PLC COMMUNICATIONS IN A PROCESS CONTROL SYSTEM

PLC COMMUNICATIONS IN A
PROCESS CONTROL SYSTEM


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 the problem 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 the master, 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 shared between 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 participants has the token for an amount of time equal to or less than a pre-defined maximum time.
During this time it may send data to or request data from any other node. When it is finished, or its maximum
time has elapsed, it will 'pass-on' the token to the next node in the list of participants and listen, as
though it were a slave until it receives the token again.
CSMA/CD networks, or what is more commonly known as Ethernet, work on the principal of there being
no absolute control of the network. Each user on the network detects for itself whether it is connected to
the network. This is known as carrier sense. Once it detects a carrier on the network it sees the network as
alive and accesses the network, sending to or requesting data from another user. Clearly as there is more
than one user on the network, more than one user may try to access the network at the same time, multiple
access. Electrically these two messages will corrupt each other, so when this occurs, both users which have
transmitted data will detect data on the network other than what it sent, collision detect. Both users then
stop communicating for a random amount of time and then try again.