A
programmable logic controller
(PLC) is a standard unit
having no dedicated application.
The unit has to be connected
to the various plant input
and output devices. The
controller then requires
programming with the tasks
that the system must perform,
this is achieved by a
set of software instructions.
The software contains
sequences that are initiated
by inputs from the plant,
which then prompts the
outputs to change the
plant status.
The
programmable controller
can consequently be applied
to an extensive range
of diverse applications.
Once configured as part
of the system , the controller
will be able to handle
tasks as diverse as: |
- Car
paint spraying and body
production
- Food
processing and production
- Nuclear
fuel rod reprocessing
- Steel
reduction and coating
- Airport
baggage and runway lighting
- Cable
manufacture
- Cable
manufacture
- Petro-chemical
processes
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| Programmable
Controller Background |
Before
the computer age began,
industrial automation
existed in the form of
hardwired logic panels.
These panels comprised
of many hundreds or thousands
of relays, cam switches,
and timers. This allowed
increased production capability
compared to hand produced
items. Unfortunately however,
due to the mechanical
nature of these devices
they were prone to failure.
Not only that but with
so many individual units
required to gain the necessary
control, they were very
difficult to maintain.
It
was with these facts in
mind, and due to an ever
widening range of cheaper
silicon devices, whose
development had been driven
by the "space race"
during the 1960's, that
gave rise to the original
concept of the programmable
controller.
The
following background information
introduces the ideas behind
the initial design concept
of the programmable controller
as we know it today.
The
original specification,
issued in 1968, by General
Motors Engineering listed
the following features:
(1)
Easily programmed and
reprogrammed, preferably
without removal of the
hardware, to alter its
sequence of operations.
(2)
Easily maintained and
repaired, preferably using
plug in modules.
(3)
More reliable and smaller
than the relay equivalents
it is designed to replace.
(4)
Competitive on cost with
the relay and solid state
logic then available.
Following
this initial specification
the first controllers
were basic relay replacement
units. As time progressed
control demands became
greater, the instruction
set increased to meet
the extra requirements.
|
| Facilities
Of A Programmable Controller |
Comparing the facilities
of a programmable controller
with a relay system
the following 3 advantages
can be established.
|
| 1.
Reliability |
Although
programmable controllers
may comprise of complex
electronic circuits, the
fact that the components
within the controller
are solid state silicon
devices, makes them inherently
more reliable than their
electro-mechanical relay
counterparts. |
| 2.
Flexibility |
Changing
of plant sequences is
far easier and can be
implemented quickly via
a programming unit without
any hardwired changes
to the control logic. |
| 3
Communications |
Controllers
can communicate with each
other over a variety of
different communications
systems. The controllers
contain data which is
easily transferred and
may therefore be shared
by other equipment. This
facility is almost impossible
for a relay system to
achieve, as "data"
does not exist as it does
in a controller. |
| GEM80
Controllers |
he
GEM80 range of programmable
logic controllers were
originally developed in
the late 1970’s
it’s presence in
the PLC market was quickly
established and it became
the PLC of choice for
major manufacturing industries.
These industries are as
diverse as Metals, Automotive,
Water, Mining, Power Generation,
Food, and Oil and Gas.
Under
the GEC Industrial Controls
banner, the uptake of
GEM80 PLC’s in the
Mid 1980’s was extensive
and as such they boasted
approximately a quarter
of the UK’s PLC
market.
Early
GEM 80 controllers operate
in the same way as the
current models, in all
but a few specific areas.
GEC/CEGELEC/ALSTOM have
a general policy of forward
compatibility within the
software for the GEM 80
range of controllers.
This means that newer
GEM80 controllers will
need very few modifications
to allow older systems
to be replaced by the
current 400 and 500 series
systems. |
| Controller
Architecture And Operation |
The
basic components for all
GEM 80 controllers are
microprocessor modules,
memory modules and a power
supply unit. These components
are connected onto the
central highway, to allow
data communication between
them.
The
central highway consists
of a printed circuit board
(PCB), containing a number
of edge connectors into
which the GEM 80 controller
modules are fitted. This
PCB forms a "backplane"
through which memory and
processor modules can
interchange data. |
|
| Microprocessors |
The
majority of GEM 80 controllers
contain two main microprocessor
modules, the Input/Output
Processor and the Ladder
Processor. Information
is exchanged between these
processors via the memory
modules (RAM). |
| I/O
Processor |
The
I/O highway ribbon that
connects the GEM 80 I/O
subrack to the controller
is connected to this processor.
