Control Methods: PLC vs Relay Logic

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Today’s PLC’s (programmable logic controllers) are cheaper and more flexible than ever before.

Still, some OEM’s and system builders prefer relay logic for their electrical controls – and in some cases, this may be justified.

Let’s take a look at some of the advantages and disadvantages of PLC controls versus relay logic.

Flexibility and ease of service

Changing functionality or adding features to a relay logic board will generally always require wiring changes. A PLC system, on the other hand, can be easily reprogrammed without needing to physically change any wiring at all.

Programming a PLC however, is a specialised task that many OEM’s aren’t comfortable with. But today’s remote access solutions can remove that hassle by allowing a programmer to securely update and debug the program remotely over the Internet.

In terms of fault-finding, it’s about a tie – tracing wires around a relay logic board can be just as maddening as troubleshooting a PLC panel without the programming software.


For small and simple control systems, relay logic can be ideal – it’s low cost and easy to service.

But as process complexity grows, a point is quickly reached where both cost and wiring complexity of relay logic overtakes a PLC solution.

Add a few analog instruments and a variable speed drive (VSD) or two, and a relay logic system rapidly becomes not only more expensive but technically impractical.

Space advantages

A small PLC can be roughly the size of a house brick. On the other hand, a row of 10-20 relays can be twice the length, and require twice the amount of wiring (i.e. larger cable ducts).

Where space is at a premium, a PLC is often a better option.

Communication and interoperability

It’s becoming more far more common that OEM’s packages are required to be integrated into existing onsite control and SCADA systems. Relay logic panels fall back on simple hard-wired signals – one pair of wires per output – for this.

PLC control systems on the other hand, can be designed to easily integrate and communicate with other systems using standard protocols such as Ethernet/IP or Profibus, over a single Ethernet or serial cable.

To find out more about how you could improve your control systems with PLC’s, contact our engineers.