DCS is a control system that works with several controllers, each designed to control a separate factory. Each controller can be a programmable logic controller.
Basically, we can say that PLC is a machine control system, while DCS is process control.

PLC is typically used in conjunction with on/off (digital) control and can be extended to analog input/output modules for special tasks. It controls the switch of the only machine. DCS controls many PLCs simultaneously.

PLC can be a subsystem of DCS. PLC dominates the control of high-speed machines, while DCS has an advantage in complex continuous processes.

DCS has a wide communication range, while PLC does not have as many.

DCS is an automated distributed control system that can provide instructions for different parts of a machine. PLC is a digital computer used for automating electromechanical processes.

PLC must be offline to change any parameters. In DCS, it can be completed online.

Programming:
PLC programming is ladder logic because DCS uses the following graphic function blocks SAMA standard

In DCS, Boolean logic diagrams are easier to understand than separate analog logic diagrams and ladder logic diagrams. For PLC, using ladder logic to write analog PID control is challenging.

PLC requires programmers to establish logical conditions and equations, and it does not have standard algorithms. In addition to having a standard algorithm library such as PID, lead lag, etc.,

Fault handling:
The diagnostic status of PLC is relatively limited and may only be indicative. It limits the use of diagnostic bits in logic.

DCS can perform on-site diagnostics on instruments, including HART and fieldbus devices. When a fault occurs, DCS I/O can choose to remain in the last or fail safe position.

PLC does not support working in remote areas, DCS card has complete diagnostic and hot plug functions.