ControlLogix DHRIO Block Transfer Timeout logic

Hello all, I couldn't find a solution through the search tool so hopefully someone out there can help me along.

I'm working with a converted PLC5 program running in a 1756-L82E. There are RIO block transfer messages for talking to four analog cards in a 1771 rack via a 1756-DHRIO/1771-ASB setup. The old logic had a timer for each analog card that would indicate an alarm if the BT message .DN bit didn't transition within 5 seconds. I'm thinking we would be better served by triggering a message timeout to keep the other BTs from hanging up. However, I've never programmed one of these before.

My thought was to:
1. Use a timer that examines the message .ST bit instead of the msg .DN bit.
2. Latch the MSG.TO bit via the above XIC timer.DN bit & XIO MSG.ER bit.
3. My understanding is that the MSG instruction will automatically clear the .TO bit when the .ER bit turns on.

Thoughts? Thanks in advance.

That sounds about right.

Manually setting the .TO bit causes the MSG to manually error and indicate a Timeout in its error code. It is the only status bit you should ever write to in an A-B controller.

I think you're correct that once the .ER bit for the MSG becomes true, your manual setting of the .TO bit = 1 with an OTL output latch instruction will be turned off and the .TO will return to 0, and .ER will stay = 1 (and .ERR and .EXERR will hold codes) until the MSG is re-triggered.

Thanks Ken! I saw your responses to older MSG timeout posts and internally questioned whether "set" meant OTL or OTE. I was probably over-thinking it but paranoia is the better part of valor, or something like that.

I would have to go back and review my old programs, but I think I actually use a one-shot and OTL for this purpose, because that effectively "pokes" a value =1 into the MSG control subelement bit just once.

If you use an OTE instruction, you will be setting that bit =1 or =0 at all times, depending on the preconditions on the rung, which isn't your goal.