Real time without additional hardware and as a system basis

TwinCAT Realtime Extension ensures real time capability

The Windows NT operating system for the PC is not capable of real time operation: it was designed for optimised performance and not for control purposes. Tasks are interrupted by various events, or their calls are delayed. The diagram shows these interruptions in a high-priority, cyclic Windows NT task:

Windows NT is not real time-compatible

Windows NT and Beckhoff Realtime Kernel Extension

As Windows NT is not real time-compatible, the operating system is extended for automation tasks: such a real time extension is the basis for TwinCAT. With a background of more than ten years of experience with PC software solutions for the PLC/NC and more than ten thousand installations under DOS, Beckhoff has developed a proven and independent real time extension for Windows NT as the basis of TwinCAT and has harmonized it with Microsoft as a system partner. In a cyclic mode of operation, 64 tasks are executed with priority control, preemptively and deterministically with a maximum of +/- 15 µs jitter.

TwinCAT is real time-compatible

With this Kernel Extension, TwinCAT has an exact time basis which executes programs with maximum priority, and independently of other processor tasks. To demonstrate this, launching of a TwinCAT task was recorded over a period in order to measure time deviations of the system. During this time, in which 1 million PLC cycles run, the PC is loaded with diverse tasks such as network operation, hard disk access, video display and mouse movements. The diagram shows a rising signal edge, triggered by a task, in an integrating representation that shows the time deviations (jitter). TwinCAT tasks are called up deterministically (timed exactly in 100 % of all cases) with slight fluctuations (jitter) of only a few µs (in this case: +/-12 µs), which are negligible for general applications:

TwinCAT offers a compatible real time basis

The Beckhoff implementation operates without a second operating system thanks to the fact that the real time tasks have been integrated completely in Windows NT:

• It does not require any hardware in addition to that of a standard PC (currently: Intel single-processor architectures) or a second operating system,
• Windows NT can be purchased, installed and used without modification throughout the world, i.e. it does not require any modifications ("patches") or replacement of its parts,
• The user does not require a knowledge of the operating system or its programming to run automation tasks,
• The implementation is also compatible with future releases of NT (e.g. NT 5.0),
• It permits the reservation of a remaining CPU capacity for Windows NT, which is observed under all circumstances,
• Even under a high real time load, it guarantees that important NT tasks are executed and so no characteristics of NT are restricted or influenced,
• It offers a load display indicating CPU utilization by the automation tasks,
• It displays the current and maximum real time jitter online and generates messages in the event of violation.

Automation solutions for practical applications

At the same time, TwinCAT does not focus on the real time implementation. Instead, it represents a complete application solution for the editing, analysis and execution of control programs in real time: no one needs to be an operating system expert to only use real time on a PC. The focus is on practical application in the field of automation.

Open programming Open I/O link Open Windows interfaces

An easy-to-use environment permits:

• Creation of real time programs to IEC1131,
• Execution on the same PC or remotely,
• Analysis with modern methods,
• Integrated interfacing to programs, e.g. visualization applications,
• Integrated, open and cross-manufacturer I/O link for field buses and PC hardware.