RSLogix 5000 - Purpose of the readback member for ouputs?

Hi all. I'm curious to know what the purpose is of the '.Readback' member for output modules. I'm looking for example of its purpose, and how it could be used in logic.

Thanks in advance.

Since you did not specify what PLC and modules used, I will give you example from the ControlLogix Digital Output module User Manual:

Off/On status for the output point echoed back from the output module.
This is used to verify proper communication only.
No field side verification is done.
For field side verification, see OutputVerifyFault.


Output Verify—Diagnostic that indicates that the input has been commanded to the On state but the output has not
been verified to be On.

Click to expand...

Contr_Conn said:

Since you did not specify what PLC and modules used, I will give you example from the ControlLogix Digital Output module User Manual:

Click to expand...

Cheers. So, I should be using the readback status when checking if the output is on, as opposed to the output status. I imagine this will cater for the issue where the module has failed, and more accurately reflects the output state?

I'm using a CompactLogix, but I thought the readback status would be defined the same for both. Would this not be the case. The CompactLogix manual doesn't explain it well (unless I'm simply looking at the wrong manual).

The OutputVerifyFault status sounds interesting, but I gather this feature is only for certain IO modules?

OutputVerifyFault is available on diagnostic outputs in ControlLogix and other platforms.

But I am not CompactLogix I/O expert, I will let Ken or other guys to reply.

You might use the .Readback member in the case where your report interval is greater than the scan time of the routine. If you want to verify that the module has indeed been updated before going to the next step.
You need to remember that with CompactLogix the IO updates are asynchronous to the scan.

NevisGroup said:

You might use the .Readback member in the case where your report interval is greater than the scan time of the routine. If you want to verify that the module has indeed been updated before going to the next step.
You need to remember that with CompactLogix the IO updates are asynchronous to the scan.

Click to expand...

Ah, good point. Thanks. I'll have to look into the diagnostic modules. I was only made aware of them recently, and haven't got a good idea as to what they do.