RTU Server works by mbpoll but not with pymodbus

[srolf]

Hi all, I'm pretty new to Modbus and pymodbus.

Problem: A self-created RTU server providing a switch status (ESP32 with modbus-serial lib) can be accessed via mbpoll but not via pymodbus (Raspberry Pi2 with a RS485 HAT).

Running mbpoll on RPi:

$ mbpoll -m rtu -b38400 -P none -a13 -t1 /dev/serial0 -v
debug enabled
Set device=/dev/serial0
mbpoll 1.0-0 - FieldTalk(tm) Modbus(R) Master Simulator
Copyright © 2015-2019 Pascal JEAN, https://github.com/epsilonrt/mbpoll
This program comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to redistribute it
under certain conditions; type 'mbpoll -w' for details.

Opening /dev/serial0 at 38400 bauds (N, 8, 1)
Set response timeout to 1 sec, 0 us
Protocol configuration: Modbus RTU
Slave configuration...: address = [13]
                        start reference = 1, count = 1
Communication.........: /dev/serial0,      38400-8N1 
                        t/o 1.00 s, poll rate 1000 ms
Data type.............: discrete input

-- Polling slave 13... Ctrl-C to stop)
[0D][02][00][00][00][01][B9][06]
Waiting for a confirmation...
<0D><02><01><00><A2><D8>
[1]: 	0
-- Polling slave 13... Ctrl-C to stop)
[0D][02][00][00][00][01][B9][06]
Waiting for a confirmation...
<0D><02><01><01><63><18>
[1]: 	1
-- Polling slave 13... Ctrl-C to stop)
[0D][02][00][00][00][01][B9][06]
Waiting for a confirmation...
<0D><02><01><01><63><18>
[1]: 	1
-- Polling slave 13... Ctrl-C to stop)
[0D][02][00][00][00][01][B9][06]
Waiting for a confirmation...
<0D><02><01><01><63><18>
[1]: 	1
-- Polling slave 13... Ctrl-C to stop)
[0D][02][00][00][00][01][B9][06]
Waiting for a confirmation...
<0D><02><01><00><A2><D8>
[1]: 	0
-- Polling slave 13... Ctrl-C to stop)
[0D][02][00][00][00][01][B9][06]
Waiting for a confirmation...
<0D><02><01><00><A2><D8>
[1]: 	0
^C--- /dev/serial0 poll statistics ---
6 frames transmitted, 6 received, 0 errors, 0.0% frame loss

The pymodbus RTU client implementation looks like this:

import pymodbus
import serial
from pymodbus.pdu import ModbusRequest
from pymodbus.client import ModbusSerialClient as ModbusClient #initialize a serial RTU client instance
from pymodbus.transaction import ModbusRtuFramer

client=ModbusClient(method="rtu", port="/dev/serial0", stopbits=1, bytesize=8, parity='N', baudrate=38400)

#Connect to the serial modbus server
connection = client.connect()
print (connection)

#Starting add, num of reg to read, slave unit.
result=client.read_discrete_inputs(address=1, count=1, slave=13)

print(result)

#Closes the underlying connection
client.close()

The response looks like this:

True
Modbus Error: [Input/Output] No Response received from the remote slave/Unable to decode response

Any idea about what I'm doing wrong?

[srolf]

The upper implementation didn't include logging, so I've updated the python script to:

import pymodbus
import serial
from pymodbus.pdu import ModbusRequest
from pymodbus.client import ModbusSerialClient as ModbusClient #initialize a serial RTU client instance
from pymodbus.transaction import ModbusRtuFramer

import logging
logging.basicConfig()
log = logging.getLogger('pymodbus')
log.setLevel(logging.DEBUG)

client=ModbusClient(method="rtu", port="/dev/serial0", stopbits=1, bytesize=8, parity='N', baudrate=38400)

#Connect to the serial modbus server
connection = client.connect()
print (connection)

#Starting add, num of reg to read, slave unit.
result=client.read_discrete_inputs(address=1, count=1, slave=13)

print(result)

#Closes the underlying connection
client.close()

The log out looks like this:

True
DEBUG:pymodbus.logging:Current transaction state - IDLE
DEBUG:pymodbus.logging:Running transaction 1
DEBUG:pymodbus.logging:SEND: 0xd 0x2 0x0 0x1 0x0 0x1 0xe8 0xc6
DEBUG:pymodbus.logging:New Transaction state "SENDING"
DEBUG:pymodbus.logging:Changing transaction state from "SENDING" to "WAITING FOR REPLY"
DEBUG:pymodbus.logging:Changing transaction state from "WAITING FOR REPLY" to "PROCESSING REPLY"
DEBUG:pymodbus.logging:RECV: 0xd 0x82 0x2 0x1 0x62
DEBUG:pymodbus.logging:Getting transaction 13
DEBUG:pymodbus.logging:Changing transaction state from "PROCESSING REPLY" to "TRANSACTION_COMPLETE"
Modbus Error: [Input/Output] No Response received from the remote slave/Unable to decode response

[janiversen]

please add the debug log as pr documentation, call pymodbus_apply.....

[janiversen]

Please also write which version you are using, hardware etc.

[janiversen]

It is much easier if you read the documentation, encode() is an internal function.

[janiversen]

Just a little thing, why do you encode a result, I really do not see the sense of that....normally you encode a request and decode a response.

[srolf]

As I tried to explain, the way the response looks like is confusing me. While looking into the pymodbus documentation, I was not able to find the definition of ModbusResponse as returned by read_discrete_inputs.

If I use in the code
print(result.bits)
and the switch on the ESP32 is closed during request,
the presentation of it looks way more straight:
[True, False, False, False, False, False, False, False]
But I derived it only from the the code of given examples in the documentation.

[janiversen]

Which is why examples are part of the documentation. The examples contains e.g. a template call showing how to to look for error conditions. All modbus calls are shown in one example, so I am not sure what you want more.

[srolf]

Please don't get me wrong: Looking into the examples is fine. Covering also ModbusResponse in the API documentation might make the start with pymodbus for newbies like me little easier.

[janiversen]

Pull requests are more than welcome.