Scan Rate

Hi forum ,
I am Parag Uike, working on Fail Safe controllers(Safety System). one of our customer has requested me to have system scan rate 1ms per 1000 instructions.
But we are using Function Block language Programming.
the above feature will be considered when Ladder Language is used.
Also the scan time of Processor will also depends upon I/O's used , Configuration, Length of Program etc.

Query is how we will come to know the scan rate under such condition for our system as the Programming language is diffrent than specified.

Please let me know if you have any Query ?

Thanx in advance.

Regards,
Parag Uike
The Game of PLC

Some controller manufacturers do advertise "scan time per N instructions" parameter but I have always thought that not all instructions are created equal. Therefore, I used to think of this value as an estimated time to perform N logical operations with bits - i.e. simple "ands" and "ors". If this is what your customer has in mind, then you have something to work on - it does not matter what you use to program A & B command - as long as a sequence of 1000 of such commands is reliably executed within 1 ms, you are OK. This also tekes care of the "program length" factor: one should assume that 5000 simple logic ops should take 5 ms to run, and so on.

Needless to say, a floating-point multiplication will most likely take much longer to execute. And, as you mentioned before, on top of the logic execution time, there always be some "housekeeping" stuff: reading inputs, updationg outputs, switching tasks, backplane and remote communications and so on.

Maybe you could discuss with your customer what are the scan time requirements for his application - if he tells you that no matter what the thing has to be able to run 2 ms scans, then you know how to cut this time in slices and how to make everything work.

Don't know what plc you are using but there are a few things to consider.

1. you could work out the scan rate of whole program by checking the instruction times etc.
2. Dependant on the code i.e. if you are calling functions are they called all the time or only when required (this becomes more awkward as calling a function will increase scan time only during the call.
3. The best way to find out is many plc manufacturers allow you to monitor the scan time or it will be put into a special register, some even tell you the min, current & longest scan time.
Some plc's allow you to call blocks on different scan priority or you could create your own i.e. process some blocks that do not require fast processing every 100ms but phase them.
also if you have loops in a function you will have to allow for the number of times the instructions are processed in the loop.

also jump instructions typically take longer on some processors in Rockwell one type of nested loop is slightly faster than another type but I cannot recall which one was the faster one. if I remember correct the differents was extremely slight but I've been unable to find a listing showing that but recall reading it somewhere

You can put an alarm if scantime exceedes a certain time, most brands have this function.