Modulebus scan setting question, see Question Details
Looking for simple advice for setting Modulebus scan time.
IF fastest Tast time (code) is 1s what is the slowest Scan Cycle time exceptable?
Assuming AI880, DI880, And DO880 timeout is set to 1024.
And AO854, DO840, and AI845 exist under ModuleBus object (no timeout config?).
It tells you right there in your screenshot what the maximum scan rate setting is.
You should set the IO scan time to half or less of your program scan time. You should set a short filter time constant of around quarter to half your program scan time on analogue IO.
If you have issues with having enough CPU time to scan all the IO channels, move your IO modules to profibus. Your IO rate can be slower (or can still be very fast) AND there is *zero* CPU load required to update the IO channels.
If I get the essense of your question, the minimum modulebus scan cycle time is calculated by a formula below, pardon me if the formula has been revised. I have recollected from my old estimations. I will give you example for SIL2 application.
ScanCycleTime (SIL2)= N1 * 0.5 + N2 * 1.3 [ms]
N1= amount of Non-safety IO
N2 = Number of AI880A, DI880 and DO880 (for redundant DO880 both shall be
accounted), these are safety IOs
From this above calculation you can find out what is the minimum scan time required for your setup if used in SIL2 application.
For latest formula, please refer to latest user manual 3BSE041434-xxx System Guide Technical Data and Configuration