Application instructions can be used to send control commands for SYNC and FREEZE between the cyclic data.
A SYNC instruction leads all slaves activated with a group address to cache output data and only forward it to the physical outputs when a SYNC instruction occurs. A SYNC instruction can be cancelled with an UNSYNC instruction.
A FREEZE instruction causes all slaves activated with a group address to cache physical input values. This cache is only loaded again with another FREEZE instruction. This operating mode can be switched off again with UNFREEZE.
The following diagrams show the example of output synchronization:
Synchronization of outputs
The aim of these synchronization instructions is the simultaneous output of data to the process by different DP slaves (field devices). With PROFIBUS, data is always transferred sequentially from the DP master to DP slaves. In normal operation, every DP slave has a chance to process this data immediately. The output data or setpoints are therefore processed at different times, which can lead to inconsistencies in a distributed application. In synchronized mode, data is still transferred sequentially but only adopted by an application with the SYNC instruction.
Synchronization of the outputs of several slaves
It is only when the isochronous cycle is used that there is synchronization not only of data adoption, but also of the application cycle in the DP slave.
The FREEZE instruction functions in an analogous way to SYNC, except instead of synchronizing the outputs, the FREEZE instruction freezes and then fetches the inputs.
These SYNC and FREEZE instructions are optional. The GSD file therefore specifies whether these functions are also supported by the DP slave concerned:
Freeze_Mode_supp = 1
Sync_Mode_supp = 1
PROFIBUS modules (ASICs) currently available on the market support these FREEZE and SYNC instructions with hardware. These functions are therefore available with most DP slaves.
The DP master must use a Set Parameter telegram to define the parameters for any planned use of FREEZE and SYNC with the DP slave.