El Progamador Logico Controlable

PLC COMMUNICATIONS IN A
PROCESS CONTROL SYSTEM
by GR MacKenzie, AEG
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 toconsider 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 atthe 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.
Figure 1(a) Point-to-point topology and (b) mesh topology.
As sites gotbigger, 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 branchesto 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.
Contact IDC
http://www.idc-online.com
© Electricity + Control
1(a) Point-to-point topology1(b) mesh topology.
Figure 2(a) Tree Topology, (b) Daisy Chain Topology and (c) Star Topology.
It is also necessary in some applications usually restricted by geographical layout to configure a
network in a star topology (Figure 2c). This works a little like a tree network with a very short trunk and
long branches.
After the development of bus type communication there immediately arose theproblem of control of the
bus. In point to point communication the control is a master-slave type control. This works well, as if either
user fails, no communication can take place anyway. In a bus configuration, however, this is not always
the case. For installations where the 'master' device is always in control and the slaves are 'dumb' devices
which need only to communicate with themaster, this topology is sufficient. In a distributed control
environment however, where all users need access to the bus, the failure of a master station and subsequent
loss of the communications network is not acceptable.
This leads to the concept of 'peer to peer' communication. In this format, no single user has control, but a
protocol is developed to allow control of the bus to be sharedbetween all participants. In PLC
communications this normally takes the form of a 'token passing' network or 'carrier sense multiple
access/collision detect' (CSMA/CD) network.
Token passing means that all participants on the network have a list of all the participants on the network
including itself, usually in the form of an address or node number in ascending order. At any time, one of
the...