pymodbus: request creation and response receiving

Question

Can anyone explain how to create the request and get the response in right way using pymodbus via Modbus TCP/IP?

I have the PLC which I want to use as slave and PC - as master.

I trying to do it in such way:

from pymodbus.client.sync import ModbusTcpClient

host = '192.168.56.9'
port = 502   

client = ModbusTcpClient(host, port)
client.connect()

#Register address 0x102A (4138dec) with a word count of 1
#Value - MODBUS/TCP Connections
#Access - Read
#Description - Number of TCP connections

request = client.read_holding_registers(4138, 1) 
response = client.execute(request)

print response

>>> ReadRegisterResponse (1)

Dilley

Answer 1

Set unit argument and use the print(request.registers) instead of print(request).

Here's an example:

request = client.read_holding_registers(4138, 1, unit=1)  # Set unit argument.

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

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

Ybarra

Any idea how we can track number of read requests coming to holding_registers? – Octa

Answer 2

You could execute dir(response) to check what the response is composed of but if pymodbus TCP master is similar to RTU serial master implementation then the data is available in Registers field so try to print response.Registers instead of response. The response.Registers should be a one-element array containing the value of register your requested for reading.

Doughty