• The CPU (central processing unit) controls all PLC activity and can further be broken down into processor and memory system components
• The input/output interface system is physically connected to field devices such as switches, sensors, lights, and motors, and provides the interface between the CPU and the field devices (inputs and outputs)

PLCs are used to control machines or processes that are sequential in nature. PLCs can also control continuous processes that use analog I/O.

See PLC's profile in the wiki

• PLC Feel
  – Modular footprint, industrial reliability
  – Wide array of I/O modules and system configurations
• PC Power
  – Large memory and high-speed processing
  – High-level data handling and enterprise connectivity
  – Extensive communications capability, multiple protocols and field networks

PACs are most often used for more complex machine control, advanced process control, data acquisition, equipment monitoring and motion control.