1769 Compact Logix Analog cards not communitcating with processor after power loss

[Redneck99]

I have a 1769 Compactlogix here with 2 analog cards in it. My problem is when there is a power loss or cycle of power the system starts fine but the processor does not communicate with the cards. The values are stuck in last state. In order to get the cards talking again I have to inhibit the cards or switch the processor from Run to program and back to run, then everything is fine again. Is this a firmware problem? i am running version 15 and everything is in one chassis. Any help would be appreciated.

 

[widelto]

Exactly the same happened to me this week on monday. The value got stuck, I had to restart the plc.

 

[Redneck99]

The firmware version is 15.3
Resetting the PLC is not a viable option for operators....

 

[Ken Roach]

If you can, check the Connection Status in RSLogix 5000 for those modules next time you get into that circumstance.

See also if you can determine the number of CIP connections the controller is using. I've had similar (but not the same) problems on systems where MSG instructions or HMI traffic would take up all the CIP connections and the controller didn't have resources to re-make the connection to a field device. Those were networked I/O, though, not 1769 I/O.

What exact model of CompactLogix controller, and what exact model of analog module ? Can you post a summary of the I/O modules connected to the controller ?

Are these analog modules getting their 24V power across the 1769 bus from the 1769 power supply, or do they get it from an external 24V supply ?

Are the power losses momentary (under a second or so), or are they sustained for several seconds or minutes ?

My instinct is to think this is a powerup sequence issue with an external power supply, so that's the direction I'm heading.

 

[Ken Roach]

Addendum: Input values will always remain at their previous value when a connection is broken to an I/O module.

This is intentional and correct behavior. The value is not "stuck": it is remaining at the last available state.

The controller always has a way to determine the "Quality" of an input data item. Sometimes this is a status bit, sometimes a GSV instruction.