Prevent bad value of analog input in Controllogix PLC

We have L72 controller of Rockwell make with 4 nos. analog inputs from pressure transmitters. Due to break in cable connection OR transmitter fault,etc. bad value shows in PLC program as -200 which causes automatic startup of standby equipments.
Is there any method by using i can avoid bad value coming from transmitters (non-smart) so as to prevent auto startup of standby equipment?
I checked in Controllogix PLC but could not identify such FB with which requirement can be fulfilled.

Add a redundant measurement and switch over between them when one is at fault.

Hi Dirt,
I assume that you are suggesting for 1 out of 2 logic, If, so, it is not feasible at site. With single pressure transmitter, how the requirement can be fulfilled.

If the reading is less than zero then it's a bad value (assuming 4-20mA). So check for this condition and use it to prevent equipment starting.

My scaling blocks set a "LoopFault" bit if the analog is outside of acceptable limits. If my process needs to ignore bad sensor readings then I use this bit to take appropriate action.

kamit patel said:

Hi Dirt,
I assume that you are suggesting for 1 out of 2 logic, If, so, it is not feasible at site. With single pressure transmitter, how the requirement can be fulfilled.

Click to expand...

Nah, I suggested 2 out of 2 (for availability). 1 out of 2 is for extra safety.

Hi Saffa,
Can you pl. send me the snapshot of the logic how did you configure scaling block with "LoopFault" bit in PLC logic?

kamit patel said:

Hi Saffa,
Can you pl. send me the snapshot of the logic how did you configure scaling block with "LoopFault" bit in PLC logic?

Click to expand...

I have my own custom scaling block which won't

But you can do this just in plain logic. I assume your analog is scaled for 4mA = 0 and 20mA = 1000 if you're getting -200 when there is 0 mA.

So 3.5mA would be -175. I like to have a bit of a buffer rather than exactly 4mA. Up to you what threshold you want.

All I would do is compare your analog with a LEQ to -175. If it's less than that for more than say, 3 seconds, then latch a loop fault. Up to you how you reset it.

Just make sure your loop fault delay is less than your standby equipment start delay, and use your loop fault bit to inhibit startup of the standby unit

Pretty simple stuff.

Kamit Patel, you didn't mention which module you're using but many modules permit you to configure alarm bits for each channel.

Go to the Controller Organizer -> IO Configuration and double click the analog module to bring up the Properties popup.

Click on the "General" tab. Make sure the Comm Format selection doesn't contain "No Alarm" in the description.

Click on the "Alarm Configuration" tab. Here, you can set up high & low process alarms for each channel individually.

Check with your module user/installation manual for details specific to that module.