First scan bit in ControlLogix

[godfrey]

When I monitor a N/C bit addressed to the S:FS bit in RSLogix5000 online, I expect it to be "green" indicating power flow to the right, but it isn't. I know this bit is set only for the first scan. The processor (1756-L55) is in Run Mode. The logic to the right does solve so I know there is power flow. Why is this? I am using v13.

 

[kamenges]

I suspect you came across a display anomoly in RSLogix5K.

Since the logic solves correctly and you know what the bit is, you may just want to comment the living dallights out of it and move on. You could also look at the RSLogix releazse notes for V15 and V16 and see if this comes up. However, bumping firmware to get a bit to display correctly when you know what it's status is anyway seems a bit excessive.

Keith

 

[Ron Beaufort]

that’s today’s entry for my “never noticed that one before” book ...



first of all - a basic concept ... green on the screen doesn’t really indicate “power/logic flow” ... in the case of an XIO (or as you said, an NC “bit”) green on the screen actually means that the software (RSLogix5000) sees a ZERO in the bit/box in the processor’s memory ...



from here on, I’m going out on a limb ...



since there is NO bit/box for S:FS located in the processor’s memory, then I’m guessing (yes, guessing) that the software can NOT see a ZERO there ... and that’s why the XIO doesn’t show up as green ...



size of wager: significantly more than pocket change ... but MUCH less than the rent ...


EDIT: same effect in version 15 ...

 

[TConnolly]

...
since there is NO bit/box for S:FS located in the processor’s memory, then I’m guessing (yes, guessing) that the software can NOT see a ZERO there ... and that’s why the XIO doesn’t show up as green ...

I'm going to wager that Ron is right.

Godfrey, if its really an issue (such as making the program readable for Bubba) then program an XIC S:FS OTE FIRST_PASS and use XIO FIRST_PASS instead of XIO S:FS.

 

[chit gyi]

S:V S:Z S:N S:C S:FS S:MINOR are status keywords supported by controller.you can use in your logic to monitor spcific event.The status flags can change so quickly,L5k software does not display the status of the flags and you cannot define a tag alias to this keywords.

 

[Lancie1]

I know this bit is set only for the first scan.

Unless your scan time is 2 or 3 seconds (instead of 5 or 10 miliseconds, then why would you expect to see S:FS turn green? The speed of light is simply not fast enough to transmit the data and display it in a manner that the human eye can detect. I have never seen the First Pass bit on an SLC turn green, although I do see the results of it in many programs.

 

[Ron Beaufort]

Greetings Lancie1 ...

from godfrey:

When I monitor a N/C bit addressed to the S:FS bit in RSLogix5000 online, I expect it to be "green" indicating power flow to the right, but it isn't. I know this bit is set only for the first scan.

by "N/C" bit, I think he means that he was using and monitoring an XIO instruction ... in most cases we'd expect to see that type of instruction turn green on everything EXCEPT the first scan ... but it won't ...

I think that chit gyi has posted along the right lines ... seems like I remember reading about this somewhere ... I'll try to look it up later if I can get the time ...

 

[TConnolly]

Maybe sketching it will help for the sake of clarity

   S:FS                    BoolTagA

----]/[----------------------( )----

NOTE the XIO S:FS instruction.

The rung is true at all times except on the first scan. However the -]/[- does not turn green on the computer screen, even though XIO S:FS is true at all times except the first scan.

With a SLC500 first scan bit we would see this

-----]/[-------------------( )----

on all scans except the first. But you do not see it in a ControlLogix, even though the rung is true because of the differences in the way the CLX handles status words.

 

[Lovell]

Just Reading from Help file about Status Flags S:FS

from the help section:

"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)."

My take on this is they simply do not even try to show the status of these bit. The execution is still the same, just no graphical representation.

Interesting. I also found that a Counter or Timer does not respond to the input.

I was unable to count using the S:FS bit as the test (toggling the keyswitch from program to run).

A latch bit, however, worked fine.

 

[harryting]

Holy thread resurrection!