Modem hanging intermittently

[Billy_d]

Our modem has just started intermittently hanging. No pattern, sometimes several times a day, sometimes less than once per day.
There are no error messages in Lookout, it's just not dialing the modem. The problem does not correct itself, it requires user intervention.
There is nothing wrong with the modem (USR 56K external), several new units have been tried and the setup dips are correct.
Power to both the PC and the modem appears to be fine.
First started about the 12th of Jan.
Problem clears when Lookout is shut down and restarted. No action is taken with the modem, when Lookout restarts it just dials normally until the next hang.
Changing the poll rate does not help restart the dial out.
PC is dialing 2 remote AB SLC503 (4800baud KE/KF/Serial) and 2 TeleSafe (1200baud Modbus) all on the PC's mother board single RS232 port set up as COM1.
Usually 2min poll rate for AB's, 10min for TS's.
There is one other SLC503 on a direct line which continues to poll every 2 min without problem. This direct connection is using an add-on PCI National RS485 card set up as COM5. (note, no problem with this direct connection).
PC has been working fine in this setup for several years. I suspect an auto update to XP or perhaps a change to a firewall for the LAN downloaded to the PC by the IS dept.
PC is running XP (latest version) Lookout 3.8 b12 (or maybe b16). No previous problems with Lookout on this PC. No changes were made to the Lookout code coincident with this problem.
We're going to try rolling back XP to a one year old mirror and disconnecting the LAN so it can't be updated during the test period.
Anyone got any other ideas or suggestions?

[Khalid]

Hi Billy,

If nothing else has changed except the Windows' updates, your test with an older version should be decisive, hopefully.  Also, you will want to test this with an NI or a 3rd party serial card.  See the following thread for a similar, intriguing, issue: 

http://forums.ni.com/ni/board/message?board.id=190&message.id=965

Regards,

-Khalid

[Billy_d]

Thankyou Khalid, I hadn't considered the possibility of a UART problem, certainly the NI RS485 card has been rock solid over the last 3 or more years.
Hopefully the roll-back of XP will do the trick. But putting in an NI RS232 card is something for us to try now if all else fails.

[Paul M.]

Billy,

I would also check the windows event viewer, to see if there is anything going on there (Start -> Settings -> Control Panel -> Administrative Tools -> Event Viewer).  If there is a windows/driver-level problem, an error or warning will often show up there.

[Billy_d]

Excellent suggestion Paul, thankyou. I'll give that a go.