Have you missed something?

The thought that a PC can be capable of providing reliable control is sometimes overlooked by system integrators. Automation systems get more diverse but many are still based on a PLC that ultimately connects to PC-based HMI, SCADA system or DCS. There is a belief in the field that a PLC is more reliable than a PC, and people's experience with PCs tends to confirm this belief. But might this be more to do with the way we use them and the software we load, rather than any inherent faults?

Anyone can develop applications that run on a PC and we are all guilty of downloading or installing applications without questioning their suitability and testing status. Studies carried out by Microsoft on NT4 showed that 43% of failures were caused by third party applications. In comparison, a PLC is a dedicated controller that is more difficult to crash, but it still has an operating system susceptible to illegal instructions. There are some commands which are known to cause many a PLC to crash or lock-up - problems which have to be resolved by either reloading the system or the failing program. But it's the PC which is still not tolerated in the control environment.

Look at the architecture of a PC or PLC system and you see more similarities that differences - the modern PLC is a powerful computer and, like the PC, it has a CPU, RAM, ROM, backplane bus and I/O. The main difference between them is the operating system. A PLC uses a dedicated operating system that can limit the potential of the underlying architecture. Maybe it would make sense to use an open operating system, such as Microsoft Windows or Linux. Many PLC manufacturers already use PC architecture displays with Windows CE, calling these 'Soft HMI' products. The fact that manufacturers use open standard operating systems for visualisation surely suggests limitations in dedicated operating system control devices.

Windows CE is not a deterministic operating system and the scan times are nowhere comparable to a PLC. The argument therefore goes that for deterministic control you should use a PLC. A standard PLC can only achieve 20ms scan times for I/O updates and if you want to beat this you can use a limited range of interrupt controlled instructions. PLC manufacturers have been saying that users need two pieces of hardware from them, one for control and another for visualisation. The visualisation link to the PLC is usually over RS232 so the 20ms update can be maintained. However, once you start using industrialised operating systems like NT/2000 and Embedded XP for your visualisation, then suddenly you're between two camps, with companies like Intellution, Wonderware, CiTect and Steeplechase vying to sell you high-level alternatives to PLC systems.

Windows NT-based operating systems on their own are not deterministic for control, but they are robust enough to handle high data throughput with true multi-tasking using multiple processors. Steeplechase software, though, lets a standard desktop PC provide deterministic control by using the real-time operating system Tenasys INTime. The Windows NT environment is treated as a task that can only run when INTime execution is complete. The real-time program resides in RAM and is never removed or refreshed by NT, with INTime driving all necessary real-time I/O. In the event of an NT crash, control is still maintained. Windows components run as normal and with this software scan times can be as low as 0.2ms while the same platform is available for Data Acquisition as well.

Putting both control and visualisation systems on a standard desktop computer can lead to enormous cost savings. A reasonably specified standard desktop can be purchased in the UK for around £500: that leaves only the cost of developing software for running it. So why is your PLC manufacturer trying to sell you a lot of boxes, when everything you need can be achieved on a single PC?

Terry Price can be contacted on +44 (0)1892 785755 or at terryprice@lineone.net