DIFFERENCE BETWEEN PLC AND DDC (Distributed Digital Control system)?

PLC is defined as Programmable Logic Controller. It is a hardware, Includes processor,I/P & O/P Modules, Counters, Function Blocks,Timers,,, etc. The I/Os are either Analogue or Digital or both. PLC can be configured to suit the application and to programmed programmed in a logic manner by using one of the programing language such as Statement List, Ladder Diagram,, etc Interaction in real time between inputs and the resultant of the outputs through the program logic - PID - gives the entire Control System.While the Digital Control System I believe it is Software/ System that uses only Digital Signals for control and PLC/PC/Server/Central Unit may constitutes an Integral part of this system.