Name: Beckhoff BK3120
Brand: Beckhoff
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PROFIBUS DP communication

BK3xx0 53Version: 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

bin

Slow FreeRun

bin

Fast FreeRun

bin

Synchronous

bin

Synchronous with optimized input update,

one cycle

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)

Beckhoff BK3120 User Manual

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