Problems with sending commands over pySerial

Question

I'm trying to talk to a home made card over a serial port, and is therefor using pySerial. In Hyperterminal, everything works fine. I can write:

$ audio on

and the audio is enabled, but if I use

ser = serial.Serial("COM1", 38400)
ser.write("audio on\r\n")

nothing happens. I can read incoming data however, so it's not something wrong with the communication. I doesn't help if I change \r\n to just \n or \r either.

EDIT: Sometime I actually get the feedback: No such command when sending the exact same command as works from HyperTerminal. The setup is also the exact same as in HyperTerminal.

Solved: To make it work, I had to send one and one character, and ending the transmission with \r.

Answer 1

Get an oscilloscope (you've got one, right?) and watch the serial line. Send one character per second through it and see what comes up on the scope (set it to trigger on the start bit). Serial port bits are in the order: start, LSB .. MSB, parity, stop.

See if there are characters that don't get through, or if there's a pattern. Another possibility is that everything is actually making it out the port, and your board is dropping characters.

Answer 2

1. Triple check that the baud rate of the device is indeed 38400
2. Triple check parity, stop bits, etc
3. Be aware of signal degradation for serial transmissions over long distances (probably not your issue)

If all the above checkout try putting the string into a byte array and sending that through the write command. Just a guess.

Answer 3

Sending characters via Hyperterminal deliver characters at the speed you type them. Sending characters through pyserial they are delivered as a continuous stream. The receiver (especially at high baud rates) could drop them, depending on the nature of the receiver.

Also, when you send commands to an interpreter, you only need the \r terminator (without the \n), (this is all that is sent by hyperterm, normally). HOWEVER, if you are just displaying the values, you may need the \n to generate the new line.