PanelView Plus 7 and 1769-L18 CompactLogix

[arlenjacobs]

Oh, pushbuttons in HMIs are trouble.

Ken nailed it; this happens on EVERY HMI that uses momentary buttons.
Use a startup routine in the controller to set your critical outputs to a know good state.

The same problem happens if only the HMI loses power, or if there is a communication loss (ex. router fails, loses power, someone unplugs a network cable, etc).


The exception to that rule is if the PLC can detect a communication loss to the HMI.
The new PanelView5000 solves that problem with a special instruction in the PLC as well as using class1 communications with the controller.

The older PanelViews had class1 messaging, so the PLC could then detect a communication loss and act on that. Such as stop (clear) the jog output.
You can also setup class1 messaging on the PanelView Plus, but it's a bit more complicated.

Another way to solve the problem is to use a heartbeat from the HMI. The HMI sets a bit in the controller every few seconds, then the PLC clears it and resets a timers. If the HMI ever misses an update then the PLC can clear all jog outputs (or whatever fail safe routine you want. maybe the startup routine).


Anyway it's not a flaw in the PanelView. It's a design limitation with all HMIs that use momentary push buttons. They are not the best choice for fail safe actions.

 

[Codenamecody]

This is the reason why I put OTU instructions after all my pushbuttons. The HMI turns the bit on, and then the PLC turns it off. If you need to see the bit on more than one rung do this:
''PB'''PB''''''PB_OS
--||--(U)-----( )--

Then just use the PB_OS tag everywhere in your code.

Not many programmers do it this way, but I prefer it just for the reason you mentioned. The HMI is SUPPOSED to turn pushbutton bits ON then OFF, but it is possible for the off transition to be missed due to crashes, power failures, etc.

Clearing all the pushbutton inputs on first scan would work in most instances, but what if the HMI loses power and the PLC keeps running? It would be required to power cycle the PLC, maybe a big deal maybe not...

edit: fixed the spacing in my "ladder logic" (ignore all the quotation marks)

 

[rupej]

What about unlatching all of the PB bits at the end of every scan, like as a last subroutine. Wouldn't that accomplish the same thing as the ONS bits but without extra tags, and easy to add to an existing program?

 

[brendan.buchan]

As with the posters above I also use the method where I have my momentary HMI button turn on another bit in the PLC, then unlatch the HMI button on the same rung.

e.g.

hmiStartPB                    hmiStartCmd     hmiStartPB
-----||-----------------------------()--------------(U)

Then use hmiStartCmd in your logic, no risk of the HMI momentary button getting stuck on.