First Pass Bit RSLogix 5000

[DBLD99]

Hello guys, does anyone know if there is such a bit as a first pass bit like in the 500 family, in the 5000 family. I tried searching ab's site but everyone knows how great that is. Thanks for your time

[rdrast]

For bit addresses, S:FS is a first scan flag for Logix processors.

http://domino.automation.rockwell.co...F?OpenDocument

Is a listing of other status file equivilants.

Be warned on the S:FS though, if you have additional timed or other tasks, you should latch your own global bit in a scheduled cylic task, or better, one global bit for each task you have configured. Then use the global 'per task' first scan bits if needed in each task.

[Bitmore]

http://domino.automation.rockwell.co...F?OpenDocument

This should answer your question

Bitmore

[Raflex]

I have the same question I need to know how to set or reset some parameters at power up

[Ken Roach]

Since you have the same question, the same answer should apply, right ?

"S:FS" is a boolean system tag in the Logix 5000 family of controllers that is true only on the first scan of the continuous task. It is often used as the address for an ordinary Examine If Closed (XIC) instruction in ladder logic. " ---] [----"

If you need to use a first-scan function in a conditional subroutine or periodic or event-driven task, use the First Scan tag to latch another global tag that can be used in those tasks.

I often use the S:FS bit to perform RES, COP, and FLL instructions, as well as resetting indexes, timers, and counters to their initial state.

[danny.michael]

Ken,
I almost never use a continuous task, but the S:FS has always worked in a periodic task as well. According to KB #34980, each program in the controller has its own first-scan bit. Isn't that the case?

[Ken Roach]

Tip of the hat to Danny.... he's right. My mind was on conditional subroutines, which don't have their own Program databases.

The most important sentence in that Technote is "Each Program in the project has its own individual first scan bit".

That means each Program, in each Task (Continuous, Periodic, Event) has its own First Scan bit.

[rdrast]

Just adding, I still use a global set of bits (a DWORD, actually) to trigger first scan initialization of multiple tasks.

Rung 0 of the first continuous routine uses s:fs to set the DWORD so that one bit is set for each task initialization that must be done. As each individual task completes it's initialization, it sets a bit false.

I can tell when my initialization is completed by seeing that the DWORD equals zero.

That also gives me a handy way to re-trigger initialization logic should the need arise.