SLC5/03 to laptop comm headache

Forgive me brothers, for I have sinned...

Not really, but I am about to. I have a project with a SLC5/03 processor, W98 laptop with a serial port (so rare now I dare not give it up) and RSLogix 500. When I first power up my SLC system AND do a fresh boot on the laptop, I can download my program into the SLC. Then I go on line and everything is fine. After being on line I can view my program (troubleshooting it right now), but as soon as I go into my Alarm subroutine for any reason, I loose comms! Once lost, I can't re-establish it. I keep getting the timeout on the RSLogix program essentially telling me it can't find the processor at that node / address. At first the PLC processor kept locking up on hardware watchdog timeouts, but I think I figured that out because I had forces in the test program, and when it loses comms when forces are enabled, the SLC CPU must go into lockdown as a fail-safe condition. I removed all the forces, killed power everywhere, was able to reload the program, and it worked. But again, as soon as I went into the Alarm subroutine, I lost comms again! I have a backup processor card and stabbed it in, does the same thing. I tested the RS232 cable, no apparent problems. And like I said, I can load the new program ONCE when everything is fresh, but sooner or later I lose comms, and it seems to only happen when I enter that Alarm subroutine. Arrrrrgghhh!

Any ideas?

What will be Comm LED status before & after the Problem occurs

The only LED labeled "Comm" is on the RIO scanner, but there is an RS232 LED and a DH485 LED on the CPU card. I am using the RS232 port, but the DH485 LED is constantly flashing. When I am able to download the program from the laptop to the CPU card, the RS232 light is flashing (fast). When I lose comms to the laptop, (assuming the PLC CPU doesn't lock up), there are no flashing LEDs.

You can access the Alarm routine in the off line file? diagnostics seems to be ok try to go online through DH485 port check the same

Thanks. I don't know how to configure the DH485 adaptor unit (PIC?) to work with RSLogix and do not have the manual (taken by my predecessor). AB wants to sell us a new manual, we need to ship the system too soon for that.

what error message you are getting check the same thing in the rslinx rswho if same problem select the driver AB_DF1 & press Auto configuration try this

I have seem similair problems like this before. It may not like one of your rungs for whatever reason. I would save a copy, delete all of the alarm rungs and start adding them in a group at a time, testing each group until the problem reoccurs.
I would look for a branch within a branch as the problem.

I once narrowed a problem down this way, the rung looked fine, but deleting the rung and them manually retyping the contacts fixed the problem. It may have had something to do with the revision level of the 503 but I don't really remember.

Thanks for the suggestions. It was a grounding problem. They had a 480V MCC on the machine system, but only 208V in the facility, so they used a transformer to step up to 480V for the MCC bus, but apparently didn't think grounding was a big deal. We hooked up a ground cable and the problems went away.

what are the exact versions of RSlinx, RSlogix, and 5/03 part number are you using?