Serial Port Communication in Python

pyserial is a python module that allows one to access serial port. It’s easy to use and one can start test serial port communication in minutes.

1. Install pyserial

If your machine doesn’t have pyserial module installed, you can install the module using easy_install.

sudo easy_install pyserial

In case you don’t have easy_install installed, you can install it by entering the following command,
sudo apt-get install python-setuptools
Then you can run the command above to install pyserial. 
2. A Sample Program
The sample program use pyserial to communicate with a modem device with its serial port interface at /dev/ttyUSB0 in Ubuntu Linux. 
#!/usr/bin/python

import serial, time

#initialization and open the port

#possible timeout values:

#    1. None: wait forever, block call

#    2. 0: non-blocking mode, return immediately

#    3. x, x is bigger than 0, float allowed, timeout block call

ser = serial.Serial()

ser.port = "/dev/ttyUSB0"

#ser.port = "/dev/ttyS2"

ser.baudrate = 9600

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 = 2     #timeout for write

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 

                     #and discard all that is in buffer

        #write data

        ser.write("AT+CSQ=?\x0D")

        print("write data: AT+CSQ=?\x0D")

        time.sleep(0.5)  #give the serial port sometime to receive the data

        numOfLines = 0

        while True:

            response = ser.readline()

            print("read data: " + response)

            numOfLines = numOfLines + 1

            if (numOfLines >= 5):

                break

        ser.close()

    except Exception, e1:

        print "error communicating...: " + str(e1)

else:

    print "cannot open serial port "
 
The program send out AT commands to query modem signal strength. If you want to know more about modem AT commands, refer to post here. 
If you set the timeout as 2, the readline() method is blocking call with timeout, the output will be as below,
write data: AT+CSQ=?

read data: AT+CSQ=?


read data: +CSQ: (0-31,99),(99)


read data: 


read data: OK


read data: 
If you set the timeout as None, the readline() method is non-blocking call, the output is,
write data: AT+CSQ=?

read data: AT+CSQ=?


read data: +CSQ: (0-31,99),(99)


read data: 


read data: OK



Note that there is less lines of output, program hangs and cannot exit.
If you set the timeout as 0, the readline() method will return immediately, the output is,
write data: AT+CSQ=?

read data: AT+CSQ=?


read data: +CSQ: (0-31,99),(99)


read data: 


read data: OK


read data: 
 
Reference:
pyserial documentation: http://pyserial.sourceforge.net/pyserial.html