Continuation of a problem, PYModbus can't replicate Modpoll register read

[Hos73]

I am continuing a discussion of a problem from stackoverflow, I have a modbus register I can write to with pymodbus and modpoll, however I cannot read from that register with pymodbus, but I can read from it with modpoll. Discussions that were had on the stackoverflow can be found here
Data sheet of what I am connecting to via usb can be found here
Any assistance is greatly appreciated!
pymodbus version: 3.2.2

Code is as follows

from pymodbus.client import ModbusSerialClient as RTUClient
import logging
logging.basicConfig()
log = logging.getLogger()
log.setLevel(logging.DEBUG)

PowerSupplyMB = RTUClient(method='rtu', port="/dev/ttyUSB0", baudrate=9600,stopbits=1,bytesize=8,timeout=1)
PowerSupplyMB.connect()
#PowerSupplyMB.write_register(0x0030, int(10.2*100), unit=1)
mb_response  = PowerSupplyMB.read_holding_registers(0x0030, 1, unit=1)

if not mb_response.isError():
    '''isError() method implemented in pymodbus 1.4.0 and above'''
    print(mb_response.registers)  # Your problem is here.

else:
    # Do stuff for error handling.
    print('Error message: {}'.format(mb_response))

---

Output:

DEBUG:pymodbus.logging:Current transaction state - IDLE
DEBUG:pymodbus.logging:Running transaction 1
DEBUG:pymodbus.logging:SEND: 0x0 0x3 0x0 0x30 0x0 0x1 0x85 0xd4
DEBUG:pymodbus.logging:New Transaction state "SENDING"
DEBUG:pymodbus.logging:Changing transaction state from "SENDING" to "WAITING FOR REPLY"
DEBUG:pymodbus.logging:Transaction failed. (Modbus Error: [Invalid Message] No response received, expected at least 4 bytes (0 received))
DEBUG:pymodbus.logging:Frame - [b''] not ready
DEBUG:pymodbus.logging:Getting transaction 0
DEBUG:pymodbus.logging:Changing transaction state from "PROCESSING REPLY" to "TRANSACTION_COMPLETE"
Error message: Modbus Error: [Input/Output] Modbus Error: [Invalid Message] No response received, expected at least 4 bytes (0 received)

[MattBrittan]

As suggested on stack overflow; remove the write_register call for now. You have established that you can read from the device using modpoll so start by having the python code do exactly the same operation. Once that works you can add extra operations and see if that leads to failure (some devices require a longer delay between requests).

[Hos73]

Sorry I should've mentioned and commented that out. But the error that is attached to that is from the version where that write is commented out.

DEBUG:pymodbus.logging:Current transaction state - IDLE
DEBUG:pymodbus.logging:Running transaction 1
DEBUG:pymodbus.logging:SEND: 0x0 0x3 0x0 0x30 0x0 0x1 0x85 0xd4
DEBUG:pymodbus.logging:New Transaction state "SENDING"
DEBUG:pymodbus.logging:Changing transaction state from "SENDING" to "WAITING FOR REPLY"
DEBUG:pymodbus.logging:Transaction failed. (Modbus Error: [Invalid Message] No response received, expected at least 4 bytes (0 received))
DEBUG:pymodbus.logging:Frame - [b''] not ready
DEBUG:pymodbus.logging:Getting transaction 0
DEBUG:pymodbus.logging:Changing transaction state from "PROCESSING REPLY" to "TRANSACTION_COMPLETE"
Error message: Modbus Error: [Input/Output] Modbus Error: [Invalid Message] No response received, expected at least 4 bytes (0 received)

[MattBrittan]

OK - Please edit the question to add that. I'd also suggest stating that modpoll -m rtu -0 -1 -a 1 -b 9600 -p none -r 48 /dev/ttyUSB0 works.

Now looking at your modpoll command you include -a 1 (Slave address) but looking at the debugging info the request is being sent to slave 0 ("0x0 0x3 0x0 0x30 0x0 0x1 0x85 0xd4") so that would explain the lack of response.

From the docs the function definition is read_holding_registers(address: int, count: int = 1, slave: int = 0, **kwargs: Any) → ModbusResponse; in your code you say unit=1 not slave=1... (I did not pick this up because you said the write_register works - I doubt it actually does).

[Hos73]

This did work! The write was properly working as I could visually confirm on the LCD that the voltage was changing. I appreciate the assistance!

[janiversen]

Looking at the log, your device does not respond, you are probably using a wrong framer.

[janiversen]

rtu framer is the default but many times you need to use the ascii framer.

Apart from that if your device never answers it can be a number of serial parameters that are wrong.

[janiversen]

and just for the record I do not follow thread on stackoverflow, normally problems with pymodbus are reported here.

You have a link to the documentation so it should be easy for you to check the serial parameters and the type of framer. I cannot offer that kind of support.

[janiversen]

you do not write which version you use, but with never versions your code is wrong, unit= no longer exist, it is slave= (please read our documentation)

[Hos73]

Thank you for the response! This did in fact work, I appreciate the assistance!