CPU ADAM 821x
Chapter 6 Deployment of the CPU 821xDP
ADAM 8000 Manual CPU 821x – Rev 1.1
6-3
The bus communication protocol provides two procedures for accessing the
bus:
Communication with the master is also referred to as token passing
procedure. Token passing guarantees that the station receives access
permission to the bus. This access right to the bus is passed between the
stations in form of a "token". A token is a specific message that is
transferred via the bus.
When a master possesses the token it also has the access right to the bus
and is allowed to communicate with all other active and passive stations.
The token retention time is defined at system configuration. When the token
retention time has expired, the token is passed along to the next master
that acquires the bus access rights with the token so that it may
communicate with all other stations.
Data is exchanged in a fixed repetitive sequence between the master and
the slaves assigned to the respective master. When you configure the
system you define which slaves are assigned to a certain master. You may
also specify which DP slave is included in the cyclic exchange of
application data and which ones are excluded.
The master slave data transfer is divided into parameterization, con-
figuration and data transfer phases. Before a DP slave is included in the
data transfer phase the master verifies during the parameterization and
configuration phase, whether the specified configuration agrees with the
effective configuration. This verification process checks the device type,
format and length as well as the number of inputs and outputs. This
provides you with effective protection against configuration errors.
The master handles application data transfers independently. In addition
you may also send new configuration data to a bus coupler.
If in the status DE „Data Exchange“, the master is sending new basic data
to the slave and the responding telegram of the slave transfers the recent
input data to the master.
Communication
Master to Master
Master slave
procedure
