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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:

1. Processor – consists of the CPU, main memory, possibly some communications and interface capability.
2. I/O – short for Inputs and Outputs – sometimes included in small quantities on a block with the processor
3. Power Supply – Provides power for the various components of the system -sometimes built into a block with the processor.
4. 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.
5. Communication interfaces – used to communicate with the programmer, operator interfaces, data monitoring tools, SCADA systems, remote I/O, etc.  Sometimes built into the processor.
6. 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 don’t 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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