Published on: 15.04.2018
My motivation for monitoring RS232 with Python.
I had PC and LG TV screen communicating via RS232 cable.
PC was turning TV screen on and off via RS232 commands.
But TV screen was not acting normally, it was always in standby mode.
So, either PC was not sending RS232 commands or TV screen was not responding to them.
In order to diagnose the problem, I decided to spy on their communication.
half-duplex and full-duplex RS232
There are two types of communication in RS232: half-duplex and full-duplex.
In half-duplex while one device is talking another one is listening, just like people communication should be.
In full-duplex both devices are talking and listening at the same time, people think that they can do this also, but it is not working in practice.
Hardware
The first step was to make special half duplex RS232 spy cable because I did not have software access on PC.
So the only solution was to sniff communication between PC and TV.
Schematics are available from https://www.lammertbies.nl/comm/cable/RS-232-spy-monitor.html.
On the same website, there are also Schematics for full duplex RS232 spy cable.
Software
For redirecting, all communication data from the RS232 compliant serial port device into a text file Eltima RS232 Data Logger can be used.
The program is working fine, the only problem that I had was that there is no output on screen just to text file.
No real-time watching of communication data was possible, due to that I decided to make my own program for viewing RS232 communication data in real-time.
Python to the rescue
The best thing about Python program language is that there is a package for everything.
For access to the serial port in Python, there is pySerial package.
My Python program was reading communication data and showing it on screen in real-time with timestamps of reading, and time difference between the last read.
Python source code for RS232 sniffing:
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import serial # pyserial import datetime def to_hex(input_str): outpur_str = " ".join("{:02x}".format(ord(c)) for c in input_str) # 4b 41 20 30 20 46 46 0d return outpur_str def main(): ser = serial.Serial( port='COM1', baudrate=9600, parity=serial.PARITY_NONE, stopbits=serial.STOPBITS_ONE, bytesize=serial.EIGHTBITS ) # open if not opened if ser.isOpen() == False: print "OPENING port." ser.open() # header fmt = "{0:15}{1:35}{2}" print fmt.format('TIME', 'RAW HEX', 'TIME DELTA') # so to have first value time_of_last_read = datetime.datetime.now() while True: data_to_read = ser.in_waiting if data_to_read != 0: # read if tehre is new data time_now = datetime.datetime.now() data = ser.read(data_to_read) hex_with_space = to_hex(data) time_with_miliseconds = time_now.strftime("%H:%M:%S.%f")[:-3] time_delta = time_now - time_of_last_read print fmt.format(time_with_miliseconds, hex_with_space, time_delta) # last time time_of_last_read = time_now ser.close() if __name__ == '__main__': main() |
