PLC and DCS

The programmable logic controller

A PLC is an industrial computer built for discrete logic, sequencing and machine control. Its architecture is a processor, input and output modules, a power supply, and a communications interface, usually in a modular rack.

It executes a scan cycle, continuously: read all inputs into an image table, execute the program logic from top to bottom, write all outputs from the output image table, then perform housekeeping and communications. The cycle repeats in milliseconds.

Two consequences follow from the scan architecture and both matter in practice. First, an output does not change the instant the logic decides it should — it changes at the end of the scan, so scan time is a real delay that must be considered for fast processes and for safety response times. Second, inputs are sampled once per scan, so a pulse shorter than the scan time may be missed entirely unless a high-speed input or interrupt is used.

The distributed control system

A DCS grew from process control rather than machine control. Its emphasis is continuous regulatory control of many loops, an integrated operator interface, a single plant-wide database, integrated historian and alarm management, and redundancy throughout.

Its characteristic features are a single engineering environment covering controllers, graphics, alarms and history together, a consistent tag database used everywhere, controller and network redundancy as standard, and online modification without stopping the process.

Convergence

The historical distinction has largely dissolved. Modern PLCs handle PID loops well and come with capable SCADA packages; modern DCS platforms handle sequences and discrete logic. The practical selection criteria today are:

• Number of loops and degree of integration — a DCS earns its cost on a large, integrated, continuous process.
• Discrete and machine control content — PLCs remain strong here, and packaged equipment arrives with them.
• Redundancy and availability requirements.
• Lifecycle support and obsolescence — a control system outlives several generations of computing hardware, and support availability over twenty years matters more than features at purchase.
• Existing site standard — a site running one platform gains far more from consistency in spares, training and engineering than from a marginally better product.

Input and output

• Analogue input — 4-20 mA, RTD, thermocouple, with resolution and isolation as key specifications.
• Analogue output — 4-20 mA to valves and drives.
• Digital input and output — switches, solenoids, contactors, with correct voltage and isolation.
• Fail-safe behaviour on module failure and on loss of communication must be configured deliberately for every output, not left at default.
• Spare capacity — a system with no spare I/O forces an expensive expansion for a small modification. Twenty per cent spare at design is normal and repays itself.