plc cycle time

[gulle]

Hello,

I have to make some analysis for uni project. We have a PLC which has a Com. Processor. This is the master on the profibus , and there are two DP slaves. We have a diagnosis software called Amprolyzer. It shows all the telegrams happening on the bus. It seems that Master polls the slaves cyclic.My question, this slave polling interval is the same as plc cycle time or what shows actually this slave polling interval ? where does it come from ?

 

[controlsdude]

PLC cycle time = logic to execute + update outputs + update inputs + housekeeping + asynchronous comms.

Not necessarily in that order.

Asynchronous comms only get updated if their is time within the cycle, predetermined by timeslice. This is what your talking about above.

example: Lets say their is a 20% timeslice for comms in the overhead of these processors. (Usually you have control on the amount of timeslice in your processor)
Your Logic executes = 13mSec, Update Outputs = 1mSec, Update inputs = 1 mSec, therefore your comms has 3mSec to execute during the plc cycle. If it does not complete then it waits for the next timeslice. If you do not have enough timeslice set aside sometimes the comms will not get executed in a timely manner. If you have too much timeslice the plc cannot execute its program and real world outputs will be missed.

I know in AB contrologix if you have an event driven task it takes priority over any continuous plc task, in PLC5 or slc500 this is called an STI interupt, not sure what it is in siemans. If you have a periodic task, this is below event driven but above continuous. Your comms between processors or hosts is the lowest priority on a PLC.

 

[PNachtwey]

The Profibus DP master usually has a dedicated processor to just the Profibus communications. The cyclic updates are usually much faster than the PLC scans. I know the SPC3 chips that are used for the slaves will automatically handle the cyclic data exchanges between the master and slave so the slave CPU is needs to update or read the I/O images when it has time.

The master's job is more difficult because the whole I/O image can't be loaded into the master's Profibus chips so the master must be constantly copying the output image to the Master's profibus chip and reading the slave data and putting that into the input image.

I have seen a Profibus DP slave get updated 3 times a millisecond when there was only one slave. This is much faster than a PLC scan.

I have worked with many PLC Profibus DP masters. You should have indicated which one you use.

 

[controlsdude]

So Profibus DP Master would be like adaptor processor mode in PLC5 over remote IO? This gives you a real world input/output data table? This is not asynchronous comms but part of scanning the inputs and outputs part of the PLC cycle time in a siemens processor?