then this request will fail with Exception Code 0x02 “Illegal Data Address” since it attempts to operate on registers 96, 97, 98, 99 and 100, and there is no register with address 100.

Illegal Data Value

A value contained in the query data field is not an allowable value for the server (or slave). This indicates a fault in the structure of the remainder of a complex request, such as that the implied length is incorrect. It specifically does NOT mean that a data item submitted for storage in a register has a value outside the expectation of the application program, since the MODBUS protocol is unaware of the significance of any particular value of any particular register.

Server Device Failure

An unrecoverable error occurred while the server (or slave) was attempting to perform the requested action.

Specialized use in conjunction with programming commands.
The server (or slave) has accepted the request and is processing it, but a long duration of time will be required to do so. This response is returned to prevent a timeout error from occurring in the client (or master). The client (or master) can next issue a Poll Program Complete message to determine if processing is completed.

Server Device Busy

Specialized use in conjunction with programming commands.
The server (or slave) is engaged in processing a long–duration program command. The client (or master) should retransmit the message later when the server (or slave) is free.

Gateway Path Unavailable

Specialized use in conjunction with gateways, indicates that the gateway was unable to allocate an internal communication path from the input port to the output port for processing the request. Usually means that the gateway is misconfigured or overloaded.

Gateway Target Device Failed to Respond

Specialized use in conjunction with gateways, indicates that no response was obtained from the target device. Usually means that the device is not present on the network.

16.2. Modbus Poll error messages

The response is not received within the expected time. Check the following:

Serial settings such as Baud rate, parity, Data bits, Stop bits etc.

Modbus mode, RTU or ASCII

Check that Host and Port are consistent with the slave

The response is not the expected one. Different slave ID.

The CRC value of the received response is not correct.

This is an error reported by the serial driver. This could happen if a USB/RS232/485 converter is used and the USB cable is unplugged. There are 4 types:

Serial connection error

Output buffer overflow

Write error using TCP/IP connection is normally caused by lost connection.

This is an error reported by the serial driver. There are 6 types:

Character buffer overrun

Input buffer overflow

Read error using TCP/IP connection is normally caused by lost connection.

Insufficient bytes received

The response is not the expected length.

Byte count error

The byte count in the response is not correct. Compared to the expected.

Transaction ID error

It is used for transaction pairing, the MODBUS server copies in the response the transaction identifier of the request.

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ID	Endianness

I’m facing some issues with a Modbus RTU implementation. I have 2x Arduino MKR Zeros with RS485 hats/expansions as my 2 slave devices (using the ArduinoModbus library). I am trying to poll the devices from my PC (Windows) using python and the pymodbus library, running at 9600 baud.

I can succesfully transfer data. The initial sanity test was a simple analogRead() on one of the Arduinos (sensor 1), writing to it’s internal holding register and then having the pymodbus master poll/request that register.

I’ve now connected a second Arduino (sensor 2) which has an I2C connection to a flow sensor. That arduino is running reads of the sensor over I2C and updating 5x holding registers with the data. The master (PC) is polling both Arduinos (sensors 1 and 2) one after the other. It is always successful in acquiring sensor 1’s data (only 1 register) but intermittently fails getting sensor 2’s data (5 registers). The python console looks like this:

Sensor 2: 0,25000,0,0, 0
Sensor 2: 0,25000,0,0, 0
Modbus Error: [Input/Output] No Response received from the remote unit/Unable to decode response
Modbus Error: [Input/Output] No Response received from the remote unit/Unable to decode response
Sensor 2: 0,25000,0,0, 0
Modbus Error: [Input/Output] No Response received from the remote unit/Unable to decode response
Sensor 2: 0,25000,0,0, 0
Modbus Error: [Input/Output] No Response received from the remote unit/Unable to decode response

A deeper look at the logs reveals the issue is not all the Bytes are making it accross, see below:

