scan time

Quick question if you have a scan time critical application, does it matter were you but that section of logic in the program? Should it be in ladder2 or can it be put into any subroutine

I dont quite get the question,

Given that, if you want to enhance scan time, generally the carefull and proper use of subroutines will help. For example you might want to place all your initial setups in a subroutine that will only be executed when the PLC is powered up.

If it is critical, I would put it in an Interrupt routine, so that each time you trigger it, it interrupts the processors normal operation and scans, performs the operation, then jumps back to normal scan.

I guess it all depend on application.



1. If it is of collecting small input pulses, options are to use hardware interuppts, high speed counters etc,



2. If it is of a specific control logic, you could use timed interrupt. [I don't like these anyway, because if you interrupt to this sub routine frequently, the overall scan time increases tremendously]



3. At times you are able to do those even without interrupt, just by how you design the system.



Maybe you could elaborate more on your process, a lot of people here are willing to help.

It can depend on the PLC you're using and what you're trying to do. When using GE Fanuc you can read selected inputs on command during the scan make decisions and then write selected outputs on demand. You can create a small subroutine doing this and call it more than once giving that routine better performance at the cost of a longer overall scan.

Or if nothing else helps, get a different CPU. Or a different PLC.

I don't think anyone mentioned the STI routine. I have used that numerous times with great success in PLC5s. I recommend using extremely compact code in the routine. Only put in what's absolutely necessary. You can then set the scan rate of the routine, and it will be executed at that interval even if the main ladder has to be interrupted.

Don't have access to info for details, but there are just a couple of thing to set up in the processor status file to make it all work. (STI file numner, interval, and I think there's an enable bit) Haven't done this in a SLC, only PLC5, but I think it's available in SLCs too.