Serial port failure

[Leo Kislov]

Rob wrote:
> Hi all,
>
> I am fairly new to python, but not programming and embedded.  I am
> having an issue which I believe is related to the hardware, triggered
> by the software read I am doing in pySerial.  I am sending a short
> message to a group of embedded boxes daisy chained via the serial port.
>  When I send a 'global' message, all the connected units should reply
> with their Id and Ack in this format '0 Ack'  To be certain that I
> didn't miss a packet, and hence a unit, I do the procedure three times,
> sending the message and waiting for a timeout before I run through the
> next iteration.  Frequently I get through the first two iterations
> without a problem, but the third hangs up and crashes, requiring me to
> remove the Belkin USB to serial adapter, and then reconnect it.  Here
> is the code:
>
> import sys, os
> import serial
> import sret
> import time
>
> from serial.serialutil import SerialException
> ####################################################################
> #### GetAck Procedure
> ####################################################################
> def GetAck(p):
>     response = ""
>
>     try:
>         response = p.readline()
>     except SerialException:
> 	print ">>>>>Timed out<<<<<"
> 	return -1
>     res = response.split()
>
>     #look for ack in the return message
>     reslen = len(response)
>     if reslen > 5:
>         if res[1] == 'Ack':
> 	    return res[0]
> 	elif res[1] == 'Nak':
> 	    return 0x7F
> 	else:
> 	    return -1
>
>
> >>>>> Snip <<<<<<
> ####################################################################
> #### GetNumLanes Procedure
> ####################################################################
> def GetNumLanes(Lanes):
> 	print "Looking for connected units"
> # give a turn command and wait for responses
> 	msg = ".g t 0 336\n"
>
> 	for i in range(3):
> 	    port = OpenPort()
> 	    time.sleep(3)
> 	    print port.isOpen()
> 	    print "Request #%d" % (i+1)
> 	    try:
> 	        port.writelines(msg)
> 	    except OSError:
> 		print "Serial port failure.  Power cycle units"
> 		port.close()
> 		sys.exit(1)
>
>             done = False
> # Run first connection check
> 	    #Loop through getting responses until we get a -1 from GetAck
>             while done == False:
> 	        # lane will either be -1 (timeout), 0x7F (Nak),
> 	        # or the lane number that responded with an Ack
> 	        lane = GetAck(port)
> 	        if lane >= '0':

Your GetAck returns either string or number and then you compare it
with a string. If you compare string with a number python currently
returns result you probably don't expect

>>> -1 >= '0'
False
>>> 0x7f >= '0'
False

This is a wart and it will be fixed in python 3.0 (it will raise
exception) I think you should rewrite GetAck to return a tuple (state,
lane)

def GetAck(p):
   response = ""

   try:
       response = p.readline()
   except SerialException:
       print ">>>>>Timed out<<<<<"
       return 'Timeout', 'NoID'
   res = response.split()

   #look for ack in the return message
   reslen = len(response)
   if reslen > 5:
       if res[1] == 'Ack':
           return 'Ack', res[0]
       elif res[1] == 'Nak':
           return 'Nak', Does Nak response contain lane id?
       else:
           return 'Unknown', 'NoID'

And then instead of

lane = GetAck(port)
if lane >= '0':

use

state, lane = GetAck(port)
if state == 'Ack':

  -- Leo