plc scan time

how do I arrive at what my plc scan time is working at?

It would help if you post what type of PLC you are using.

I know that AB SLC/500's have status regusters which contain the latest and max scan times.

A generic method for a PLC that doesn't have an accessible scan time register:

Set up a 'toggle' bit like this:

 Toggle       Toggle

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

'Toggle' will be true every other scan. Use a contact from 'Toggle' to increment a counter with a preset of 500. While the counter is counting, enable a timer. The first scan that the counter is done, the accumulated value in the timer is the elapsed time for 1000 scans. Divide the time in seconds by 1000 to get the average scan time over 1000 scans.

Note that this method is easier if your PLC has a retentive timer instruction. By that I mean a timer with an independent reset function, and not one that resets when the enabling logic goes false.

Some (such as Hitachi s10/2a) have specified time comspumtion for each instruction type. Count the instrutions in each catagory, do the math.

Hello Steve

Maybe I´m wrong, but is´nt the elapsed time for 998 scans ?

Counter = 1 Time = 0
Counter = 2 Time = 2 scan
Counter = 3 Time = 4 scan
Counter = 4 Time = 6 scan
Counter = 5 Time = 8 scan

What about Counter = 0 (zero)?...

Counter = 0 Time = 0
Counter = 1 Time = 2 scan
Counter = 2 Time = 4 scan
Counter = 3 Time = 6 scan
Counter = 4 Time = 8 scan

beerchug

-Eric

Differences in the way the ladder logic is constructed could mean that the preset for the counter might need to be some number other than 500 in order to time 1000 scans.

That's why I chose to count such a large number of scans. If your logic actuallly times 999 or 1001 scans, the average value will only be off by a fraction of a percent. There will be more variation from one scan to another than the variation introduced by the timer's ACC value representing the time for 999, 1000, or 1001 scans.

Remember, this is a procedure for calculating the average scan time. If you need the precise scan time for a specific scan, this is not the method to use. But congratulations to you for thinking deeply enough about the details to come up with that question.