Hi Experts

What could be the cause of 2 or more interlock *first signals* displaying simultaneously. 

Yes it is possible this can happen if the input conditions change state on multiple interlock inputs on exactly the same scan. 

Examples of how this happens:

1. if you have connected both inputs to the same exact source  (this is the easiest way to reproduce it for demonstration purposes)

2. having unique/divergent logic that happened exactly simultaneously.  I think its a quite rare event in real time. (unless you have logic that makes both input conditions happen at the same time due to code scan order originating from a common initiating event, then maybe it always happens.  In this case you may want to evaluate your ILK conditions further)

To diagnose the issue of multiple "FirstIn" signals displaying simultaneously, first verify that the "FirstIn" logic is correctly configured. It should display only the initial trip signal, and any subsequent signals must remain inactive unless the first one is reset.
Next, check whether multiple interlock signals are mapped to the same data block or memory area. Overlapping or shared memory assignments can lead to conflicting updates and result in simultaneous signal displays.
Review the Continuous Function Chart (CFC) for any improper connections or shared inputs between interlock blocks. Misconfigured links or unintended connections can cause multiple blocks to trigger at the same time.
Also, confirm if the CPU has experienced a warm restart recently. A warm restart can retain the last known values of interlock signals, which may cause multiple "FirstIn" indicators to remain active.
Inspect the logic to ensure there are no unintended interactions between interlock blocks. For example, if the output of one block is feeding into another block’s input, this could create a chain reaction of signals.
Lastly, if the system has been recently upgraded or migrated, make sure that all interlock blocks and associated components are updated to versions compatible with the current environment. Using outdated versions may result in unexpected or faulty behavior.