PROFIBUS Manual
The PROFIBUS-DP cycle time determines the possible reaction times of a reactive system. In practice, this cycle time features twice in the system reaction time and influences system reaction time by a percentage of 2 to 20, i.e. as a rule, the proportion of bus cycle time in a system's overall reaction time is relatively small.
In a PROFIBUS-DP network it is possible to differentiate between three different cycle times:
1.The class 1 DP master (controller) processes its control program in a specific cycle time. Before each cycle, input data is read and after each cycle, new output data is available. This is called the master application cycle.
2.The DP slave takes the output data, processes its device functions and makes input data available within a defined cycle time. This is called the slave application cycle.
3.PROFIBUS copies output data cyclically from DP master to DP slave and input data from DP slave to DP master. This is called here the bus cycle.
With PROFIBUS DP it is now possible to differentiate between three operating modes:
Free running bus cycle
The cycles of the master, the PROFIBUS and the DP slave are not coupled to each other. In order for such a sytem to function, the cycle time of the DP slave should be shorter than the cycle time of the PROFIBUS which, in turn, should be shorter than the cycle time of the controller. The cycle time of the PROFIBUS-DP should therefore lie between the two other cycle times. Here we provide a simple estimation formula for the bus cycle time and the calculation for a single master system. Project development tools automatically calculate the cycle times for any planned system.
Equidistant bus cycle
The duration of the bus cycle is fixed in the project and observed by the DP master to the microsecond. This is only possible with an ASIC ASPC2 from version E (approx. 1999).
Isochronous cycle
In addition to the fixed bus cycle, the slave application cycle and - in a qualified sense - the master application cycle are synchronized with the bus cycle.
