Read RUN status from HMI

seeigecannon

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Join Date
Sep 2018
Location
Colorado
Posts
7
Hello. I am new with PLCs, and I am working on modifying code for a non-production system with no safety implications.


We are using an Allen-Bradley Micro850 and a PanelView800. We are using Connected Components Workbench for the programming.


We have gotten burned a couple of times before with the PLC being in programming mode and not working, but the display has no indication that the PLC is not running. I have tried linking an indicator on the display to "__SYSVA_RESMODE" which indicates what state the PLC is in (and is a global variable), and is independent of the run state, but when I try to link it in the display tags it says "System Variables not supported".


I have considered copying the global system variable to a regular global, but it wouldn't be updated if the program were to stop.



Does anybody have an idea how to read system variables with a display? If somebody knows of a better workaround then I am game for that too.


Thanks for taking the time to read this.
 
If the HMI won't let you link a tag to a system variable, then in the PLC program copy the system variable to a variable that the HMI can access.

PLC goes to program mode, last status is maintained for that bit, display possibly lies.


You can use a heartbeat to indicate life. say a word/dword just run a add function in your PLC, so that as long as that number is changing you know your in run mode. let it hit 32000 and set it back to zero.
 
Hello. I am new with PLCs, and I am working on modifying code for a non-production system with no safety implications.


We are using an Allen-Bradley Micro850 and a PanelView800. We are using Connected Components Workbench for the programming.


We have gotten burned a couple of times before with the PLC being in programming mode and not working, but the display has no indication that the PLC is not running. I have tried linking an indicator on the display to "__SYSVA_RESMODE" which indicates what state the PLC is in (and is a global variable), and is independent of the run state, but when I try to link it in the display tags it says "System Variables not supported".


I have considered copying the global system variable to a regular global, but it wouldn't be updated if the program were to stop.



Does anybody have an idea how to read system variables with a display? If somebody knows of a better workaround then I am game for that too.


Thanks for taking the time to read this.

What I do is to toggle a bit in the PLC once per second, with a 0.5 second update on the HMI (you can choose how often you want this to run) and display it in the upper left corner of each display.

Our Operations people seem to be OK with the explanation that it is a 'heart-beat' ... if it is not changing, the PLC is not running, call me on the radio.

That's the only consistent way (across all of the various PLC and HMI brands) that I've gotten to work.
 
The problem is, if the PLC isn't running, it can't execute the GSV instruction that tells the HMI that it's in program mode!

The best thing to do is have the PLC write its seconds value into a register, and have the HMI monitor that. Some HMI scripting can time if the current value continues to equal the stored value (if not, store the new value), and trigger an alarm if it is.
 
The problem is, if the PLC isn't running, it can't execute the GSV instruction that tells the HMI that it's in program mode!

The best thing to do is have the PLC write its seconds value into a register, and have the HMI monitor that. Some HMI scripting can time if the current value continues to equal the stored value (if not, store the new value), and trigger an alarm if it is.

if the heartbeat stops changing there is a problem. however the heartbeat is established, the end result is if its changing its good, if it stops its bad.
 
Sure thing. Use them alot for interplc comms to verify the remote PLC is alive. the PLC reading the remote PLC will throw an alarm for Lost Heartbeat when it doesn't change for a few seconds.
 

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