How to show "invalid" data??

[belias]

I'm fairly new to the world of PLCs and HMI panels, so I'm hoping someone can point me in the right direction.

I have 2 BRX PLCs connected via PeerLink.  PLC A (the remote unit) is connected to a 4-20ma well-depth level sensor and shares its real-time data with PLC B (the local unit) via PeerLink.  The current value is displayed on a bar chart on the HMI panel (along with a calculated value of the number of gallons remaining).  There's also some logic that controls alarms, automatic water shut-off valves, etc.

OK, so question:  What do I do when the data becomes invalid for some reason (let's say the 4-20mA cable gets broken, or the PeerLink connection drops).  I can't simply show a value of 0 since it's not correct and it would trigger lots of alarms...and I can't show "100%" for the same reasons.

Is there a way I can set the data to "null" or similar until the problem is fixed?  Ideally this would cause the HMI graph to display an error similar until the problem is fixed.


Thanks,

Brian

[Bolt]

You should be able to address this issue using the X bits from the analog module.  There are "input" bits that enable when a module fails self test, is missing 24V, or has data not valid.  additionally, there are bits for each channel's transmitter being broken.  You could use those X bits to show an error, send an email, alert on the HMI, etc.

Note, I'm not sure if these bits exist for on board analog inputs, I'm basing this off my analog input expansion module.

See page 8-23 in this manual.
https://cdn.automationdirect.com/static/manuals/brxuserm/ch8.pdf

[belias]

Great info - thank you.  That solves about 75% of it.  The remaining issue is how to detect a failure in the link between the 2 PLCs.   Is there a way to detect this in a similar way?

Thanks again.

[racrowd]

Cyclic bit states.  Send another bit from the remote plc and make the bit cycle between on and off for 1 second each (or some other slower frequency).  If it doesn't change on the local plc within the amount of time + some error, then trip the connection dropped alarm and start reconnection routine.  I'm sure there are better ways, but that should work for the most part.

[MikeS]

peerlink has some structure members that you could use to detect the lack of communication, for example PL.B0Rate - PL.B15Rate is the transfers / second for each data block, if the link fails that value will go to 0. Look at help topic DMD0300, toward the bottom, for a full list of the PL fields you can use.

[belias]

Just wanted to say thanks.

The combination of checking the X10 status bit for a valid analog signal and using the PL.__Act and PL.__Rate values is working great.  I have these set to trigger a status bit that shows a warning on the HMI panel.

Thanks for help.


- Brian