Beckhoff BK3 0 Series, Documentation

Page: 53 / 90

PROFIBUS DP communication
BK3xx0 53 Version: 4.3.0
5.1.2 K-bus Cycle
The K-bus cycle can be set to run freely ( FreeRun mode [ } 53] ) or synchronously ( synchronous mode
[ } 54] ) with respect to the DP cycle. The K-bus cycle for the DP coupler consists of the following parts:
Fig. 38: Configuration of the K-bus cycle for the DP couplers
The K-bus cycle time can be calculated with a precision of around 10 % using the following formula (4-
channel terminals or terminals with more than 6 bytes data (exception: ASI terminal KL6201: more than 12
bytes of data) require two or more K-bus cycles).
Tcyc (in µs) = number of K-Bus cycles x (600 + number of digital channels x 2.5 + number of analog
input channels x 32 + number of analog output channels x 42)
The K-bus cycle time can be read via DPV1 [ } 60] . If TwinCAT is used, this is possible on the "Beckhoff" tab
of the DP coupler in the System Manager.
K-bus modes
The K-bus mode (the type of synchronisation between the K-bus cycles and the DP cycle) is set via the
UserPrmData [
}
35] :
Byte 9, bit 4 Byte 9, bit 6 Byte 12, bit 0 Byte 12, bit 1 K-bus mode
0
bin
1
bin
0
bin
0
bin
Slow FreeRun
1
bin
1
bin
0
bin
0
bin
Fast FreeRun
0
bin
0
bin
0
bin
0
bin
Synchronous
0
bin
0
bin
1
bin
0
bin
Synchronous with optimized input update,
one cycle
0
bin
0
bin
0
bin
1
bin
Synchronous with optimized input update,
two cycles
FreeRun mode
Slow FreeRun (default setting)
In the FreeRun mode there is no synchronisation between the K-bus cycle and the DP cycle. It is a
characteristic feature of the Slow FreeRun mode that the K-bus cycle is called from the main task. Acyclic
communication or events result in heavy jitter in the K-bus cycle (KS2000, DPV1, terminal diagnosis, etc.),
because all of these functions are also called from the main task.
Fig. 39: K-bus - Slow FreeRun mode (default setting)