MSG problems on slc network

lalion

Member
Join Date
Jul 2003
Location
Ballarat, VIC, AUST
Posts
36
Hi, ive been having problems with existing messaging system on a dh-485 network with 4 SLC5/03 plc's.

The system has been in place running happily for years but this morning we found that 2 of the plc's would no longer talk via their rj45 port, I ended up replacing with 2 spare cpu's.

Now I can communicate with all plc's over network again with rslinx, but I have found that 2 MSG instructions on 1 of the plcs (one i didnt have to replace) is now no longer working, with error code 37 "Message timed out in local processor" on read msg instruction, and error 7 "Target Node does not respond" on write MSG, find screenshots attached.
read-msg.jpg
write-msg.jpg

Also I have noticed that neither EN or DN bits are energizing on the MSG rung when im online, even when using a manual toggle bit to trigger msg instruction, but S:2/7 "outgoing msg pending" bit is staying high on the SVC instruction rung.
SVC instruction.jpg

Does anyone have any ideas of what might be happening or can give advice on things to try?

Thanks, leon
 
It's probably worth it to examine how the logic that triggers the MSG instructions is written. Although all the logic on those rungs is hidden in the screenshots, the presence of OTU instructions on the Timeout (bit 08) bits makes me think this logic might have been written as a combination of the SLC-5/02 style manual timeout and the simple self-retriggering examples found in the instruction set reference.

I suspect that the messages are all written to re-trigger themselves immediately on completion. This works great until one message fails, just like you can tailgate in traffic until somebody hits the brakes.

If you're happy with the power cycle results, go ahead and leave it as-is, but if you're interested in preventing such a logical buffer overflow in the future, post the logic, preferably in *.RSS format.
 
Hi, ive been having problems with existing messaging system on a dh-485 network with 4 SLC5/03 plc's.

The system has been in place running happily for years but this morning we found that 2 of the plc's would no longer talk via their rj45 port, I ended up replacing with 2 spare cpu's.

Now I can communicate with all plc's over network again with rslinx, but I have found that 2 MSG instructions on 1 of the plcs (one i didnt have to replace) is now no longer working, with error code 37 "Message timed out in local processor" on read msg instruction, and error 7 "Target Node does not respond" on write MSG, find screenshots attached.
View attachment 14658
View attachment 14659

Also I have noticed that neither EN or DN bits are energizing on the MSG rung when im online, even when using a manual toggle bit to trigger msg instruction, but S:2/7 "outgoing msg pending" bit is staying high on the SVC instruction rung.
View attachment 14660

Does anyone have any ideas of what might be happening or can give advice on things to try?

Thanks, leon

I ran into this problem a couple years ago. Ultimately, I resolved the problem by switching to SLC5/05 processors, but I did solve the problem on the 5/03 DH-485 network by changing power supplies until the problem went away.

Has the MSG routing always worked well? Can you post the logic you use to trigger the instructions?
 

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