PLC5 and CLX

Saul#75 · Oct 21, 2020

I have a CLX L72 that I want to monitor a PLC5 PLC (L40E) and determine whether or not it is up and running.
I can get the PLC5 to let the CLX know its up via messaging triggered by the first scan bit S:1/15 but its knowing when the PLC5 is off. There is no controlled shutdown, just off like in a power cut.
CLX is currently reading data from the PLC5 so I'm currently using the error bit but as the messaging is sequenced the error bit is reset every cycle.
Any ideas?

PS
All comms is ethernet based.

alan_505 · Oct 21, 2020

Create a heartbeat routine with messages between both processors.

Alan.

Saul#75 · Oct 21, 2020

I was thinking that but its making it robust for all eventualities. Do you read or write?

One idea is for the CLX to trigger a response from the PLC5 and the PLC5 to return a random value from S:24...
If the value is different from the last one then all is good...

I_Automation · Oct 21, 2020

What i do is have one PLC (CLX here) send a number that increments every write, then the other PLC (PLC-5) moves that value to a different tag that is read by the first PLC.

Run a timer that does not reset itself for a decent time to detect comm's loss in each PLC. When the CLX reads a new value RES the timer, when the PLC-5 gets a new value RES its timer.

If the readback value for the CLX does not change within a timer PRE then comm's are lost. If the written value in the PLC-5 doesn't change then it knows the comm's are lost.

Saul#75 · Oct 21, 2020

Another good idea.
Just to add, the CLX is never off but the PLC5 is off quite often.

I've got a bit of tinkering to do

Saffa · Oct 21, 2020

We do pretty much what I_Automation suggests on all our PLCs used in remote telemetry applications where we only read from them every few minutes.

Using the incrementing heartbeat lets you confirm that:

- You are getting healthy reads
- You are getting healthy writes
- The processor is in run (you can still read from Rockwell PLCs when they are faulted or in prog mode - this catches lots of people out who rely only on .ER bits of a message instruction. )

Ken Roach · Oct 21, 2020

Write logic in the ControlLogix to periodically read a block of Status information from the PLC-5's Status data file (data file 2).

If the /ER bit is set, you know there's a failure somewhere in the network path or the controller is shut off.

I would read at least S:1 through :23.

S:1 is the controller status and flags
S:10 and 11 are the Minor and Major Fault bits
S:12 is the Error Code
S:18,19,20,21,22,23 is the Real Time Clock (Y:M

:H:M:S)

rupej · Oct 21, 2020

I_Automation said:
What i do is have one PLC (CLX here) send a number that increments every write, then the other PLC (PLC-5) moves that value to a different tag that is read by the first PLC.

Run a timer that does not reset itself for a decent time to detect comm's loss in each PLC. When the CLX reads a new value RES the timer, when the PLC-5 gets a new value RES its timer.

If the readback value for the CLX does not change within a timer PRE then comm's are lost. If the written value in the PLC-5 doesn't change then it knows the comm's are lost.

This is what I've done for years and it has never let me down. Even simpler and easier to follow than a boolean heartbeat toggle or whatnot, in my opinion.

I_Automation · Oct 22, 2020

I_Automation said:
If the readback value for the CLX does not change within a timer PRE then comm's are lost. If the written value in the PLC-5 doesn't change then it knows the comm's are lost.

Actually that should say:

If the readback value for the CLX does not change within a timer PRE then comm's are lost or the PLC5 is not running to move the value to the other tag.

TheWaterboy · Oct 22, 2020

I use a date constructed similar to Ken suggestion (YYMMDDHHMMSS). I read it at some interval and it has the advantage of showing a time stamp displaying the last time you heard from it.

PLC5 and CLX

Saul#75

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alan_505

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Saul#75

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I_Automation

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Saul#75

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Saffa

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Ken Roach

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rupej

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I_Automation

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TheWaterboy

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