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Recently I have been tasked with a project involving reading Modbus data, specifically with the pymodbus package. before diving into this project I wanted to simulate some reading and writing of Modbus data (without having to use a machine) with python. Using the sample data they have on their homepage, I try to write using the client but encounter a Modbus error

Ive tried looking into the matter to see what I could dig up wonder if I need a server to write or read. However I am unsure as with my experiences with socket and serial I just needed an established connection with the correct port to simply write (however I understand that Modbus is different).

Here is the code

client = ModbusTcpClient('localhost')
client.write_coil(1, True)
result = client.read_coils(1,1)
print(result.bits[0])
client.close()

and here is my error "pymodbus.exceptions.ConnectionException: Modbus Error: [Connection] Failed to connect[ModbusTcpClient(localhost:502)]"

I expect the output to simply write Modbus without needing anything no necessarily listen on the other end, but instead I keep getting an error when trying to connect/write. (sorry if this is hard to comprehend, my brain is all over the place and I am extremely new to Modbus in general).

Fastas
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1 Answers1

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If you want to send Modbus queries and you don't have any Modbus hardware yet you need to run a dummy Modbus server on your computer.

You can take a look at the examples.

You might also need to add a rule to your firewall for port 502. If you are on Linux you can just switch ports to a higher number like 5020 on both ends to avoid this problem.

Marcos G.
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