PLC on a chip - fact or fantasy?

What is a Programmable Logic Controller? It is a highly specialised industrial computer, running highly specialised software designed for ease of use and reliability in the industrial control environment. At heart it is just another variety of computer running software, says Chris Hazlewood, product manager at Mitsubishi

COMPUTERS AND SOFTWARE ARE EVOLVING at a terrifying rate and the PLC is no exception. Amongst the most exciting developments are Soft Logic, embedded PLCs, Windows CETM and JAVA-based embedded controllers. There has been a lot of debate recently about whether PLCs could be replaced by Soft Logic (a PC running a program that performs the function of a PLC), but most of this ignores the fact that a huge proportion of the market for PLCs is for so called 'Micro PLCs'.

These are PLCs with small numbers of I/O designed to control, say, a pump topping up a tank or a sausage slicing machine. For these applications, many of which are stand alone, the complexity and overhead cost of a PC cannot be justified. Furthermore, the major benefits of screen and keyboard have no value in unattended installations where an operator interface is not required. Also, the present rather unstable operating systems for PCs leave a lot to be desired, especially for such unattended locations where a re-boot could mean a journey to a remote site during the night.

Soft Logic has its market, but it lies in medium to large control installations where the operator interface of the PC is appreciated, where links to corporate IT systems and archiving to hard disk is a benefit, where networking is involved and where an alternative to the PLC solution is considered an important element. Change in the micro PLC market will come from a quite different quarter: embedded control.

What is an embedded control? This comes from a device that has been stripped down to its essential components and suitably packaged for building directly into a product needing control facilities. At the extreme it could be a PLC or PC on a chip. It is often assumed that this is the domain of PC/104, the new emerging standard which effectively minimises the deployment of the traditional computer onto a circuit board the size of a beer mat. Manufacturers of these products believe this is the future of control technology.

Rugged controllers

But let's not forget that PLC manufacturers have been developing and will continue to develop small, faster, and wholly more rugged control products not only for the present, but for five years, ten years and 20 years hence. The question we should actually be asking is not "Will the PC replace the PLC?" but probably "What technology will be absorbed in tomorrow's industrial controllers?"

To answer this we really need to look in more detail at what problem we really want to solve: how to make effective embedded control which allows development, expansion and maintenance, while still performing its core but essential task of controlling! So what would we expect to see performing the task of embedded control today? Usually, the device would be packaged in a small PCB sub-assembly, probably with a plastic case, that can conveniently be built into a product needing control. The concept will be familiar to designers involved with the development of consumer durables such as washing machines, TVs and video recorders which commonly use embedded micro controllers.

However, embedded micro controllers are typically fixed function, with the program built-in by the maker. Programs are normally written using C++ applications and are then compiled to suit the target device. This approach is practical because the products normally require no variation from unit to unit.

But industrial machine makers will often need to customise individual machines, and that usually means customising real-time programs. Recently, some manufacturers of industrial PLCs have offered a PLC programmed in C, but there are inherent technical issues with this.

Essentially, C++ is a non-starter for an electrical engineer commissioning a one-off machine in his customer's factory in a foreign country. For real-time user programming, you need a familiar high level language such as traditional ladder logic, or more recently, the flexible structured text and sequence function charts of IEC1131-3. Fortunately, these languages can now be implemented conveniently in PC programming tools, so if you create a device that can run the resulting code, you have the PLC on a chip concept described above.

In fact, if you look closely inside a micro PLC, you will find a PLC on a chip. The challenge is to make the device 'OEM friendly' by packaging it so that an OEM can assemble it into his product conveniently and without risk of damaging it, but without adding significant cost or size. Accomplish that and you have an embedded controller that can be programmed like a PLC. More accurately, you have a full strength PLC packaged like a micro controller.

Tiny PLC

An example is the Mitsubishi MELSEC FX2NC. In a housing just 30mm by 90mm by 75mm, this PLC packs 16 inputs and 16 outputs and has space for 8K program steps in RAM, plus an optional 16K program steps in EPROM. Up to 256 I/O can be supported in larger housings. To program it, OEMs simply use the familiar MM+ or PCS WIN programming tools that they use for the full size PLCs. Ladder logic, and even IEC61131 to some extent, were developed to suit historical approaches to programming industrial controllers. IEC61131 also bravely tried to help the user by developing a common programming language which could be ported across PLC vendors. But not all IEC61131 compliant software stores programs on the hard disk in the same way - and not all compliant software has all manufacturers' drivers. The ability to take a program written for a PLC from one vendor and run it on a PLC from another vendor is still a long way off. In the IT industry, market pressure for applications that will run on Mac, PC or UNIX platforms, or other environments such as the Web has given rise to Java byte code. Java provides truly platform-independent code, because the programming interface has been removed from the direct control of the hardware. It is also highly re-usable because it is object oriented and because its structure breaks code into discrete component applications.

But Java is more than a programming environment. Essentially users develop their program as a Java application which then calls in resources or classes to do certain specific tasks. Each class is only accessed as and when it is needed, and if not in use it is discarded. The active class in turn triggers the native code on the JVM to enable the action or event to occur. The example shown supposes the user's program calls the Java.net class which in turn triggers the TCP/IP functions of the JVM to be used.

Where's the advantage in this? Well, the Java code is compiled into standard byte code which is then interpreted by the JVM. This means there is no need to write code to suit a specific chipset, but just to stick to the standard and have truly portable applications. If you are struggling to picture this in your mind, think of the classes as a similar concept to DLLs, except the Java classes operate directly with a chip's lower level services.

Java is not the answer to everything but it does offer us all some interesting alternatives!

• Mitsubishi Electric
  Email c113@industrialnetworking.co.uk