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PLCs are continuously expanding in
capabilities and the architecture or hardware that is part of that is
also expanding with new functionality. There are some hardware
components that are basic to every system. A PLC system generally has
the following components:
- Processor
consists of the CPU, main memory, possibly some communications and
interface capability.
- I/O short for
Inputs and Outputs sometimes included in small quantities on a
block with the processor
- Power Supply
Provides power for the various components of the system -sometimes
built into a block with the processor.
- Rack/Chassis
Modular systems generally need a common backplane to plug into.
Racks are designed to at least accept a processor and multiple I/O
cards. Communication from the processor to the I/O is generally
achieved through connections on the rack and power from the power
supply is generally distributed through the rack.
- Communication
interfaces used to communicate with the programmer, operator
interfaces, data monitoring tools, SCADA systems, remote I/O, etc.
Sometimes built into the processor.
- Programming Tool
used to program and access the processor.
 The processor is generally what
most people refer to when discussing a PLC. Many controls specialists
(including myself) tend to interchange PLC, processor and controller
when referring to the module that receives the program and handles
processing the logic. All of the components together are generally
referred to as a PLC system. The processor is where a PLC program
would reside and is responsible for receiving data from the input
cards and distributing it to the output cards
I/O cards or Input and Output
cards are the connections to the real world for the PLC system. As
discussed there are discrete I/O and analog I/O. Both come in many
different types. Discrete I/O can come in various voltages: 24 Volt
DC or AC; 120 VAC; TTL; sourcing; sinking; and on and on. Analog I/O
has even more variety. There are Voltage inputs/outputs, Current
inputs/outputs and cards that handle both. There are cards that just
grab inputs or outputs and cards that have a mix of inputs and
outputs. There are cards that are specially designed to connect to
Thermocouple and RTD sensors and to compensate for the non-linearity
of the signal. I/O can also be local or remote. Local I/O is located
in the same rack or chassis as the processor, whereas remote I/O can
be located in a separate location that is more convenient to wire
into.
The Rack or Chassis is used to
house the PLC processor, I/O cards, and other cards that are needed
with the system. Some smaller block style PLC systems dont use racks
and even some modular style systems have cards that stack together
with each card having the required connectors to pass processor
communications and power to each subsequent card.
There are many available
communication interfaces ranging from cards designed to communicate
via Ethernet to specialized cards designed to communicate on a
manufacturers proprietary communication link. Generally those cards
are used to allow the programmer to interface with the controller,
another device to interface with the controller, or to allow the
controller to manipulate and read I/O located in a remote location.
Programming interfaces vary from
handheld programmers (yuck) to more modern programs that can be
installed on a laptop to interface and program the PLC.
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