PROFIBUS
CX8030 / CX8031
81
Version: 1.6
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.
9.4
PROFIBUS DPV1
PROFIBUS DPV1 refers primarily to the acyclic read and write telegrams, with which data sets in the slave
are acyclically accessed. A distinction between a Class 1 and a Class 2 master is also made for DPV1. The
difference between acyclic Class 1 (C1) and Class 2 (C2) connections is that the acyclic C1 connection is
established during the DP StartUp phase of cyclic DP operation. Once the slave has reached the WAIT-CFG
state it is possible for acyclic DPV1-C1 read and write telegrams to be sent from the master to the slave,
whereas the C2 connection is established separately, independently of the cyclic DP connection. This is
usually carried out by a second (C2) master so that, for instance, a manufacturer-specific project
configuration and diagnostic tool can access the slave's data.
When two masters are used, however, is must always be borne in mind that these share bus access (a
token is exchanged), so that time relationships are less favourable than in the case of a single master
system.
