Virgo PLC

Virgo 2000 Virtual-PLC

In the Virgo 2000 Suite, automatic control is configured and monitored on a Microsoft Windows-compatible computer but runs on a PC-based controller operating under an industrially-proven real-time operating system (RTOS). This architecture lets you develop, configure and monitor automatic controls from a familiar desktop environment but gives you the peace of mind that comes from knowing that the control code is running on the world's most reliable and fault-tolerance RTOS.

The Virgo concept A group of programs (previously configured in the workbench) with a common scan cycle acts as a virtual PLC, also known as a virgo. You can simulate a virgo off-line or download it to a PC-based controller of your choice. Since a PC is typically many times more powerful than a PLC, you can run more than one virgo on a single physical controller or you can run a single virgo at a faster scan rate.

Automatic controls execute according to a PLC-style scan cycle. Each virgo executes the following steps in a deterministic cycle:

• read field inputs,
• respond to service requests,
• write field outputs,
• retain persistent data.

The PLC-style scan cycle is the most widely used model for real-time control. Under the RTOS, this cycle can run deterministically as often as once per millisecond. Virgo 2000 lets you run several virgos, each with a different scan period, on a single PC-based controller.

PLC-style scan cycle

Real-time power
The Virgo 2000 runtime executes on the QNX RTOS, the most reliable and fault-tolerant technology available for the PC platform. QNX has been proven in mission-critical applications for more than 15 years. Its micro-kernel architecture is fully debugged and it offers the microsecond deterministic task switching needed to build a modern control system. AlterSys is committed to keeping up with RTOS technology, and will release Virgo 2000 for Windows CE when the long-awaited real-time version is released by Microsoft. As always, ALTERSYS believes in providing the best technology available to our customers for their critical applications!

True Real Time, No So-Called "extensions
A number of PC-based control systems use so-called real-time extensions to achieve acceptable response times while running under Windows NT. Virgo 2000 has not used this architecture for the following reasons:

• The real-time extension must be rebooted following a Windows NT crash, whereas a true RTOS running on a separate CPU is unaffected by a NT crash. Thus, the real-time extensions don't eliminate having to reboot, they only let you choose when. To our way of thinking, your controller is not doing its job while it is rebooting. PLCs don't have to reboot, why should a PC-based virtual PLC?
• Virgo 2000 virtual-PLC has a small footprint that can be embedded in economical form-factors such as PC104, various single-board computers and even PLC cards such as the Allen-Bradley Open Controller. It doesn't require the latest in PC technology Eit even runs happily on 80386 or 80486-based machines. On the other hand, there simply isn't enough room on a typical flash drive for Windows NT plus a real-time extension plus a useful control application.

Business benefits
The primary business benefit of Virgo 2000 Virtual PLC is the ability to run control on any standard computer available from a wide range of suppliers. You are no longer captive to a given supplier for hardware maintenance and upgrades. The competition between suppliers means that you get higher performance hardware at lower prices. What's more, your upgrade path is guaranteed: backwards compatibility assures that Virgo 2000 will run on any future PC. You can choose the form factor best suited to your application, whether it be a single-board computer, a PC processor card in a PLC rack, an industrially-hardened PC or an off-the-shelf commercial unit. The choice is yours, as are the savings.

Hardware and Software Requirements

• A computer with an Intel 80386 or compatible processor
• 8 MB of RAM
• A hard disk or flash disk with 14 MB storage space*
• A VGA monitor, a diskette drive, and a keyboard for installation purposes
• A serial port or Ethernet adapter

*Custom configurations could be optimized to less than 4 MB
The QNX operating system is provided by AlterSys
For high-performance control, processor and storage subsystem should be chosen according to the requirements.

AlterSys Inc., 555, rue d'Auvergne, Longueuil, PQ J4H 4A3. Tel: 450-674-7774; Fax: 450-674-7344.