Serial port failure

[Rob]

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':
                    if False == Lanes.has_key(lane):
                        Lanes[lane] = True
	        else:
	            done = True
	    port.close()
	    time.sleep(3)

# Report number of lanes found
        NumLanes = len(Lanes)
	if NumLanes == 1:
    		print "\n\nFound 1 unit connected"
	else:
    		print "\n\nFound %d units connected" % NumLanes

	return NumLanes


>>>>>> Snip <<<<<<
####################################################################
#### Main Program Code Section
####################################################################

#open the serial port
# capture serial port errors from trying to open the port

port = OpenPort()

# If we got to here, the port exists.  Set the baud rate and timeout
values

# I need to determine how many lanes are on this chain
# First send a turn command

#Create a dictionary of lanes so I can check each lane's responses
Lanes = {}
#<><><><><><><><><><><><><><><><>
# Call the lane finder utility
NumLanes = GetNumLanes(Lanes)
#<><><><><><><><><><><><><><><><>

#if no lanes responded, exit from the utility
if 0 == NumLanes:
    print "I can't find any units connected."
    print "Check your connections and try again"
    sys.exit(1)

# list the lanes we have in our dictionary
for n in Lanes:
    print "Lane - %s" % n

Now, here is the error message that I get

dad at nb29:~/py$ ./Thex.py
Looking for connected units
True
Request #1
True
Request #2
Serial port failure.  Power cycle units
dad at nb29:~/py$ ./Thex.py
The serial port is unavailable.
Disconnect your USB to Serial adapter, Then
reconnect it and try again.
dad at nb29:~/py$

Does anyone have any ideas?

Thanks,

rob < Amateur.N7TZG at gmail.com >