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PROGRAMMABLE LOGIC CONTROLLERS
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UNIT 21799P PROGRAMMABLE LOGIC CONTROLLERS
The material is quite suitable for anyone wishing to study this interesting subject and
does not require a lot of mathematical knowledge. Obviously, access to suitable
computer software such as Pneusim Pro or Bytronics simulation software will be
a great help. You do need to have a reasonable knowledge of computer technology
and a good background understanding of industrial processes.
PURPOSE AND ORIGINS
The PLC has its origins in the motor manufacturing industries. Manufacturing processes were partially
automated by the use of rigid control circuits, electrical, hydraulic and pneumatic. It was found that when
ever a change had to be made, the system had to be rewired or reconfigured. The use of wiring boards on
which connections could be changed by unplugging them and changing them around followed. With the
development of micro-computers it was realised that if the computer could switch things on or off and
respond to a pattern of inputs, then the changes could be made by simply reprogramming the computer and
so the PLC was born.
There are still many applications of automated systems with permanent connections to perform a single
control action. Often the system uses logic components to produce the correct action (electronic and
pneumatic). The PLC mimics this process by performing the logical operations with the programme rather
than with real components. In this way cost savings are produced as fewer components are needed and
more flexibility is introduced as programmes can be changed more easily than reconfiguring a hard ware
system. Programming is covered in Outcomes 2 and 3.
ARCHITECTURE AND TERMINOLOGY
The PLC activates its output terminals in order to switch things on or off. The decision to activate an output
is based on the status of the system’s feed-back sensors and these are connected to the input terminals of
the PLC. The decisions are based on logic programmes stored in the RAM and/or ROM memory. They
have a central processing unit (CPU), data bus and address bus. A typical unitary PLC is shown below.
THE CPU
The next diagram shows the internal structure of the Central Processing Unit in its simplest form. It usually
contains (but sometimes it is external and separate) an Arithmetic Logic Unit. This is the part that performs
operations such as adding, subtracting, multiplying, dividing and comparing. The Buffers act as switches
that isolate the lines on either side if required. A, B and C are latches that passes the data from one side to
the other when told to do so.
Digital data is passed around through busses. The busses were originally 4 parallel lines but as technology
progressed this become 8, then 16 and now 32. Digital numbers and how they are put onto busses is
explained in outcome 2.
The busses are connected to memory chips. In a memory chip, digital numbers are stored in locations. The
number is the data and the location is the address. Data can be sent to or brought from memory locations
by either writing it or reading. The lines labelled R and W are signal lines that makes the CPU read or
write.
TESTING
The PLC has certain diagnostic, monitoring and testing facilities within the software. Light Emitting Diodes
(LED) shows the status of the inputs and outputs. It is also possible to fix a bank of switches to the input
side and test a programme by setting the switches to a certain state and seeing if the appropriate output
action is taken. The most advanced method connects the PLC to a computer with appropriate software
and runs a complete simulation of the system being controlled showing the status of everything.
PROGRAMMING METHODS
The P.L.C. is programmed with logical commands. This may be done through a programming panel or by
connection to a computer. There are several types of programming panels varying in complexity from a
simple key pad to a full blown hand held computer with graphics screen. Computers are able to run
programming software with graphics, simulators, diagnostics and monitoring. This could be a laptop carried
to the site or a main computer some distance away. Often the programme is developed and tested on the
computer and the programme is transferred to the PLC. This could be by a communication link, by a
magnetic tape, compact dusc or more likely with an EEPROM. The EEPROM is a memory chip to which
the programme is written. The chip is then taken to the PLC and simply plugged in. The memory cannot be
overwritten but it can be erased by exposure to UV light and reused.
TWISTED PAIR
When information is sent along two wires, often a twisted pair is used. An example of this is the ordinary
copper wire that connects your landline telephone to the network. To reduce the chances of picking up
stray electro-magnetic signals from other lines running along side it, the two insulated copper wires are
twisted around each other. More than one twisted pair may be placed inside an outer insulated layer and
sometimes the cable is screened or shielded by a grounded outer layer. Twisted pairs come with each pair
uniquely colour coded when it is packaged in multiple pairs. Different uses such as analogue, digital, and
Ethernet require different pair multiples. Although the twisted pair is often associated with home use, a
higher grade of twisted pair is often used for horizontal wiring in LAN installations because it is less
expensive than coaxial cable.
COAXIAL CABLES
Coaxial cable is the kind of cable used to connect a TV set to the aerial. It is also used to connect
telephone exchanges to the telephone poles near to users. It is also widely used to connect computers and
PLC’s with systems such as Ethernet and other types of local area network (LAN).
The cable has an inner conductor surrounded by a concentric conductor (coaxial with it) made from
copper mesh and separated by a layer of insulation. The outer layer is usually grounded. They can carry
information for a great distance.