1756-IF4FXOF2F/B card goes to fault

Yesterday, on Christmas Eve at 22:00 customer called me to check why some machine faulted and stopped production.
I went there as happy as you can be when you receive such a call on Christmas Eve to see what it was.
Basically 1756-IF4FXOF2F/B fast analog input/output card went to fault and customer reset it just little before I arrived there.
So I could not see diagnostics and why it went to the fault (card surely went to the fault, analog input signals all flat-lined at same time and it was for sure card problem).


Now, I'm used to Siemens where you have diagnostic buffer in CPU, and you can scroll down for an error which might be it.
Reading Allen-Bradley manuals seems that there's no similar thing, rather when you go online on to Connection tab of a card you could see what is Module Fault code and check in manual for it. It seems it doesn't keep last stored fault or I'm wrong?


What I'm curious is there a way to catch an error number, by using some Message instruction and storing it in internal variable of PLC? I'm sure this will happen again, as customer already complained that it happened on another card on another PLC (forgot to ask if it was same type, wasn't really in mood to do proper analysis on Christmas Eve), and I'm sure they will most likely just reset it again by power off/on, rather than checking for error number.


Another question is is there a way to do software reset, just in case we can't discover reason for fault quickly enough?

You can monitor the individual status of each IO card with a GSV instruction, and retrieve any error codes if they exist. GSV | MODULE | EntryStatus will give the overall condition, and attribute FaultCode or FaultInfo will return the exact fault.
If this is causing a hard controller fault, you can handle the condition in the controllers "Controller Fault Handler" routine, but you have to create that first.

Some module faults can be cleared by triggering a MSG instruction.

Ok, I made some logic with GSV to store FaultCode into variable in PLC.
FaultCode is what I'm looking for, not FaultInfo? FaultCode will give me a hex number I can check in manual?

doomsword said:

Ok, I made some logic with GSV to store FaultCode into variable in PLC.
FaultCode is what I'm looking for, not FaultInfo? FaultCode will give me a hex number I can check in manual?

Click to expand...

Yes, but Info will give additional information if it is provided by the card.

My IO monitor routine checks the EntryStatus first, and then, if there is a fault flagged, it goes and grabs the other two fault words. It is important to use EntryStatus as a gateway, as if the module is not communicating with the backplane, it won't show an actual fault.

The default option is the default for good reason - in most applications, that's the best option to use. It means that you can remove a faulty series A input module and replace it with a newer series B input module, and have it continue to work without requiring downloads etc. I almost exclusively use "compatible module".

There are very, very few good reasons to use "disable keying". I've used it occasionally to get me out of trouble in a pinch when I just needed to get modules communicating to get things back up and running, but I always circled back to fix it properly and set it back to "compatible module". Generally, if you're disabling keying, you're probably doing something unsupported or not best practice.

"Exact Match" is generally used in a couple of scenarios:
1. When using ADC (automatic device configuration), which requires it
2. When you are using a very particular feature of your module which absolutely requires exactly that module and no other, and it's not possible to e.g. replace a series A module with a series B without re-configuring or re-testing to ensure proper functionality. I've never needed to use "exact match" for this scenario, but I'm sure others have

There may be other reasons to use one option or the other, but these are the main reasons I'm aware of that factor into my decision making on how to configure my electronic keying.