Viewpoint: DCS and PLC – spot the difference

The best place to start in debating the differences between a distributed control system (DCS) and a programmable logic controller (PLC) is to think like this: in the majority of cases, a PLC controls a machine and a DCS controls a plant.

Nowadays, a DCS is used when the value of the product is high, the production is continuous and failures in the system result in damage to process equipment.

This is because a DCS normally has built-in redundancy, ensuring a higher level of system insurance. All upgrades are made online while the system runs continuously.

Diagnostics in a PLC system will alert an engineer when something is broken. Whereas in a DCS, asset management software provides alerts of what might break before it does

Conversely, PLCs are often used when the value of the product is relatively low and production needs to be flexible. Systems can be shut down for maintenance, troubleshooting or upgrades without damage to equipment or significant downtime costs.

Diagnostics in a PLC system will alert an engineer when something is broken. Whereas in a DCS, asset management software provides alerts of what might break before it does, so a fitting substitution can be made.

This kind of predictive analysis is particularly important for obsolete part replacement and avoiding costly downtime.

Meanwhile, PLCs are highly customisable. They have standard libraries and routines built in, but also have the capability to be specially programmed using custom code from scratch.

Modern industrial applications are demanding the reliability of a DCS system but the flexibility of a PLC

On the other hand, engineers expect a DCS to offer an out-of-the-box control system with features such as sophisticated alarms and logic from pre-existing function blocks.

The highest priority of a DCS is to deliver reliability and availability. In fact, their designs often trade high levels of functionality for repeatability and dependability.

Another key difference between a DCS and a PLC is the speed of logic execution. PLCs are designed to meet the needs of applications that require scan rates of 10 milliseconds or less. This allows them to accurately control motors and drives running at high speeds.

However, there is not the same kind of pressure on a DCS to be as quick because they control systems rather than individual devices. A DCS has regulatory control loops that generally scan in the 100 to 500 millisecond range.

For all their differences, though, PLCs and DCSs are becoming more alike. The DCS was originally developed for analogue control.

However, the latest generations of PLCs are increasingly capable of delivering simple to complex proportional integral derivative (PID) control.

Reliable control

In addition, today’s DCS hardware is not as expensive as it was a couple of decades ago and is less difficult to implement. It is also no longer as cumbersome either – modern DCS hardware resembles a PLC in size.

This is why a DCS can now be used in smaller applications, whereby it is not spread across the whole plant, but rather a complex subsection that needs reliable control.

For example, take the server halls of data centres: these types of control systems are often seen as hybrids of both DCS and PLC.

Modern industrial applications are demanding the reliability of a DCS system but the flexibility of a PLC. This change has brought about a certain level of technological convergence between PLCs and DCSs that defies traditional ideas of the two.

In essence, some industrial plants are now employing more than one approach to ensure critical systems are well managed.