Python Serial Read Not Working

Question

I am attempting to control a projector using RS232 from python. This link has the needed information about the port settings and expected responses. http://www.audiogeneral.com/Optoma/w501_rs232.pdf

To summerise it baud = 9600, Data bits = 8, No parity, 1 stop bit, no flow control.

when the command "~00124 1\r" is sent the projector should respond okn where n is the power state.

when the command "~0000 1\r" is sent the projector should power on

From Putty I am able to send the power on command and other commands and see that the projector does what it is supposed to. I can also send the read command and get the appropriate okn response back to putty.

From python I can send the power on command and see the projector power on. however when I send the power state command I never see any character come into the read buffer.

Here is the code for a test script i wrote trying to debug this.

import serial
ser = serial.Serial("/dev/ttyUSB0")
print ser.baudrate
print ser.bytesize
print ser.parity
print ser.stopbits
print ser.xonxoff
print ser.rtscts
print ser.dsrdtr 
print ser.name
print "Power State"
ser.write("~00124 1")
while ser.inWaiting() > 0:
 response = ser.read(3)
 print response
output:
9600
8
N
1
False
False
False
/dev/ttyUSB0
True
Power State

I expect an okn after the power state line but it does not show up

• user4171906: I don't see anything print after "Power State" which suggests that ser.inWaiting() return is <= 0, so you should print it and it's type() as it may be a string not an int.
• CrimsonKnights: I Printed that its type is int but its value is 0 so the loop wont run. I tried it with while True: and it just hangs until I kill the program.

Answer 1

Putty emulate a serial terminal, like minicom in Linux/Unix or HyperTerminal on Windows. Try adding \n\r at the end of the string to act as a real serial terminal.

I suggest you to try reading your data byte per byte instead of 3 bytes at time. Better if you use readline method.

• CrimsonKnights: don't think readine will work since as best I can tell the projector does not send\n to cause a new line it only sends \r after the characters. Ill try 1 byte and adding \n to the write statement now
• CrimsonKnights: The \r was part of the problem I also had to add a time.sleep(0.05) between the write and the read to give the projector time to respond
• sardylan: You can also try setting port timeout to None, which cause all read calls to be blocking. In this way yout python script will wait until at least n bytes ar received. Try also calling method flush on serial port to ensure that all bytes are sent before calling the read method