Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Fieldbus and SCADA communications systems 351
It is very easy to connect Profibus FMS, DP and PA versions together on the same
system, as the main difference between them is only the physical layer. This allows
deployment of lower cost devices in most of the plant (FMS) and fast devices (DP) in
parts of the plant that need the speed. Intrinsically safe devices (PA) are used in the areas
of the plant where intrinsic safety is required.
13.2.3 The data link layer
The data link layer is defined by Profibus as the fieldbus data link layer (FDL).
The medium access control (MAC) part of the FDL defines when a station may transmit
data. The MAC ensures that only one station transmits data at any given time.
Profibus communication is termed hybrid medium access. It uses two methods of
operation, namely token passing and master/slave operation.
The token passing method ensures the assignment of the bus access right within a precisely
defined time interval. The token is circulated between all masters with a maximum
(and configurable) token rotation time. Token passing is especially useful for communication
between complex automation masters that require equal rights on the bus. The
token is passed in a defined sequence (in order of increasing addresses).
The master/slave method allows the master that currently has the token to communicate
with the associated slave devices. The master can then read from or write data to the slave
devices.
A typical configuration is shown below.
Figure 13.2
Typical architecture of a Profibus system
During the bus system startup phase, the task of the active station’s media access control
layer (MAC) is to detect logical assignment and establish the token ring. The MAC also
handles adding or deleting stations (which have become inactive), deleting multiple nodes
with the same address or deleting multiple or lost tokens.