Scan Time - Ladder,Function Blocks,ST, SFC

[BITS N BYTES 66]

In reviewing some of the newer features of CX Programmer I am confused as to how they affect the overall scan time and performance of the PLC. Function blocks provide a method of encapsulating processes with multiple iterations [Ladder and ST] without having to repeat the same ladder code. ST programming is particularily suited to handling Math and String based applications, however complex ST programs still seems to suck up large amounts of processor memory. The SFC method of programming offers a graphical method of programming that on the surface is very appealing when it comes to program organization and debugging. Trying to understand how each method of programming affects the overall performance of the processor! What [if any] overhead [scan time] does each method of programming add to the PLC scan time. Currently all our existing code is in ladder with multiple tasks. If we were to utilize the SFC programming how would this affect performance of the overall program? NOT AT ALL! A LITTLE! A LOT!

[Guest anonymous]

Hello BITS N BYTES, The short answer is that there should be little or no effect on the overall scan time. Whatever method you choose to program with, what you see in CX-P is compiled down to basically the same binary mnemonic code that the PLC runs. Obviously, depending on the process, one method will be more scan - effective than the other, depending on the number of internal instructions generated, but the difference really shouldn't be that intrusive. A little vague I know, but hope it helps!

[Andy_P 7]

I would agree with that mostly, but it does seem that when using FB's on a CP1L, there is quite a significant hit taken when actually calling them, especially if there are several inputs/outputs to the block. The spec given (in manual W451-E1-03 page 1088) states that when calling an FB, the startup time on a CP1L is 320 microseconds, and each boolean operand is a further 60 microseconds, all before the contents of the block are even executed. Several FB's in a program can soon add to the cycle time significantly. Andy.