K-bus modes, Freerun mode, K-bus cycle – BECKHOFF BK3xx0 User Manual

Page 51: Slow freerun (default setting)

Notes on the Documentation

Fieldbus Components

K-Bus Cycle

The K-Bus cycle can be set to run freely (FreeRun mode) or synchronously (synchronous mode) with respect to the
DP cycle. The K-Bus cycle for the DP coupler consists of the following parts:

The K-Bus cycle time can be calculated to a precision of approximately 10% by means of the following formula (4-
channel terminals or terminals with more than 6 bytes of data (exception: ASI terminal KL6201, which has more than
12 bytes of data) require two or more K-Bus cycles. The number of K-Bus cycles is in register ?? of table 90):

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. 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:

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 optimised input update, one
cycle

bin

Synchronous with optimised 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.