Traditionally PLC's were control systems that had a single controller and local IO.  The IO was mainly digital and allowed for start/stop control of machinery based on binary logic. 

The PLC's roots came from the evolution of microprocessors and it replaced binary logic being done by electromechanical relays and timers.  Consequently, the speed of the PLC's had to be fast.

The first PLC was invented in 1968 by Richard E. Morley.  He went on to start the company Modicon and you can still buy Modicon PLC's today through Schneider Electric.

The first PLC's were programmed using 'ladder logic' which mimicked electrical schematic drawings; where the power in a circuit flows from left to right or hot to neutral.

At first, besides the program used to program the controller, there was no way to look at the logic that the controller was running.  So, along came software to not only view the logic happening in the controller but to also allow an operator to input signals.  This essentially replaced physical pushbuttons and switches in circuits.  This software became known as HMI or Human Machine Interface.

HMI software does not traditionally come with a PLC like it does with a DCS

Here are some links to good PLC resources on the web:


PLC Professor

Contact and Coil

PLC Manuals

PLC Software Downloads

AutoCAD Drawings