I’m using a python program to communicate with a PLC using OPCUA as well as a mass flow controller to dose some substances, using modbus. The python script reads and writes to both.

async def read_input_registers(modbusclient, start_address, num_elements, slave, datatype):
    response = await modbusclient.read_input_registers(start_address, num_elements, unit=slave)
    if response.isError():
        logging.error(f"Read Input Registers Error for {datatype}: {response}")
    else:
        if datatype == 'B':
            data = response.registers[0]
        else:
            data_bytes = b''.join(pack('>H', register) for register in response.registers)
            data = unpack(datatype, data_bytes)
        print(f"{datatype} Data:", data)
    return data

This is the function i use to read from the modbus registers and the following is where I use this function

MFC_01DosedQuantity_g = await read_input_registers(modbusclient, 8, 2, 1, '>f' ) #Float datatype (big-endian)
await record_variables('MFC01_DosedQuantity_g', MFC_01DosedQuantity_g)

MFC_02DosedQuantity_g = await read_input_registers(modbusclient, 18, 2, 1, '>f') #Float datatype (big-endian)
await record_variables('MFC02_DosedQuantity_g', MFC_02DosedQuantity_g)

MFC_01DosingStatus =  await read_input_registers(modbusclient,1, 1, 1, 'B')  # Unsigned8 datatype
await record_variables('MFC01_DosingStatus', MFC_01DosingStatus)

MFC_02DosingStatus = await read_input_registers(11, 1, 1, 'B')  # Unsigned8 datatype
await record_variables('MFC02_DosingStatus', MFC_02DosingStatus)

status = await read_input_value(client, 'ns=4;s=|var|C6 S14 PRO.Application.GVL.Stage')

    if MFC_01DosingStatus == 512 or MFC_02DosingStatus == 512:
        print(f"Error detected at stage {status}, shutting down...")
        sys.exit()

My problem is that my if condition does not seem to work. The code always goes to sys.exit() despite the value of MFC_01DosingStatus and MFC_02DosingStatus being 256.

I’ve tried to check for a type mismatch, they seem like integers. The record_variables function records them into a .csv file as integers. I’m not sure what else to try.