PLC 5 message problems

[FluxCap]

Having a hard time with this one:

The attachment has the controller and message error details.
The controller is sending a message to an ancient DOS PC. The PC has a specialty board that has a read and write output hardwired to 2 PLC inputs. I can see both of these bits toggling at the correct times. The PLC uses read and write message instructions over channel 0 that is configured as per the attachment. The PC receives the data when it is sent, but when the PLC tries to read the data from the PC nothing is returned. Either read or write message returns the error "you used the wrong station number", yet the PC receives the data. The channel status for channel 0 always shows that msg sent and received are the same, with no errors, unless I unplug the channel. Any suggestions?

 

[Ken Roach]

The important question here is "what changed" ?

I noticed that your MSG instruction configuration has the Local Station Address set to 3.

In a point-to-point connection, the destination address is usually set to 1. Many DF1 implementations just ignore the destination node address because they know they are the only device connected.

It's also possible that the PC's implementation of DF1 Full Duplex is not really giving the appropriate exception code.

Have you tried putting in some test code to run just one of those messages at a time, to be sure they're triggering correctly and completing quickly ?

 

[BachPhi]

Check if cable plug into correct port in the back of PC.
Check PC application directory for any new 'modification' or if you have a backup , copy and restore it. Look around for pkunzip and such.

 

[FluxCap]

This system has been mothballed for years, so I don't have any idea what might have changed or gone bad. Since the PC is receiving data, I assume it is plugged into the correct port. I will try test code with just the one msg instruction to see if that makes a difference.
Thanks for the pointers!

 

[Ken Roach]

This is an appropriate time to mention my favorite serial troubleshooting tool. For years we used splitters and dual-port forwarders and other stuff, and this is a compact, inexpensive tool that replaces them easily:

https://www.stratusengineering.com/hardware-software-products/

DF1 Full Duplex with PLC-2 style addressing is so simple that you can decode it by hand, much like Modbus.


If you do any serial troubleshooting work, the Serial Tap is a wonderful tool. Being able to see what's actually going across the wire on a serial link is the difference between guessing and knowing.

 

[FluxCap]

I have a competitors serial analyzer and it ain't cutting it. Ordering what you suggested as soon as I finish posting.
Thanks again.

 

[BachPhi]

You stated: PLC write data, PC received it OK.
PLC read data, nothing received.
But both times you received "wrong station".

Look for ini, dat file and change station number.

 

[Ken Roach]

Rhetorical question: how would the PLC-5 know that "you" used the wrong station address when sending the message ?

Rhetorical answer: if the PC application responded with an error code that indicated that the DF1 frame's DST byte didn't match its Station/Node address.

If the PC application uses the DST byte for some other purpose (like a KF2 box did for DF1 -> DH+) it would return a timeout or not respond at all.

If you can intercept the serial data, you'll find the Source and Destination bytes at the start of the DF1 payload.

 

[Ken Roach]

The DF1 Reference Manual hasn't been updated in years, but it certainly covers the legacy PLC-2/3/5 commands well.

Check out the error status sections in Chapter 8.

https://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1770-rm516_-en-p.pdf

I'm looking through the error status byte codes that could be generated by a remote station and trying to figure out which one could have the plain language description "you used the wrong station number".

Maybe Local STS (generated by the PLC-5) code 7, "Station is Offline", which would be a timeout when there's no reply.

Or the remote node might be responding with STS nibble 30, meaning "Remote node host is missing, disconnected, or shut down".

Obviously this is not a PLC but a DOS application implementing DF1 and emulating a PLC, so the only way to tell what's going on is to pop the hood and pull the rocker valve cover.

 

[FluxCap]

Ken & BacPHI,
I'm doing this remotely, so I can't see into the DOS PC. I don't have a PLC 5 in my shop, so I set up a SLC505 and a ML1500 messaging each other over the serial port to see if I could duplicate the error. No luck. Should have stuck with BCSS.
I called the site and asked them to swap the transmit and receive because I had run out of ideas. While on the phone, I hear the engineer say to himself "I can't believe...". He gets back on the phone and tells me the receive wasn't connected. Hmm, thought that was the first thing we checked. Well, I only checked the other end....
So you were dead on Ken. How could the PLC 5 know it was using the wrong station address? Why wasn't I getting a timeout or no response. It's working right now.
Thanks for both of your help. It kept my spirits up.

 

[BachPhi]

Nice! I cant believe that sh## either. lol

 

[Ken Roach]

FluxCap, BachPhi, Ken Roach, and the On-Site Engineer all raise their glasses at the end of business on a Friday night:

To your Rx, and to your Tx, sláinte !