CompactLogix PLC not Showing Open Wire Fault

[mdklassa]

I'm working on troubleshooting a PLC for a piece of manufacturing equipment. The issue I'm trying to resolve is an open wire fault not getting communicated back to the PLC.


The system is running an Allen Bradley 1768 CompactLogix PLC that connects via ethernet to a 1734-AENT adaptor and then to a 1734-OE2C analog output card. This card controls a field device (a control valve). Everything appears to be wired correctly. However, when an open wire fault occurs (i.e. the wiring between the field device and the card is disconnected), no fault information gets communicated back to the PLC itself.

The output card (the OE2C) is reading the fault and begins flashing red for the channel disconnected. However, the adaptor does not show any fault information. When I use a GSV block in RSLogix 5000, the status value that is returned for the channel is 16#4201, which to the best of my knowledge indicates that the card/channel are operating normally without any errors.

Has anyone run into this issue before? Is there a setting somewhere in the configuration that needs to be adjusted to read the fault? Or is there some other issue?

 

[Ken Roach]

Welcome to the PLCTalk forum community, and thanks for the thoughtful and good question !

The GSV checks the logical connection between the 1734-OE2C module and the CompactLogix CPU, which goes through the POINTBus, then the 1734-AENT's network connection, and the network physical layer, and the 1768-ENBT module before it gets to the CompactLogix. And that's all in good shape.

The status of the analog channel's diagnostic features are part of the ordinary Input data connection. The module has an Input connection, and a Configuration connection, as well as the Output connection that you're using to send values to be used by the A/D converter on the 4-20 mA signal terminals.

Check out the Controller-scope tag database and you will find a named connection for the OE2C module with a structure that includes sub-elements like

POINTAdapter:1:I.Fault

POINTAdapter:1:I.Ch0Status
POINTAdapter:1:I.Ch0Fault
POINTAdapter:1:I.Ch0Calibration
POINTAdapter:1:I.LAlarm
POINTAdapter:1:I.HAlarm

It is my understanding that ".Ch0Status" is a 1-byte SINT, and the Ch0Fault, Ch0Calibration, LAlarm, and HAlarm tags are BOOL type sub-elements, each one indicating that conditino.

I don't think the module can distinguish between Open Wire and other possible types of faults

 

[mdklassa]

Hi Ken,

Thank you for your response! It was very helpful. Thank you for pointing me in the right direction for where to look for the fault code. After some more finagling, I was able to get our HMI to display the fault correctly. Thanks for your help!