granan granan - 1 month ago 9
Linux Question

How to write on serial port in python that ttyUSB0 will be interpreted commands?

I have raspberry PI B+ with connected Telegesis ZigBee module(ETRX3 USB sticks) via USB. Using commands:

debian:~# stty -F /dev/ttyUSB0 -raw ispeed 19200 ospeed 19200
debian:~# cat < /dev/ttyUSB0 &
debian:~# echo "ATI" > /dev/ttyUSB0


the ZigBee module executed ATI command and I can see correct output:

Telegesis ETRX357

R308C

OK


The same thing I want to do with python script. I was written python script with code:

#!/usr/bin/env python

# based on tutorials:
# http://www.roman10.net/serial-port-communication-in-python/
# http://www.brettdangerfield.com/post/raspberrypi_tempature_monitor_project/

import serial, time

SERIALPORT = "/dev/ttyUSB0"
BAUDRATE = 19200

ser = serial.Serial(SERIALPORT, BAUDRATE)

ser.bytesize = serial.EIGHTBITS #number of bits per bytes

ser.parity = serial.PARITY_NONE #set parity check: no parity

ser.stopbits = serial.STOPBITS_ONE #number of stop bits

#ser.timeout = None #block read

#ser.timeout = 0 #non-block read

ser.timeout = 2 #timeout block read

ser.xonxoff = False #disable software flow control

ser.rtscts = False #disable hardware (RTS/CTS) flow control

ser.dsrdtr = False #disable hardware (DSR/DTR) flow control

ser.writeTimeout = 0 #timeout for write

print 'Starting Up Serial Monitor'

try:
ser.open()

except Exception, e:
print "error open serial port: " + str(e)
exit()

if ser.isOpen():

try:
ser.flushInput() #flush input buffer, discarding all its contents
ser.flushOutput()#flush output buffer, aborting current output

ser.write("ATI")
print("write data: ATI")
time.sleep(0.5)
numberOfLine = 0

while True:

response = ser.readline()
print("read data: " + response)

numberOfLine = numberOfLine + 1
if (numberOfLine >= 5):
break

ser.close()

except Exception, e:
print "error communicating...: " + str(e)

else:
print "cannot open serial port "


and get results as on the screen

ATI


but i want to command be execute by ZigBee module, as like in shell commands. What am I doing wrong?

write on ttyUSB0

Answer
  1. you need to append an end-of-line to your write()

    ser.write("ATI\r\n")

  2. you should change the timeout to:

    ser.timeout = None

Otherwise readline() will return after 2 seconds, even if nothing has been read.