-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
Hello,
When transmitting via UDP to a PLC, I run into a strange problem
where socket.sendto returns double the number of characters sent in the
datagram. I thought this was an error and used Wireshark to sniff the
connection and discovered that it did, in fact, include two copies of
the string I am transmitting in the same packet. The only
thing differentiating this string from prior commands is its length.
The functional ones are between 5 & 7 bytes (with 16 byte responses
received successfully) but transmitting a 66-byte message actually
results in 132 bytes being sent!
I am running 2.6.6 on Windows XP, which I understand has a default
minimum buffersize of 576 bytes which I would think is sufficient.
Apologies if this is somewhat incoherent, I'm cross-eyed from staring
at this!
- - Todd
- -- code --
def PLC_Functions(command, argument):
"""
Command is one of: GetTray, Status, SetText
Argument is one of: tray number (as string), None, message (as string)
The PC transmits and receives on socket 2260 (pcSocket)
The PLC transmits and receives on socket 2002
The protocol used is UDP
"""
MegaMatPLC =
(config.get('megamat','plcip'),int(config.get('megamat','targetport')))
# at some point it will be necessary to wrap these in TRY/EXCEPT to handle
the socket errors
# create UDP socket
pcSocket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM,
socket.IPPROTO_UDP)
# bind it
pcSocket.bind((config.get('megamat','pcip'),int(config.get('megamat','sourceport'))))
# make them blocking (i/o is 'fire-and-forget' to a PLC that may or may not
respond in a given time.)
# non-blocking fails and raises windows exceptions for some reason
(20-Oct-2010)
pcSocket.setblocking(True)
if command == 'CallTray':
# frame 1 (see docs for frame sequence)
print 'Received call tray command for tray %s' %(argument)
getReadyMsg = '0145\r'
totalsent = 0
while totalsent < len(getReadyMsg):
sent = pcSocket.sendto(getReadyMsg[totalsent:],MegaMatPLC)
if sent == 0:
raise RunTimeError('Transmission failure. Please confirm
MegaMat settings in megapy.ini')
totalsent = totalsent + sent
# Frame 2
response = ''
while response.find('\r') < 0:
response = pcSocket.recv(17)
if response == '':
raise RunTimeError('PLC did not respond')
#this should be changed. The MIF software will wait until it
receives a response or timeout, not ABEND
print response
if response[0:4] == '0135':
print 'Drive ready received.'
#Frame 3
getReadyMsg2 = '016040\r'
totalsent = 0
while totalsent < len(getReadyMsg2):
sent = pcSocket.sendto(getReadyMsg2[totalsent:],MegaMatPLC)
if sent == 0:
raise RunTimeError('Transmission failure. Please confirm
MegaMat settings in megapy.ini')
totalsent = totalsent + sent
# Frame 4
response = ''
while response.find('\r') < 0:
response = pcSocket.recv(16)
if response == '':
raise RunTimeError('PLC did not respond')
#this should be changed. The MIF software will wait until
it receives a response or timeout, not ABEND
print response
if response[0:4] == '0130':
# Frame 5
# Transmit tray request
if int(argument) < 10:
shelfPrefix = '0000'
else:
shelfPrefix = '000'
shelf = shelfPrefix + argument
unit = '001'
stack = '01'
"""
There is a 10 digit number plus 40 blanks spaces after the
shelf description built above.
It is possible this is for storing shelf descriptions and
barcodes on the PLC. We will
follow the example of the MIF software and put a phony b/c and
blank description.
X is a field terminator and \r is the end of transmission
(EoT.)
"""
fakeBarcode = '1234567890'
fortySpacesField = ' '*40 + 'X\r'
getShelfMsg = '0105' + shelf + unit + stack + fakeBarcode +
fortySpacesField
print 'Transmitting shelf request as follows: \n' + getShelfMsg
print 'Get shelf length %i' % (len(getShelfMsg))
totalsent = 0
while totalsent < len(getShelfMsg):
sent = pcSocket.sendto(getShelfMsg[totalsent:],MegaMatPLC)
print sent, totalsent, len(getShelfMsg)
if sent == 0:
raise RunTimeError('Transmission failure. Please
confirm MegaMat settings in megapy.ini')
totalsent = totalsent + sent
'''
listen for success code here; not really necessary as the PLC
either does it or doesn't (due to the safety
interlocks the PC doesn't control squat. Given that the
control PC is currently sitting next to it a more
practical form of feedback is whether or not the damn shelf
shows up in the doorway. There is a door-open command
available but it isn't really useful for our purposes.
'''
#Frame 6
response = ''
while response.find('\r') == -1:
response = pcSocket.recv(16)
if response == '':
raise RunTimeError('PLC did not respond')
#this should be changed. The MIF software will wait
until it receives a response or timeout, not ABEND
print response
if response[0:4] == '0130':
# Frame 7
LCDLine3 = '01613' + ' ThetaMegaMat ' + '\r'
totalsent = 0
while totalsent < len(LCDLine3):
sent = pcSocket.sendto(LCDLine3[totalsent:],MegaMatPLC)
if sent == 0:
raise RunTimeError('Transmission failure. Please
confirm MegaMat settings in megapy.ini')
totalsent = totalsent + sent
# Frame 8
response = ''
while response.find('\r') == -1:
response = pcSocket.recv(16)
if response == '':
raise RunTimeError('PLC did not respond')
#this should be changed. The MIF software will
wait until it receives a response or timeout, not ABEND
print response
if response[0:4] == '0130':
# Frame 9 - Final transmission
LCDLine4 = "Shelf " + shelf
totalsent = 0
while totalsent < len(LCDLine4):
sent =
pcSocket.sendto(LCDLine4[totalsent:],MegaMatPLC)
if sent == 0:
raise RunTimeError('Transmission failure.
Please confirm MegaMat settings in megapy.ini')
totalsent = totalsent + sent
# Frame 10 - Final response, final frame
response = ''
while response.find('\r') == -1:
response = pcSocket.recv(16)
if response == '':
raise RunTimeError('PLC did not respond')
#this should be changed. The MIF software will
wait until it receives a response or timeout, not ABEND
print response
pcSocket.close()
if command == 'Status':
print 'Received command for PLC status'
if command == 'SetText':
print 'Received command to set display text as: \n\n %s' %(argument)
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.16 (GNU/Linux)
iEYEARECAAYFAky/TxoACgkQwnknPuQqPIOVkQCfUfYMnOfFVYH+H7X3CnI19UvT
vFUAnjuUHO+oikUQrwvYfLsRkszynY1s
=jthz
-----END PGP SIGNATURE-----
--
http://mail.python.org/mailman/listinfo/python-list
