Adam 821x Manual page 106

CPU ADAM 821x
Communication
Master to Master
Master slave
procedure
Data consistency
ADAM 8000 Manual CPU 821x – Rev 1.1
The bus communication protocol provides two procedures for accessing the
bus:
Communications 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 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 when the system is being configured. 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 this may
now communicate with all other stations.
Data is exchanged in a fixed repetitive sequence between the master and
the slaves assigned to this 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, configura-
tion and data transfer phases. Before a DP slave is included into the data
transfer phase, the master verifies during the parameterization and configu-
ration 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 receipt of the slave transfers the recent input data to
the master.
Data is referred to as being consistent, if it has the same logical contents.
Data that belongs together is: the high- and low-byte of an analog value
(word consistency) and the control and the status byte with the respective
parameter word, required to access the registers.
The data consistency during the interaction between the peripherals and
the controller is only guaranteed for 1Byte. That is, the bits of one byte are
acquired together and they are transmitted together. Byte-wise consistency
is sufficient for the processing of digital signals.
Chapter 5 Deployment of the CPU 821xDPM
5-3