processing speed, PLC

Hey guys, kindly help me.
formula for processing speed for PLC and how can you increase its speed? thank you.

The PLC manufacturers sometimes print tables of execution speeds for each function. To try to calculate scan time from these is a cumbersome process and not particularly accurate. The only was to increase processor speed is to buy a faster processor (if the PLC manufacturer offers one).

To reduce scan time, you have to look closely at the application for opportunities to save time. Can some functions be placed in subroutines and only called when necessary rather than every scan? Can you perform mathematical functions with integers instead of floating-point numbers? Integer math is faster. Can you get away with reducing the frequency of communications with external devices?

steve,
i appreciate your response. thanx a lot!

It also depends on the PLC manufacturers scheme for scanning the logic and the programming techniques you employ. For instance I believe Allen Bradley scans left to right for each rung of logic but will stop scanning that rung once an open contact is reached and start scanning the next rung. For that you would want to put your contact that is most likely to be open as the first element in that rung. Modicon on the other hand scans up and down each network and checks the state of each contact, but functions aren't solved unless power flows through all contacts leading to it. You can also use the 'end of scan' or 'skips' if your scan rate is critical. The PLC manufactures can give you the nominal scan rate per k of memory used but your programming techniques will also have an effect.