PaulS:
serialCom.write(str(tau).encode('ascii'))
What do you suppose the str() and encode() bits are doing? If you guessed that they are converting the value 200 to the string "200", you win no prize. If you guessed anything else, you loose.
rc = Serial.read();
This function is reading one byte/character, NOT the entire string "200". And, it is certainly NOT reading the string "200" and making an int of it
The simplest solution to your problem is to stop having Python convert the value to string.
Thanks for the reply Paul. So I will probably have to just code like: serialCom.write(tau), right?