Communication between Python and PLC through TCP

Public forum Technical questions
1

@4lloyd, a question:

I’ve got the following PLC programme:

image
image.png707×369 10.6 KB

Which is communicating with this Python code:

def check_transport_band(conn):
    number1 = 0
    try:
        data = conn.recv(512)
        print("Data received from PLC")
        print("Raw: ", data)
    except Exception as e:
        print("Error while receiving data. Error message: ", str(e))
        return None
    
    if not data: return None
    
    if (len(data) == 6):
        print("Data received from PLC")
        number1 = int(struct.unpack(">h", data[0:2])[0])
        print("Number 1: ", number1)
    else:
        print("Received too little or too much bytes. 6 needed, received: ", len(data))
    if number1== 1:
            # or number2 or number3 
            print ("owkee")
            return True
            
    else:
            print ("ownee")
            return False

Now, the first time start the programme, when the starting signal is (0/ band off), I receive data which has a length of 6 bits (as expected).

However, the second time around I receive 24 bits:

image

When start the programme, when the starting signal is (1/ band on), I receive data which has a length of 6 bits (as expected).

However, when i turn it of it starts to range between 30 and 24.

image
image.png583×623 10.4 KB

Would you have any idea what’s going on here?

2

I think that the problem is that the PLC is sending data more often then your script is checking for it. Therefor there are more than 6 bytes in the receive buffer.

If you add

data = data[-6:]

after

if not data: return None

then the “data” variable will only have the last six received bytes.

Does this solve the problem?