It is responsible for
accepting the input data
from the input modules
and storing the data in
an assigned memory area.
Once
the program has been executed
by the ladder processor
new output states will
be calculated and placed
in the memory area assigned
to output data. The I/O
Processor then moves the
output data from memory,
transferring it to the
output modules in the
GEM 80 I/O subrack.
The
I/O Processor also handles
serial communication data
transfers from the two
standard GEM 80 serial
ports. The processor transfers
data into the serial link
assigned memory area as
well as transferring data
from memory out via the
serial ports. As well
as controlling the operation
of the standard serial
links, the serial communication
link to the GEM 80 programmer,
connected to the programming
port, is also controlled
by the I/O processor. |
| Ladder
Processor |
The
Ladder Processor's task
within the GEM 80 controller
is to execute the user
program. To do this it
requires the state of
the input data tables,
these are obtained from
the memory having been
placed there by the I/O
Processor. With this information
the processor is able
to execute the program.
As
result of program execution
the updated states of
the outputs are generated.
These are then loaded
into the memory, from
which the I/O Processor
then retrieves and transfers
the data to the output
modules. |
| Memory
Modules |
The
memory modules provide
a large storage area,
that is divided into smaller
sections. These are then
used for their assigned
tasks. The different sections
of information that may
be held within GEM 80
controllers memory include
the following, program
instructions, data tables,
video memory and mailboxes,
used for inter-module
message passing. |
| GEM80
Programmers |
All
GEM80 systems can be programmed
using any one of these
7 programming devices:
1.
The original GEC Portable
programmer circa 1980,
a dedicated black box
programmer. Required a
separate cassette deck
and din plug to save and
load programs.
2.
The GEM80 System Programmer
a common site in the mid
1980's with it's black
front and yellow outer
casing. Included a digital
tape drive for loading
and saving programs. Later
models were sprayed in
CEGELEC's grey colour.
Enhanced programmers also
included a floppy disk
drive for program storage,
although the formatting
is not PC compatible.
3.
In 1986 a third party
company Advanced
Technical Software
Ltd produced the System
Programmer Emulator the
first PC DOS based GEM80
programmer. This more
or less copied the system
programmer menu screens,
apart from the copy compare
option. This is due to
the fact that the PC had
a disk drive rather than
the digital tape drive.
This product was sold
by GEC at Kidsgrove under
licence from ATS.
4.
Mid 1980's GEC projects
at Rugby developed their
own DOS based programmer
GPP (GEC Projects Programmer)
this was used primarily
on GEC projects jobs and
was therefore not sold
as a product, only as
part of a GEC project.
The format is much the
same as the ATS system
programmer emulator, the
system being a basic system
programmer emulator package.
This product was renamed
CPP when the company changed
from GEC to CEGELEC. It
is also often included
as part of the "Over"
suite of programs developed
by Projects at Rugby
5.
In 1990 the renamed CEGELEC
industrial Controls produced
a Windows GEM80 programmer
called GEMESYS 3 that
ran under Windows 3.1
(and now Windows 95,95
etc.). This product had
a very slow uptake with
most customer's preferring
to stick with the DOS
Based ATS programmer which
had now been overhauled
and named the Universal
GEM Programmer.
6.
Advanced
Technical Software
Ltd's GEM80 programming
package The Universal
GEM80 programmer is still
the current favourite
GEM80 programmer. The
company updated the basic
emulator and developed
extra menus, screens and
search options to provide
the quickest means of
programming and monitoring
a GEM80 system. Although
still DOS based if you
currently require a GEM80
programmer this is the
option to choose. The
latest version is currently
version 5.4, use the hyperlink
to access their web site
and gain full details
of this programmer.
7.
Advanced
Technical Software
Ltd's latest Windows based
GEM80 programming package.
Is produced in conjunction
with Alstom. It can read
all formats of previous
programmers including
GPP, emulator and Gemesys.
The AGP is now sold by
both Alstom and ATS and
is sold as the current
programming package for
GEM80 systems.
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