1794-ASB Local Faults detectable by PLC5?

I have a rack of flex I/O that had a bad module cauising an intermittent local fault on the 1794-ASB module. The rack is used for the main control panel I/O for the machine. The machine was in auto when the fault became persistent, and the operator was unable to stop the machine as it ran out of stock and made a small mess. The E-stop is hardwaired and was finally hit to shut it down.

The flex i/o module 1794-IB16 which caused the fault has been replaced, but before I did that, I inserted contacts in the PLC code to shut down the machine in the case of a RIO fault on that rack.

Well, when there is a local fault on the ASB, the rack fault bit remains cleared, so, as far as I can tell, the PLC cannot detect when this occurs.

Thanks
Paul C.

Does anyone have a workaround for this? I don't like the idea of losing communication with any I/O and not being able to program an orderly shutdown.

Paul,
I've never used flex-IO with a PLC5, so I don't know if there is another way, but if you were willing to consume two IO points, an input and an output, you could detect it programatically. Wire the output back to the input, then send a pulse stream to the output. If the input does not match the state of the output pulse stream (with a very brief time delay for IO to respond), then you know you have a malfunction.

When FLEX I/O is used with CIP networking adapters (DeviceNet, ControlNet, EtherNet/IP) there is an available "slot status" word that can indicate the status of each module in the rack. With old-fashioned Remote I/O, though, there's no provision in the memory addressing for this extra data.

So the 1794-ASB does all it can do; it lets you decide whether you want it to announce a slot failure with just the Local Fault status LED, or if it should assert a Rack Failure if just one module fails. That's the behavior Paul wants the adapter to have.

From the 1794-ASB Series E User Manual, page 2-9, the description of the Rack Fault Select (RFS) option:

When RFS is disabled (S2–6 on), module removal and insertion under power (RIUP) is possible. If an I/O module stops responding, the adapter declares a local fault and flashes the Local Fault indicator. The adapter also resets the output data (if any) for the module not responding. All other modules remain active.

When the RFS is enabled (S2-6 off), communication error detection is extended to the I/O module level. If an I/O module stops responding, the adapter declares a local fault, flashes the Local Fault indicator and causes the scanner to declare a Rack Fault. The adapter resets the output data (if any) for the module not responding and commands all other outputs to go to the state determined by the Hold Last State switch (S2–1).

Click to expand...

Thanks Ken,

That's exactly what I want.

It turns out I was looking at the manual for a series B! I had the series E pdf saved also, just didn't rename it as such so it went unnoticed.