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How to write PLC input registers using pymodbus

I want to write to PLC input registers using pymodbus. I can read them:

from pymodbus.client.sync import ModbusTcpClient
client = ModbusTcpClient('10.10.10.32')
client.connect()
reg = client.read_input_registers(1,5)
print(reg.registers)

But I still haven't found a way to write them any value. I appreciate any help. Thank you.

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3 answers


Input registers are read-only. You can write holding registers using Modbus functions. Writing a single register or writing multiple registers (ModbusTcpClient.write_register or ModbusTcpClient.write_registers in pimodbus).



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The PLC has a special set of registers for reading and a set for writing. The set of write registers is different from that of a PLC. You are reading registers, for example, may start with register "1". You read "1" but you don't write. You will need to match the Modbus register for your PLC.

My knowledge of Python is low, but it seems like you are trying to read up to 5 registers starting at 1? To write, you probably need to use



reg = client.write_output_registers (?, ??)

I usually use Wago 880. I can write to registers 0-999 and read from 1000-1999. I am assuming Python will take care of the functional code for you.

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Example of using Festo MODBUS / TCP:

# First digital input address
address = 40003
# Written value
value = 255

# It will send '11111111' to the output 
client.write_register(address, value)

According to the documentation:
http://pymodbus.readthedocs.org/en/latest/library/sync-client.html

// function
write_register (address, value)
// Parameters
address - starting address for writing to
value - Value for writing to the specified address

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