BECKHOFF BC3150 - Manual (Page 88)

Safety Instructions
86 Fieldbus Components
Synchronisation with Sync and Freeze
The Sync and Freeze commands in the GlobalControl request telegram (broadcast telegram) allow the master to
synchronise the activation of the outputs (Sync) or the reading of the inputs (Freeze) in a number of slaves. When the
Sync command is used, the slaves are first switched into Sync mode (a process that is acknowledged in the
diagnostic data). The I/O data is then exchanged sequentially with the slaves in the DataExchange telegram.
Transmitting the Sync command in the GlobalControl telegram then has the effect of causing the slaves to generate
the most recently received outputs. In Freeze operation a Freeze command is first sent in the GlobalControl
telegram, in response to which all the slaves latch their inputs. These are then fetched sequentially by the master in
the DataExchange telegram.
States in the master
The master distinguishes between the CLEAR state (all outputs are set to the Fail_Safe value) and the OPERATE
state (all outputs have the process value). The Master is usually switched into the CLEAR mode when, for instance,
the PLC enters STOP.
Class 1 and Class 2 DP Masters
The Class 1 master refers to the controller that carries out cyclic I/O data exchange with the slaves, while a Class 2
master is a B&B device that generally only has read access to the slaves' I/O data.