Are PLCs dead ducks?

Dave Wilson gets to the heart of the PC vs PLC debate and finds that PC-based hardware is making inroads into the PLCs domain

The Personal Computer (PC) has changed the face of factory automation. Everywhere you look, some aspect of PC functionality has crept into automation systems design. But are PC-based solutions set to completely take over the automation scene, or is there still some room left for the Programmable Logic Controller (PLC)?

At the inexpensive end of the marketplace, the £100 PLC brick is unlikely to disappear quickly. Yet when one looks at more sophisticated PLC or PLC/CNC controllers in the £3000 to £5000 price range, the challenge from the PC becomes more evident.

In the past, PLCs have had some competitive advantages over the PC. They were rugged and very suited to an industrial environment. The connectors to I/O and peripherals that extended from the front of the units made them easy to interface to. And, they sported a power supply that could be used to drive other peripherals directly from the front of the PLC. But perhaps the largest contributing factor to the longevity of the PLC is the existing software base. That is one of the key factors that influence a designer’s decision to choose between a PC or a PLC.

In the past, PLC software developers used a proprietary development environment and cross compiled their software to a PLC target, which often used a different processor and operating system to that used on the development hardware. Industrial PC users, on the other hand, use the same hardware and software environment as the target. This has allowed them to test code on their PC development system prior to downloading to a target.

Today, proprietary development systems have been replaced by PC tools. All software development environments, from PLC and PC vendors alike, now use a PC as the platform of choice for software development.

The standardisation of several programming languages under IEC 1131-3 standard for industrial control programming has fuelled the PLC vs PC debate further. The IEC 1131-3 standardises a number of different programming environments, including Instruction List, Structured Text, Sequential Function Charts, Function Block Diagrams and Ladder Diagrams.

Most of the major PLC and industrial PC manufacturers are embracing the standard: once they do so, some claim that any developer writing software that meets the basics of the standard should theoretically be able to port his application to any hardware that he likes. That means that if a designer prefers to run his ladder logic program on an industrial PC, rather than a PLC, he should be free to do so.

CAN IT BE TRUE?

While PC vendors would like to believe that scenario, in actual fact the 1131-3 language was actually developed to allow developers to structure the writing of code, not necessarily to provide interoperability between platforms. And vendors of PLCs are allowed, within the constraints of the standard, to develop additional functionality within the standard to optimise the performance of their PLCs. According to Michael Juniper of Omron, many traditional PLC vendors have done just that. While that might well benefit the PLC users, it casts a shadow over the issue of interoperability between vendors products.

Traditionally, PLC hardware has been very different from that of the PC. A PLC was based on a proprietary architecture. The operating system that ran on the PLC, the processor that the software ran on and the backplane bus that allowed elements within the PLC to communicate were proprietary. Proprietary architectures still exist. But just as the PC-based development environment has crept into the traditional programming environment, elements of the PC are also creeping into what once were tightly controlled proprietary PLC hardware architectures.

Some PLC vendors have used open backplane buses like the PC/AT (ISA) buses to open up their systems to third party controller boards, while at the same time retaining their own proprietary processor cards and operating kernels that slot into the AT bus for PLC and CNC functions. In such systems, the PC is used as a front end/user interface device that also performs housekeeping operations.

Others, such as Omron, have merged their proprietary backplanes with open architectures by allowing designers to add cards with PC functionality into the proprietary backplane. An Omron card dubbed the ISP also allows for ISA expansion so designers can add third party ISA cards into the system.

In many cases, where a PC may be integrated with a group of PLCs, developers of specialised programs, often written using low-cost PC tools, can now run their application in tandem with the more traditional PLC control programs. In a vision inspection application, for example, complex positioning algorithms can now run on the PC, passing the results to the PLC running control system code.

However, some vendors of pure PC based hardware solutions now claim that it is no longer necessary to purchase traditional PLC hardware at all: they say that all PC and PLC components can be purchased from non-traditional PLC vendors, then integrated by the systems house. Vendors like Integrated Measurement Systems, distributors of the US-Advantech product range, claim that the PC can do more than simply operate as a front end to a PLC, whether that is based on a proprietary architecture, or not.

Such vendors point to those ruggedised industrial rackmounted systems, usually based on the ISA bus as a total systems solutions. On the hardware front, these designs usually comprise a rackmounted PC, usually Pentium based, that interfaces to a range of I/O and peripheral boards across the PC/AT backplane. Like their PLC counterparts, I/O is accessible from the front panel and a power supply is provided to power up other I/O.

Hardware in these passive backplane systems is all PC-based. Newer systems are PCI bus based with 130 Mbyte/s bandwidth. Although the PCIbus itself is limited to four slots, this limitation has been overcome by bus repeater devices on the backplane that allow the bus to be expanded. On the other hand, there is the Compact PCI bus – an industrial backplane bus in a Eurocard format developed by a group of US-based companies who have industrialised the PCI specification.

Despite the fact that traditional PLC vendors might doubt that their PC counterparts have the necessary experience in industrialising computer systems, plenty of boards are available that plug into the passive backplane of an industrial PC to transform it into a controller. Carlsbad-CA based PMC, for example, offers a range of line of multi- axis motion control cards, software to handle homing procedures and tool change operations, as well as a PLC interface. Closer to home, Chapmore Controls supplies a CNC/PLC combination board on a PC card that runs real time programs and safety critical functions separately from a host PC. Here again, the PC simply acts as the interface to the user.

From the software perspective, vendors of PC based industrial systems claim that the Windows NT operating systems with a recently defined real-time extension developed by VenturCom, is now the de facto operating system for industrial control. If a real time Windows NT alone is not enough, Microsoft is also touting its Object Linking and Embedding (OLE) technology as the answer for developers who would like to break out of the stranglehold of a single vendors’ I/O offerings, and who would like to mix and match I/O from different companies. But PC hardware is a moving target. Some designers are still wary of committing to a hardware platform that is likely to change every six months. The result is that hardware and software changes in the field may be more expensive in the long run, due to the quick obsolescence of PC based hardware and software. Some system integrators, having committed to a PC solution, have returned to a PLC vendor after having tried the PC route simply because of it. What is more, the Windows NT operating system is somewhat large compared to many proprietary real-time operating systems that run in PLCs, and for smaller operations may be overkill.

In some respects, many PLC vendors products’ already offer all of the benefits of the PC without any of the disadvantages. Today, PLCs have PC-based development tools, many come with PC graphics interfaces and some systems allow designers to plug in PC based CPUs to allow PC style applications that can run concurrently with PLC control code. While PC vendors tout the PC as the entire solution to a control problem, the reality is that the PLC has already stolen many of the best aspects of the environment anyway.

{{OmronTel: 0181-450 4646Enter 532

ApliconTel: Freephone 0800 525 335Enter 533

PLC OpenTel: Netherlands +31 418 541 139Enter 534

Integrated Measurement SystemsTel: Southampton (01703) 571177Enter 535

PMCTel: US +1 760 930 0101Enter 536

Chapmore ControlsTel: Ware (01920) 466566Enter 537

VenturComTel: US +1 617 661 1230Enter 538}}