S:1/15

OregonMajor

Member
Join Date
Jun 2005
Posts
10
I have seen this used quite a bit by a certain programmer. What is this status bit. First pass? Why would it be used so much in a program?
 
First pass is correct. I typically use it to initialize registers upon power-up. How much is "so much"? Can you post the code?
 
It can also be used to prevent things from happening on first pass. My suspicion is that if it is used "too much", then the code is likely not very well organized. Of course, "how much is too much?" is a whole new question . . .

Marc

PS: It could also be that the original programmer either didn't know about (or trust) the prescan
 
To simplify it. It get used alot on the "main" file. Where all of the JSR commands are used to jump to other files. It gets used at the begining of the JSR rungs. Its always used as an XIO instruction.

So if I'm thinking right, this will look at those files for the 1st intial scan of the plc, then thats it???
 
As an XIO - it prevents execution of the JSR's on the first scan. After the first scan, your code will execute as though that XIO instruction is not present (since you will not be in the "first scan").

Marc
 
looks like he is using it as a bypass...possibly an automated process that he doesn't want to happen in the case of a power failure or power on.
 
That does sound like it might be it. I will go online and watch how it works. Maybe it will help me understand it a little better.

Thanks
 
With the greatest respec't , no you won't !! , How are you going to go online and catch this happening , if you you halt the processor , go online and fire it up sgain , I don't think you will see a thing . What you can do is put some test bits in series with the first cycle , and switch them on and off and see what happens . In this case it sound very untidy .
 
This bit can also be used to move setpoint presets into registers on power-up. It is also used to copy configuration data for I/O modules from storage registers to the configuration word(s) for the module. For instance, in earleir versions of RSLogix, configuration wizards did not exist for modules like the NI8, so you had to manually enter the config data.
 
TIC said:
With the greatest respec't , no you won't !! , How are you going to go online and catch this happening , if you you halt the processor , go online and fire it up sgain , I don't think you will see a thing .

I agree completely. An effect will ONLY be seen on the very first scan (probably on the order of 5 - 20 milliseconds) - you aren't going to see that when you go on-line (your eyes wouldn't catch it even if your RSLogix display did - which it probably won't).

Again - first pass through the logic, those JSR's are skipped. Every subsequent pass through the logic, pretend your XIO isn't there . . .

Marc
 
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