# plc cycle time



## guelle (7 November 2007)

Hello, 
I am sorry for writing in english. my german is poor.

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 Amprolyzer. It shows all the telegrams happening on the bus. It seems that Master polls the slaves cyclic.My question, is this slave polling interval the same as plc cycle time or what shows actually this slave polling interval ?  where does it come from ?  Is it possible to see PLC cycle time by looking at the telegram recordings ? 
In the PLC program DP Send and Receive blocks is under OB1.


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## Rainer Hönle (7 November 2007)

It depends on the PLC. On SIMATIC PLCs the bus cycle is not equal to the PLC cycle time. The bus will normally run as fast as possible, and the PLC too ;-). If you have a short PLC program and a lot of masters and slaves, the bus is slower than the PLC, if you have only a few slaves and a big PLC programm, the bus is faster than the PLC. You can't "guess" the PLC cycle time from the bus cycle time. All above for normal DP data exchange mode.


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## eYe (7 November 2007)

Hello guelle,

if u are using Step7 Software from Siemens u can check the PLC cycle time as follows:

PLC -> Diagnostic/Settings -> Module Information -> Scan Cycle Time (Tab)


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## guelle (7 November 2007)

thank you very much for the answers. 

i would like to ask this one too. there is one dp master. therefore, after every fdl request telegram and after every data exchange sessions, master sends the token to itself. Is a token holding time defined in such mono master systems too? or we can not mention it in mono master systems ?

thank you


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## Rainer Hönle (7 November 2007)

There are a lot of timing parameters there. And the master will search other masters obove its own adress in a specific time frame and so on. I think it is the best, if you look at Profibus-DP/DPV1 from Manfred Popp. It is a must if you are working on protocol level with profibus. It is better than reading IEC 61158 ;-)


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## guelle (8 November 2007)

Yes I have that book. But i really dont understand what "token rotation time" or "target token rotation time" in a mono master dp configuration might mean. And i can not find anything like after fdl request telegram master sends token telegram but in my recordings it seems so. 

Regards,
Guelle


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## Rainer Hönle (8 November 2007)

At any time an additional master can "enter" the bus. And that is why the master must send the token to the next higher master address. And if it is the last master in the ring, to the first master. Even it is only an single master system in the moment. Look at the fdl status telegramms. That is the search for new devices.
If Popp is not enough for you, you must read the IEC. But do you recognized all graphics, specially for timing parameters, the gap factor and so on?

What do you really want to do?


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