problems while reading with pyserial

Question

So I'm trying to communicate with with a zigbee module connected to a serial port by using AT (sometimes refered as Hayes) commands.

My problem is I can't figure out how to correctly read its answers with the pyserial module of Python (I'm using Python 2.7 on an embedded device).

Sometime the script manage to perfectly read the module's responses and sometimes it just returns a suite of 'A' characters.

Here is a sample output :

Opening serial /dev/ttyS2 ... /dev/ttyS2 is open...

Enter command or 'exit' : AT

------------- Response -------------

AT OK

Enter command or 'exit' : ATI

------------- Response ------------- aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaAaaaaaaaaaaaaaaaaaaaaaAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaAaaaaaaaaaaaaaaaaaaaaaaaaAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaAaAaaaAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

Enter command or 'exit' :

I really don't understand what's happening here but I think it's probably because I use the pyserial module incorrectly. I've already tried to use the inWaiting() command to read only the bytes contained in the input buffer but this function always returns zero and the readline() command don't seems to work either so I just read packets of 1024 bytes.

Here's the code I've written so far :

import serial, time, os

def sendAtCommand(command):

    # responseArray = []
    # i = 0

    try:
        # print('Number of bytes waiting in input buffer before write : '+str(ser.inWaiting())) -> always zero...

        if ser.inWaiting():
            # flush input buffer, discarding all its contents
            print('flushing input buffer ...')
            ser.flushInput()
            # flush output buffer, aborting current output
            # ser.flushOutput()

        try:
            ser.write(command.encode('ascii')+'\r')
            #ser.write(command+'\r')
            #time.sleep(5)
        except SerialTimeoutException as e:
            print(repr(e))

        response = ser.read(size=1024)
        # print('Number of bytes waiting in input buffer after write : '+str(ser.inWaiting())) -> always zero
        # while ser.inWaiting():
            # print('Reading response ... '+str(i))
            # responseArray.append(ser.readline(eol='\r\n'))
            # i += 1
            # time.sleep(0.2)

        print('------------------------------------')
        print('------------- Response -------------')
        print('------------------------------------')
        print(response)
        # for line in responseArray:
            # print(line)           
        print('------------------------------------')

        time.sleep(1)

    except KeyboardInterrupt as e:
        print('Closing serial '+port+' before interrupting ...')
        ser.close()
        exit()

VERSION = '0.02'
firstStart = True

port = '/dev/ttyS2'
baudrate = 19200
bytesize = 8
parity = 'N'
stopbits = 1
timeout = 1
xonxoff = False
rtscts = False
dsrdtr = False
write_timeout = None
inter_byte_timeout = None

try:
    os.system('cls' if os.name == 'nt' else 'clear')
    print('')
    print('PySerial version : '+serial.VERSION)
    firstStart = False

    print('')
    print('Opening serial '+port+' ...')
    ser = serial.Serial(port, baudrate, bytesize, parity, stopbits, timeout,
    xonxoff, rtscts, write_timeout, dsrdtr, inter_byte_timeout)
except ValueError as e:
    print(repr(e))
except SerialException as e:
    print(repr(e))

if ser.isOpen():
    print(ser.name + ' is open...')
    print('')

while True:

    try:
        cmd = raw_input('Enter command or \'exit\' : ')

        if cmd == 'exit':
            ser.close()
            exit()
        else:
            sendAtCommand(cmd)
    except KeyboardInterrupt as e:
        print('Closing serial '+port+' before interrupting ...')
        ser.close()
        exit()

Answer 1

Ok so first, thank you all for your answers. Finally I've figured out how to resolve this issue.

I don't know why but the problem was that I powered the telegesis zigbee board by using it's UART vcc pin (connected to the Linkit 7688 3v3 pin).

Now, I just connect the GND, Tx and Rx of both board and use an external source for power and it works perfectly. Please refer to the picture below if you have the same issue with the same material.

Don't use the VCC pin boxed in red, you must power the board using an external power source through the x2 socket.

(Sorry all for the poor quality of my english)

EDIT :

Ah and now I don't read random packets of 1024 bytes anymore, I use a separate thread for this as you can see below :

class readSerial(Thread):
    """This thread waits for data in the serial input buffer"""
    """This is a new subclass of the Thread class (cf. Python 2.4+ threading module"""    

    def __init__(self, serialObject):
        """Overrides the __init__(self [,args]) method to add additional arguments"""
        Thread.__init__(self)
        self.serialObject = serialObject
        self.daemon = True
        self.start()

    def run(self):

        ser = self.serialObject

        data = ''

        ser.reset_input_buffer()
        ser.reset_output_buffer() 

        while True:
            try:
                if ser.in_waiting > 0:
                    data = ser.read(ser.in_waiting)

                    print('msg received : ')
                    print('')
                    print(data.decode('ascii'))

                if 'UCAST' in data.decode('ascii'):
                    zigbeeRequest(data.decode('ascii')[26:], ser)

                    ser.reset_input_buffer()

                    data = ''
                if 'BCAST' in data.decode('ascii'):
                    zigbeeRequest(data.decode('ascii')[26:], ser)

                    ser.reset_input_buffer()

                    data = ''

                time.sleep(1)                    
            except Exception as e:
                print(repr(e))

Then just put

readThread = readSerial(ser)

in your main programm after you have opened your serial connection.

I hope this can be useful for someone even if I know well it isn't perfect programming (don't catch exceptions like I do...)