Technology | PLC

Soft PLCs vs Hard PLCs – Which is the future?

Words by Alex Matheson
Soft PLCs vs Hard PLCs – Which is the future?
A mocked up graph with notes about soft and hard PLCs

Programmable logic controllers (PLCs) have traditionally been intrinsically hardware based. Born out of the automotive industry in the late 1960s, they were as much a part of production lines as any other piece of physical equipment.

In recent times, relatively speaking, software based PLCs have emerged prompting the question; which is better placed to enable the future of manufacturing, a hard PLC or a soft PLC?

What is a programmable logic controller?

A PLC is a computer specifically designed to control different manufacturing processes such as assembly lines, production robots, automated equipment, and more.

They tend to be built for purpose, and built to withstand industrial environmental conditions including extended temperature ranges, large amounts of electrical noise, and with resistance to impacts and vibration.

The first PLC was created for the automotive industry in 1968 and helped further automation in manufacturing. They largely replaced relay logic systems which struggled automating outside of dedicated closed loop environments. They also struggled with reliability as if even one wire were out of place, the whole production process could grind to a halt.

PLCs, on the other hand, were more modular, reliable, and compact. They provided the opportunity for scalability, being extensible with additional input/output (IO) modules, and tolerated more extreme industrial environments better.

How are PLCs Programmed?

PLCs are commonly programmed using ladder logic. Ladder logic was developed initially as a means to document the design of relay racks; electrical relays wired in particular configurations.

It formed the basis for a programming language that closely reflected the actual workings of electrical relays in physical equipment like PLCs. Because it’s a computing language that so closely resembles the physical workings of these devices, it comes with a number of benefits and drawbacks.

It enables maintenance engineers to troubleshoot issues more easily by requiring less technical knowledge. Engineers don’t need to understand the intricacies of specialised programming languages, for example.

However, it limits what can be done at the CPU end of things. Ladder Logic is bound by the physical constraints of the system it’s describing, minimising the amount of adaptability the system is capable of.

What’s the difference between a soft PLC and a hard PLC?

A soft PLC is defined as software that’s capable of carrying out the functions of the CPU component in a PLC, but can exist on hardware alongside other pieces of software. A hard PLC is a specific piece of hardware that acts solely as a PLC, with its CPU functions happening on a dedicated unit.

The main thing to focus on is where the CPU is, because PLCs are such varied things that defining either type by their inputs (sensors) or outputs (actuators) would very quickly become a jumble. So, if a CPU exists as part of the PLC, that’s a hard PLC. If it exists on a separate computer, that’s a soft PLC.

Why stick with hard PLCs?

Every manufacturing process is complex. Even the most straightforward of processes is going to require hundreds or thousands of moving parts, all needing to operate in concert to create products at scale.

Having a dedicated PLC has obvious benefits here. You have one system to program, maintain, and understand. Troubleshooting is limited to a finite space and upgrades are only dependent on the system itself, not any other software that’s been loaded alongside the PLC.

They also benefit from specialised knowledge. The technicians who support hard PLCs have dedicated experience and these systems have a weight of tradition behind them that makes them somewhat more reasonable to maintain.

Why move to soft PLCs?

Given the preceding paragraphs, you’d be forgiven for thinking a move to soft PLCs is a bad one. But, as with every technological advance, there’s two sides to this story. Soft PLCs can do things that hard PLCs can’t and some of the benefits of hard PLCs can also create problems that soft PLCs can solve.

As we move into the world of Industry 4.0, we understand that flexibility is a key component of progress. The interoperability of machinery begs for concepts like soft PLCs, where the potential setups for any manufacturing operation are infinitely variable.

In the factory of the future, CPU functionality might exist in the cloud, enabling multiple manufacturing centres at once. That puts the PLC responsibility firmly into the world of IT, AI, and cloud computing.

Something important to remember is that in soft PLCs, the CPUs exist separately from the rest of the PLC. With that separation comes some really interesting advantages.

Networked systems

Hard PLCs might not always be hardwired, but they will always be limited in how well they can network. As dedicated pieces of equipment, anyone installing them is limited to the capabilities of that particular PLC. Improvements can be made, but only within the scope of what their provider has provided.

A soft PLC can, theoretically, have any amount of sensors and actuators networked in. The software that handles the PLC can be redesigned digitally, not physically, to allow new devices to be handled within the established system.


Continuing that idea, because soft PLCs can be redesigned at a software level they can infinitely adapt to changes in hardware. A hard PLC will be limited by the hardware available to it where a soft PLC wouldn’t be.

Diversification of support

With specificity comes certain issues. Custom built systems might be perfectly designed to enable certain processes, but at the same time they can be more costly to implement and maintain.

If your hard PLC encounters an issue, then there’s a limited number of people you can call to troubleshoot it. Worse still, if support for your hard PLC is ended, for whatever reason, then you might not be able to just adapt and update it, you may be looking at installing a whole new PLC.

Future proofing

A hard PLC might also hamper the adaptability of an operation. If your beautifully designed, perfectly implemented hard PLC operates within strict limits, you may be unable to capitalise on trends in the industry by adapting your production to match the needs of a shifting customer base.

With a soft PLC housing its CPU functionality in a separate PC, both the brains of the operation and the input and output devices can be expanded and adapted as technology develops.

In an Industry 4.0 world, facilities full of internet of things (IoT) devices can be linked up, those facilities can be networked, that network can exist within the cloud, all being managed by a PLC that exists digitally, across facilities, networks, nations and continents. The sky really is the limit.

Which is better: a soft PLC or a hard PLC?

At the end of the day, manufacturing will always require something like a PLC to allow it to produce large numbers of things efficiently. Hard PLCs and soft PLCs have their benefits and drawbacks and which one your company lands on will be down to your business goals, methodologies and predictions for the future.

Hard PLCs make sense in a lot of situations, but Industry 4.0 advances might rely on soft PLCs to become realised. The potential growth offered by a soft PLC is something that has to be taken into account when decisions are being made.