Because the status flags can change so quickly, RSLogix 5000 software does not display the status of the flags (i.e., even when a status flag is set, an instruction that references that flag is not highlighted).
Steve, I have been using the First Scan bit for years to reset counters and timers at start-up, so I know that indeed it does go ON.
Let me backtrack some here. I think this "trick" is similar to what a magician does with tricks that fool our eyes. We see him pull a coin out of someone's ear. Our eyes are not fast enough to get the correct signal to tell our brain what really happened. The same is true of our computer monitor screens. They do not have a fast-enough update time to show the First Scan bit as ON. Because your computer is not fast enough to record the event does not mean that it did not occur. You can set another bit that latches on the First Scan bit and indirectly know (after the fact) that First Scan has been set for 1 scan only.
Think about this. A typical PLC scan cycle is 3 milliseconds. To see a bit that is only ON 0.003 seconds, (1) your eyes would have to be like an eagles, (2) your communications link between the PLC and your computer would have to be blazing fast, (3) your monitor update time would have to be less than 3 milliseconds. I think in general that none of the above 3 events are true, and to see the First Scan bit go on, the total time for all 3 events would have to be less than the PLC cycle-scan time.
If you artificially created a PLC scan cycle that was at least 500 milliseconds (1/2 second) then you could probably see the First Scan bit go on.
For example, the S:4/4 bit is a Status bit too, and you can watch it blink on-off all day long, because it stays on LONG ENOUGH that the signal travels from the PLC to your computer, and your computer monitor has enough time to show the bit, and your brain has enough time to receive the information from the monitor screen, before it goes off again.
