Matt Ellen Matt Ellen - 3 months ago 52
Python Question

PySerial not talking to Arduino



I have written some code to simulate some hardware I'm working with and uploaded it to the Arduino board. This code works. I know this, because I get the expected response from HyperTerminal.

However, when I try to connect using PySerial the connection does not error, but I get no response to the commands I send.

Why might this be?

Python code

import serial

def main():
sp = serial.Serial()
sp.port = 'COM4'
sp.baudrate = 19200
sp.parity = serial.PARITY_NONE
sp.bytesize = serial.EIGHTBITS
sp.stopbits = serial.STOPBITS_ONE
sp.timeout = 0.5
sp.xonxoff = False
sp.rtscts = False
sp.dsrdtr = False

sp.open()

sp.write("GV\r\n".encode('ascii'))
value = sp.readline()
print value
sp.write("GI\r\n".encode('ascii'))
value = sp.readline()
print value

sp.close()

if __name__ == "__main__":
main()


NB: the code on the Arduino sends back
\r\n
at the end of a response to a command.

HyperTerminal configuration:

COM4 configuration in HyperTerminal

Edit



I have found that if I increase the timeout to 10 seconds and add a
sp.readline()
before I send anything, then I get responses to both commands.

How long is the hardware handshake usually between PySerial and an Arduino or USB RS-232 ports?

Answer

Can not verify this but it could be that you try and read before there is any data there, thus you get no reply back.

To test this you could try and poll until there is data

value = None
while not value:
   value = sp.readline()
print value

Edit

The Arduino will reset when you open a serial connection, any data written during bootup will likely go to bit heaven. You could use a sleep for 2 seconds (could not find the exact time it takes, will likely vary anyway) before you do any reads/writes.

Alternatively you could write to it until you get a response back, after you get a return you start doing "real work".