Serial server and client - No reply from server.

[1038642892]

Scenario

I'd like to use the server and console REPL to test serial communication locally on Linux (WSL2) with a socat provided link between two virtual serial ports.

Scenario diagram

Server <-> /dev/pts/7 <-> socat <-> /dev/pts/6 <-> Client

Issue

The server does not reply to the command... shown below.

Server init command

pymodbus.server --web-port 8881 run --modbus-server serial --framer rtu --modbus-port /dev/pts/7 --unit-id 1

Client init command and REPL commands

pymodbus.console serial --method rtu --port /dev/pts/8 --baudrate 9600 --bytesize 8 --parity N --stopbits 1

> client.connect
true

> client.read_coils slave=1 address=1 count=1
{
    "original_function_code": "1 (0x1)",
    "error": "[Input/Output] Modbus Error: [Invalid Message] No response received, expected at least 4 bytes (0 received)"
}

Socat pipe in between

This command below creates a link between two virtual serial ports, and prints messages out in hex format.
This has been proven to work with another piece of code (i can share this if required).
socat -x -v -d -d PTY,raw,echo=0,ispeed=b9600 PTY,raw,echo=0,ospeed=b9600
sudo chown user:user /dev/pts/7 /dev/pts/8

This is the output (requesting read coil address 1, count 1, slave 1.

2022/10/20 10:58:53 socat[1873] N PTY is /dev/pts/7
2022/10/20 10:58:53 socat[1873] N PTY is /dev/pts/8
2022/10/20 10:58:53 socat[1873] N starting data transfer loop with FDs [5,5] and [7,7]
< 2022/10/20 11:09:53.296277  length=8 from=0 to=7
 01 01 00 01 00 01 ac 0a                          ........
--

You'll see there's no reply.

Any support is appreciated to try to troubleshoot why i'm not seeing a message/correct message back from server.

[janiversen]

Socat works nicely, I use it daily. You can test serial without socat too, see test/test_examples.py

@dhoomakethu is the expert on REPL, I am concentrating on the library itself and as you can see from our test setup the library as such works.

[1038642892]

Thank you for your message, i will check out the test examples.

[janiversen]

I will however next week start using REPL client to test some a new datastore feature that allows you to build device simulations, so maybe I will see and fix your problem.

[dhoomakethu]

@1038642892 what version of pymodbus are you using ?

I just gave a try with latest on dev and it seems to be working

[dhoomakethu]

For your reference this is the socat caommand I use

socat -d -d PTY,link=/tmp/ptyp0,raw,echo=0,ispeed=9600 PTY,link=/tmp/ttyp0,raw,echo=0,ospeed=9600

[dhoomakethu]

Looks like you are missing --timeout option in the client , add a timeout like --timeout 2 and see if that helps.

[1038642892]

I think we have different socat versions
sudo socat -d -d PTY,link=/tmp/ptyp0,raw,echo=0,ispeed=9600 PTY,link=/tmp/ttyp0,raw,echo=0,ospeed=9600

2022/10/20 12:22:18 socat[1905] N PTY is /dev/pts/7
2022/10/20 12:22:18 socat[1905] E cfsetispeed(0x562559ec39c0, 9600): Invalid argument
2022/10/20 12:22:18 socat[1905] N exit(1)

But changing to this command (b9600) seems to create the /dev/pts/7 and 8 with symlink /tmp/ttyp0 and ptyp0 just fine.
sudo socat -d -d PTY,link=/tmp/ptyp0,raw,echo=0,ispeed=b9600 PTY,link=/tmp/ttyp0,raw,echo=0,ospeed=b9600

2022/10/20 12:22:33 socat[1907] N PTY is /dev/pts/7
2022/10/20 12:22:33 socat[1907] N PTY is /dev/pts/8
2022/10/20 12:22:33 socat[1907] N starting data transfer loop with FDs [5,5] and [7,7]

Here's the new server and client test with your requested change to timeout and similar params to yours.

pymodbus.server --verbose --web-port 8881 run --modbus-server serial --framer rtu --modbus-port /tmp/ttyp0 --unit-id 1 --random 2 --baudrate 9600
pymodbus.console serial --method rtu --port /tmp/ptyp0 --baudrate 9600 --timeout 2

For reference my socat version

socat -V
socat by Gerhard Rieger and contributors - see www.dest-unreach.org
socat version 1.7.3.3 on Oct 26 2019 17:42:04
   running on Linux version #1 SMP Fri Apr 2 22:23:49 UTC 2021, release 5.10.16.3-microsoft-standard-WSL2, machine x86_64
features:
  #define WITH_STDIO 1
  #define WITH_FDNUM 1
  #define WITH_FILE 1
  #define WITH_CREAT 1
  #define WITH_GOPEN 1
  #define WITH_TERMIOS 1
  #define WITH_PIPE 1
  #define WITH_UNIX 1
  #define WITH_ABSTRACT_UNIXSOCKET 1
  #define WITH_IP4 1
  #define WITH_IP6 1
  #define WITH_RAWIP 1
  #define WITH_GENERICSOCKET 1
  #define WITH_INTERFACE 1
  #define WITH_TCP 1
  #define WITH_UDP 1
  #define WITH_SCTP 1
  #define WITH_LISTEN 1
  #define WITH_SOCKS4 1
  #define WITH_SOCKS4A 1
  #define WITH_PROXY 1
  #define WITH_SYSTEM 1
  #define WITH_EXEC 1
  #undef WITH_READLINE
  #define WITH_TUN 1
  #define WITH_PTY 1
  #define WITH_OPENSSL 1
  #undef WITH_FIPS
  #define WITH_LIBWRAP 1
  #define WITH_SYCLS 1
  #define WITH_FILAN 1
  #define WITH_RETRY 1
  #define WITH_MSGLEVEL 0 /*debug*/

[janiversen]

Just for info, in respect of REPL dev is the same a release 3.0.0 for now.

[1038642892]

I have also found that using non REPL - code example has the same issue - no reply from server.

In my screenshot you can see i've enabled --verbose on server, is this DEBUG message the only one expected with the --verbose or should I see further messages?
DEBUG async_io:860 Exception while create - 'ModbusSingleRequestHandler' object is not callable

[janiversen]

This is a bug we solved quite a while ago, seems you are using an old version.

[1038642892]

OK I will fetch source and try with that. Thank you

[1038642892]

I was using the version available in pip.

I've removed pymodbus-3.0.0 and tried from source (3.0.1.dev0).

This seems to have fixed it. Thank you for your time @janiversen and @dhoomakethu. I'm impressed with the pymodbus tooling.

See below for installation steps used:

virtualenv testenv
source testenv/bin/activate
git clone <URL>
pip install -e .

[janiversen]

thanks for the report, this gives me a good reason to speedup 3.0.1