PLC COMMUNICATIONS IN A PROCESS CONTROL SYSTEM

PLC COMMUNICATIONS IN A
PROCESS CONTROL SYSTEM


Communication has become a major part of any process control automation system. Today PLC
communication is as much for data acquisition as plant control. The first thing the designer often asks is
'how'? But shouldn't he first be asking 'why'?
Before one can consider how to implement a communication system, one has to consider what the final
objective is. What is the importance of the data, what is the amount or volume of data to be transferred and
when or how often is the data required. All of these are factors of why the communication is needed. Once
all this information is known, one is much better placed to decide how this is to be done.
In order to make this final decision however, we first need to look at the options.
Topologies
The topology of a network refers to the 'structure' of the network, ie how all the machines, termed participants
or users, are connected.
The most simple topology is point to point - a single link between two machines (Figure 1a). This generally
works well in very small installations. When the installation grows and communication is required
between all the 'participants' in the system, the configuration becomes very messy, see Figure 1b. This is
commonly known as a mesh topology. As seen here, to connect eight users will require 28 lines therefore
56 interfaces. A ninth user is an additional 8 lines and 16 interfaces. This is clearly very expensive in hardware
and installation.

As sites got bigger, so the bus or local area network (LAN) was developed. The concept here is to have
one communication interface per user, and a single cable (or medium) connecting all users. Physically this
is normally achieved in a tree (Figure 2a) or daisy chain (Figure 2b) structure. The tree topology uses taps
or splitters to separate information from the main bus (trunk) and transmit it down the branches to the
users. The daisy chain topology is very similar but has the main bus cable running into and out of the
communication interfaces of the users. This method requires isolation between the electronics of the interface
and the bus itself to prevent a failure of the interface from affecting the bus.