Scan Time

Could someone be kind enough to tell me what the general rule of thumb is for calculating total scan time of a program? I read the answer to this somewhere but I can't remember the source.

It depends - on PLC brand, model, length of program, kinds of commands used, etc. etc. The typical range is a couple of msec to a couple hundred msec. Most PLC manuals have a method for calculating the scan time in an appendix or separate documentation, but the process is tedious, inexact, and for run of the mill applications not worth the effort.

Scan Time - Can it be forced SLOW

Just wondering, kind of related. Can scan time on a AB PLC 5/11 be SLOWED down to allow actual viewing of the process. This is just a stab in the dark but if it can without MAJOR hassles it would be helpfull. Trending is just not cutting it.

In every scan (old PLCs, CtrlLogix5000 is differend)
1. updates Inputs to process image.(executes part of the system program)
2. run one program cycle (executes application program)
parallelly check events as timers updating etc. (system)
3. updates process image to Outputs.(executes part of the system program)

For-Next loop with indirect addresses (fe. 1 to 100) can be
even 100 times slowlyer, than 100 times more code with direct pointers without loop.
Actually N8:10 is direct pointer to memory where its contents is.
N8:[N8:12] includes 2 pointers, processor find 1s't what is N8:12 contents (pointer to pointer) stack it etc. So it do much more work than with direct pointing. Also with loop processor must stack jump addresses and increace for-next counter etc....

scan yime

There is no general rule for calculating scan times of a process, you must consider each process on its own make-up. Take the speed of your processor then start adding in the things to be scanned like length of program, input cards, output cards, analog cards, HMIs, etc....
As mentioned earlier, usually it is not worth the effort, besides it isn't very often that you hear a request to slow down scan times, as a matter of fact I think this is the first time I ever heard of such a request. Usually you want to be as close to real time as possible.

Roger

Re: Scan Time - Can it be forced SLOW

okiebob said:

Just wondering, kind of related. Can scan time on a AB PLC 5/11 be SLOWED down to allow actual viewing of the process. This is just a stab in the dark but if it can without MAJOR hassles it would be helpfull. Trending is just not cutting it.

Click to expand...

I don't know AB, but in Omron CS1 propertyes have scanning time adjusment property fe. 10 ms.
You can do some subroutine with long loop that slowed scanning so
much you like (under limited max-time whitch is adjustable in many plcs). It works with any PLCs.

Another technique to slow things down for de-bugging is to put all the logic of ineterest in a subroutine, and have the subroutine call controlled by a timer. When you are done, you can either eliminate the timer and call the subroutine every scan, or cut and paste the logic back to the main program. You may want to leave timers and counters in the main program to insure you don't miss a transition, and for specific cases you may want to have the subroutine set internal coils and have the coil controlling the outputs in the main program. You can get as elaborate as you need to!