2 slc 5/04 communicating over dh+, had a MSG instruction reading I:2.9 in the other processor. Msg instrustion EN bit was staying on always with no DN bit and no data being received.

copied the rung that MSG was in (is in a sub routine} and pasted it in the main routine (lad2) and waalaa ti went to working. so I took it back out of main routine and it went back to working in the sub routine.??????

Can anyone offer any explaination for this,

I had this in 5/05s. I called tech support and the common responce was I overloaded the comms stack. I had been adding / editing the comms. in a small network.
I think that when I was modifing the existing MSG I must have edited it during a read or write portion of it and scrambled it's brain just for that MSG only. After a power cycle all as well once again.

Sort of typical.

My personal MSG rules are:

1) Always include a DISABLE bit in the rung, and toggle it to stop the MSG from executing when modifying it.

2) Do not run MSG instructions 'wide open'. Use a periodic task or timer to re-trigger them.

3) Do pay attention to the .DN , .ER , .ST , and .EW bits. You might also want to pay attention to the .WQ (Wait for Queue) status bit. All of those bits are defined in the help file, and located in the Control Block you assign to the MSG instruction.

3) Be aware of the maximum number of 'active' messages that can be handled. SLC 5/04's can only handle I think it's 4 messages at a time over a single channel, but some adapters can buffer up to 14 total queued messages.

4) IT IS BETTER TO READ THEN WRITE!!! The MSG block on SLC's, PLC's, and CLX's for the most part only generate a single "Request / Reply" transaction when they READ from a target device. If you WRITE to a target device, there are several other handshaking messages taking up bandwidth.

5) DO NOT ever manually manipulate the status buts (including the Timeout bit). That's sure to cause problems.