PLCs can physically be grouped into 3 categories.
Backplane Based: Consist of a CPU, a Backplane (also called Rack) and IO Cards that slide into the Backplane.
Bricks (Non-Backplane Based): ¬†Have CPU and IO built into them. ¬†Often, you can connect additional IO to their sides.
Programmable relays: Low end modules with built in screen and buttons used for programming.
They connect a CPU to IO and¬†are available in different sizes to¬†accommodate IO requirements.
Backplanes add expense to a system, while often adding little additional functionality. ¬†In some cases they allow for components to be hot swap-able. ¬†Because of the considerable expense, many PLC manufacturers are moving away from Backplane Based systems in favor of Brick systems.
In Brick based system, like the one to the right, each brick has connectors on it’s left and right that allows CPUs and IO to connect.
In this example: ¬†leftmost is a power supply, then a CPU with built in IO, followed by 6 IO units.