11/24/2022 03:52:59 PM Running transaction 4
11/24/2022 03:52:59 PM SEND: 0x1 0x3 0x0 0x0 0x0 0x5 0x85 0xc9
11/24/2022 03:52:59 PM Changing state to IDLE - Last Frame End - 1669265577.447308, Current Time stamp - 1669265579.457942
11/24/2022 03:52:59 PM New Transaction state "SENDING"
11/24/2022 03:52:59 PM Changing transaction state from "SENDING" to "WAITING FOR REPLY"
11/24/2022 03:52:59 PM {msg_start} received, Expected 15 bytes Received 11 bytes !!!!
11/24/2022 03:52:59 PM Changing transaction state from "WAITING FOR REPLY" to "PROCESSING REPLY"
11/24/2022 03:52:59 PM RECV: 0x1 0x3 0xa 0x0 0x0 0x61 0xa8 0x0 0x0 0x0 0x0
11/24/2022 03:52:59 PM Frame - [b'x01x03nx00x00axa8x00x00x00x00'] not ready
11/24/2022 03:52:59 PM Getting transaction 1
11/24/2022 03:52:59 PM Changing transaction state from "PROCESSING REPLY" to "TRANSACTION_COMPLETE"
11/24/2022 03:52:59 PM Modbus Error: [Input/Output] No Response received from the remote unit/Unable to decode response
11/24/2022 03:53:01 PM Current transaction state - TRANSACTION_COMPLETE

I’ve now removed asking sensor 1 for data at all and have my python script only requesting from sensor 2 (the problem sensor) but the problem still remains, my python script is as follows:

import serial
import time
import logging
from pymodbus.client import ModbusSerialClient
from pymodbus.transaction import ModbusRtuFramer

logger = logging.getLogger()
logger.setLevel(logging.DEBUG)
handler = logging.FileHandler('main.log', 'w', 'utf-8')
handler.setFormatter(logging.Formatter(fmt='%(asctime)s %(message)s', datefmt='%m/%d/%Y %I:%M:%S %p'))
logger.addHandler(handler)

client = ModbusSerialClient("COM12", ModbusRtuFramer, baudrate=9600, timeout=10, reset_socket=False)
client.connect()

while(1):

    c2 = client.read_holding_registers(0,5,1)
    if c2.isError():
        logger.error(msg=c2)
        print(c2)
    else:    
        print(f"Sensor 2: {c2.getRegister(0)},{c2.getRegister(1)},{c2.getRegister(2)},{c2.getRegister(3)}, {c2.getRegister(4)}")
    
    time.sleep(2)

I’m not quite sure what’s misfiring… I can stream the sensor 2 data perfectly well using a GUI like QModMaster which I know uses libmodbus under the hood. Should I be instead working with the python library pylibmodbus? It requires a compiled libmodbus locally which is a bit of a headache for Windows…

Am I missing a setting for pymodbus that could help?

I have tried varying the pymodbus timeout which did not work. I have tried decreasing the sample rate for the flow sensor to reduce the frequecy and ensure the Arduino is free/available for Modbus requests. This did not work, in fact decreasing it made the problem worse.

I have tried adding basic delays to the python code (time.sleep(2)) to slow down the Modbus requests but this had no impact on the errors.

Hope someone knows what’s going on as I’ve spent ages trawling through online sources to find the answer to no avail. If more clarification is needed from my end I can provide

Thanks!

P.S. Arduino code below for reference

#include <Arduino.h>
#include <ArduinoRS485.h>
#include <ArduinoModbus.h>
#include <Wire.h>

/**
 * Modbus slave/server
*/

#define SAMPLE_RATE 50

const int ADDRESS = 0x08; // Sensor I2C Address
const float SCALE_FACTOR_FLOW = 500.0; // Scale Factor for flow rate measurement
const float SCALE_FACTOR_TEMP = 200.0; // Scale Factor for temperature measurement

int count = 0;
unsigned long startMillis;
unsigned long currentMillis;

enum error_types{
  no_error,
  write_mode_error, 
  read_error
  };

enum error_types error; 

