DH+ Communication Problem......

I have this problem with MSG instructions on an SLC500 PLC. I have two different PLC's that are communicating through DH+.

PLC-A has a "Write" MSG instruction that writes data into a register on PLC-B. The electrical panel where PLC-B is located was powered down for some maintenance. After it was powered back up, now the same "Write" instruction located on PLC-A is stuck on transmitting, so data is not getting transfer between the PLC's.

Is there anything that I can do to fix this problem. It appears that always happens when electrical panel for PLC-B is powered down.

Never mind guys. I just read that if the "Message Timeout" on the MSG instruction is set to 0, this is what happens.

A timeout value of
0 means no timeout value. In other words, if communication is
interrupted, the processor will wait forever for a reply. If an
acknowledgement is received (as indicated by the ST bit being set), but
the reply is not received, the MSG instruction will appear to be locked
up, although it is merely waiting for the reply

Click to expand...

I just change the tiime out to 5 seconds and its working now.

I think you're filling up the transmit buffer for the SLC-5/04 controller. My recollection is that it's 4 messages in the 'buffer' and 10 messages in the 'queue'. This may vary with firmware revisions.

It is probable that your logic for repeating the transmission in PLC-A does not take message failure into consideration. I'll bet it is based on the /DN bit of a timer.

What I do is add an XIO instruction with the /EN bit of the Message Control Block (bit 15, in the SLC-5/04) as one of the pre-conditions for the MSG instruction. That way if the timer I'm using to trigger the MSG instruction fires before the MSG has completed, I don't fill up the SLC-5/04's message buffer with requests that can't be completed.

Ken,

PLC-A used to have 6 messaging instructions. I read that you can only have no more than four MSG instructions, so I had to rework the communication between various PLC's not knowing otherwise.

I was able to get PLC-A MSG instructions to start behaving properly. You are totally correct Ken. All the MSG instructions are condition by the /DN bit of a timer.

PS.
Thanks for your help......