PROFIBUS DP communication
BK3xx0
54
Version: 4.3.0
Fast FreeRun
To avoid the jitter resulting from acyclic communication or events and to achieve fast K-bus update times,
the
Fast FreeRun
mode can be activated. The K-bus cycle is called by a higher priority task, controlled by a
timer. At the end of the K-bus cycle the low-priority tasks (DPV1, KS2000 interface, etc.) are assigned
computing time corresponding to 12.5 % of the preceding K-bus cycle duration, before the next K-bus cycle
is started. In fast FreeRun mode therefore the inputs and outputs are up-to-date, but are not synchronized to
the DP cycle:
Fig. 40: K-bus
mode fast FreeRun
Synchronous mode
As explained in the table above, there are three different synchronous modes.
Standard synchronous mode
In standard synchronous mode the K-bus cycle is always started immediately following reception of the
Data_Exchange telegram from the DP master. The outputs are therefore generated as quickly as possible,
while the input cycles are always one DP cycle old.
Fig. 41: K-bus - standard synchronous mode
It is important here to ensure that the duration of the K-bus cycle plus approx. 20 % (to allow for the lower
priority processes on the coupler) is shorter than the DP cycle time (which, under TwinCAT, means the cycle
time of the associated task).
Synchronous mode with optimized input update (one cycle)
In optimized input update, the start of the K-bus cycle can be delayed following reception of the
Data_Exchange telegram, so that the inputs are more up-to-date than they are in standard synchronous
mode, whereas generation of the outputs is more severely delayed. It is important here to ensure that the
duration of the K-bus cycle, plus the delay time, plus approx. 20 % (to allow for the lower priority processes
on the coupler) is shorter than the DP cycle time (which, under TwinCAT, means the cycle time of the
associated task).