// Protoypes
int stop_continuous_measurement();
int start_continous_measurement();

void setup() {
  int ret;

  // Start Serial and I2C
  Serial.begin(9600); // initialize serial communication
  Wire.begin();       // join i2c bus (address optional for master)

  // Set up MODBUS
  if (!ModbusRTUServer.begin(0x01,9600)) {
    Serial.println("Could not begin ModbusRTU server...");
    while(1);
  }

  // configure holding registers at address 0x00, 4 registers for data
  ModbusRTUServer.configureHoldingRegisters(0x00, 5);

  // start sensor
  do {
    // Soft reset the sensor
    Wire.beginTransmission(0x00);
    Wire.write(0x06);
    ret = Wire.endTransmission();
    if (ret != 0) {
      Serial.println("Error while sending soft reset command, retrying...");
      delay(500); // wait long enough for chip reset to complete
    }
  } while (ret != 0);

  delay(50); // wait long enough for chip reset to complete

  // To perform a measurement, first send 0x3608 to switch to continuous
  if(start_continous_measurement() !=0) {
    error = write_mode_error;
  }

  startMillis = millis();
}

void loop() {

  ModbusRTUServer.poll();

  uint16_t aux_value;
  uint16_t sensor_flow_value;
  uint16_t sensor_temp_value;
  int16_t signed_flow_value;
  int16_t signed_temp_value;
  float scaled_flow_value;
  float scaled_temp_value;
  byte aux_crc;
  byte sensor_flow_crc;
  byte sensor_temp_crc;

  // measurement mode (H20 calibration), then read 3x (2 bytes + 1 CRC byte) from the sensor.
  // To perform a IPA based measurement, send 0x3615 instead.
  // Check datasheet for available measurement commands.

  error = no_error;

  currentMillis = millis();

  if(currentMillis - startMillis > SAMPLE_RATE){
    Wire.requestFrom(ADDRESS, 9);
    if (Wire.available() < 9) {
      error = read_error;
    }
    else{
      sensor_flow_value  = Wire.read() << 8; // read the MSB from the sensor
      sensor_flow_value |= Wire.read();      // read the LSB from the sensor
      sensor_flow_crc    = Wire.read();
      sensor_temp_value  = Wire.read() << 8; // read the MSB from the sensor
      sensor_temp_value |= Wire.read();      // read the LSB from the sensor
      sensor_temp_crc    = Wire.read();
      aux_value          = Wire.read() << 8; // read the MSB from the sensor
      aux_value         |= Wire.read();      // read the LSB from the sensor
      aux_crc            = Wire.read();

      signed_flow_value = (int16_t) sensor_flow_value;
      scaled_flow_value = ((float) signed_flow_value) / SCALE_FACTOR_FLOW;

      signed_temp_value = (int16_t) sensor_temp_value;
      scaled_temp_value = ((float) signed_temp_value) / SCALE_FACTOR_TEMP;
    
      // write to MODBUS registers
      ModbusRTUServer.holdingRegisterWrite(0, (uint16_t) count);
      ModbusRTUServer.holdingRegisterWrite(1, (uint16_t) scaled_temp_value*1000);
      ModbusRTUServer.holdingRegisterWrite(2, (uint16_t) scaled_flow_value*1000);
      ModbusRTUServer.holdingRegisterWrite(3,(uint16_t) aux_value);
      ModbusRTUServer.holdingRegisterWrite(4, (uint16_t) error);
    }
    startMillis = currentMillis;
  }
}

int start_continous_measurement() {
  Wire.beginTransmission(ADDRESS);
  Wire.write(0x36);
  Wire.write(0x08);
  return Wire.endTransmission();
}

int stop_continuous_measurement() {
    // To stop the continuous measurement, first send 0x3FF9.
    Wire.beginTransmission(ADDRESS);
    Wire.write(0x3F);
    Wire.write(0xF9);
    return Wire.endTransmission();
}