Add support for Modbus RTU Over TCP

[marinonjalsson]

Hi

It would be a good idea to add support from Modbus RTU Over TCP. I, for one, got device supporting that protocol and not Modbus TCP!.

Below are some statements from http://www.control.com/thread/1305797461

Where Modbus over TCP means a MODBUS RTU packet wrapped in a TCP packet.

Standard Modbus RTU is meant for transmission over serial lines (RS232 or RS485 are the most common). The message starts with a one byte Slave ID and ends with a two byte CRC. Sometimes the RTU message is run through a gateway onto ethernet without actually changing any of the bytes in the message. This is commonly called "RTU over TCP".

There is a different specification for Modbus TCP where the message bytes are modified to add the 6 byte MBAP header and remove the two byte CRC.

Best regards, Marinó

[sq5gvm]

Hi Marinó,

If you're interested in patching libmodbus you can grab "ugly patch that breaks the design guidelines of the beautiful original code" written by Johan Englund at https://groups.google.com/forum/#!msg/libmodbus/xm8S6MMDhgM/Qd97y9_mhwgJ
I'm using his patch (updated for version 3.0.6 of libmodbus) successfully since ~February 2014.
To see how to use it - please check referenced above changes in my branch of check_modbus nagios/icinga plugin.

regards
Mirek (sq5gvm)

[karlp]

I'd like to get this as an option too. PyModbus lets you use arbitrary framers with arbitrary transports, it works quite well: https://pymodbus.readthedocs.org/en/latest/examples/changing-framers.html

I had been trying to use socat to setup a virtual serial port<->tcp port mapping, but was getting "Invalid Argument" errors from some syscall inside libmodbus. Worked around it by using pymodbus, but that's not as portable to small devices as libmodbus :)

[stephane]

Related to #257

[stephane]

When I see all the forks which implement that feature I think you are legion to want it!

[cevatbostancioglu]

So , i look around and still can'not found any implementation. i implemented myself , first i am opening tcp socket and listening. and then parsing manually modbus tcp packet , find slaveid , function etc. after that polling with modbus_new_rtu , reading there and putting that values in ctx->mb_mapping then calling modbus_reply function , everything works fine.

for every tcp packet , first 5 bytes is zero. than starting the frame. i don't know why.
0x < 00 00 00 00 00 06 01 03 04 02 00 01 >
this packet offset is 7.

BUT some modbus IDE's(SCH ELECTRIC ION SETUP) , just sending this packet
0x < 03 03 0B C2 00 01 26 30 >
this packet offset is also 7. but when i write query[offset + 1] like that , gives me nothing.

so , i parsed like , " if first index is zero or not", if it's zero parse this index like that and so on.

BUT i can't send data to ION Setup program , modbus library send errors like unknown functions etc.

so do you guys any idea how i can handle this problem ?

[ElenRey]

Hello, here is an article about Modbus over ethernet that I've found

Have anybody tested this software?

[henkhoo]

+1 for Modbus-over-TCP option please.

i also found http://www.modbusdriver.com/modpoll.html can do it with the
-m enc ( Encapsulated Modbus RTU over TCP ) option.

[v-zhuravlev]

Sample implementation: #385

[DmitriyVinogradov]

Hello! I suggest a patch for use modbus over tcp and modbus over udp.
patch.zip

[DmitriyVinogradov]

Thank. I wanted to stay within the library implementation. сб, 3 нояб. 2018 г. в 4:38, Mike Purvis <notifications@github.com>:

…

Here's another example implementation: https://github.com/rscada/libmodbus/blob/master/modbus/modbus-udp.c — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#192 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AqDq7BlbokO8AfcKeY6hzvIaro_1E6IRks5urOWcgaJpZM4BbSKN> .