Werner Sonnabend compares the various features and benefits of VMEbus and CompactPCI. But what factors should users use to pick a system?

Both VME computers and CompactPCI (CPCI) systems offer substantial benefits. For this reason, users would be well-advised to use rational criteria when selecting the most suitable computer for their specific application.
A manufacturer of microcomputer packaging systems has considerable system expertise, offering tangible advantages for users. The principal objective of non-conforming mechanical systems is to facilitate the independent compilation of complex industrial computer systems.
However, for many applications in automation technology, it is often more convenient to purchase an entire tested system for 'plug and play compatibility'. Whether self-configured or ready-assembled, choosing the correct 'heart' - the CPU - ensures optimum benefits and maximum success of the finished system. Experience has shown that both systems are capable of achieving this at comparable costs. So when should users opt for VMEbus, and when is CPCI the best choice? The speed on the buses themselves is not a decision-making criteria, since neither system exhibits clear superiority over the other.

An ace with real-time applications

At best, PCs can only accomplish time-critical tasks in 'quasi-real-time', by employing various interrupt-type tricks, whereas VMEbus computers have certain system advantages which enable them to come up trumps in real-time applications, such as production control and image processing. What is more, such applications also benefit from more than10 years of invaluable expertise. Users can choose from a vast range of several hundred system cards from different manufacturers to suit every conceivable task. VME is justifiably known as 'the industrial computer'. These cost-effective, universal high-tech computer systems owe their success to the development of VME and VME64 backplanes and cards and their standardisation in IEEE1014 and IEC821, together with the more recent generation of backplanes to VME64 Extension (VME64x) and VME 320 specifications.
The advantages of real-time capabilities and cheaper hardware prices for computers based on the VMEbus system must be weighed up against possible disadvantages, including larger constructions and less popular software.

Joker comes into play

CPCI technology is pinning its hopes for future success on telecommunications applications. 'Sub-bus systems', switching processors and H110 technology are the buzzwords in this field. Input/output connections are generally connected at the rear, thus enabling the rapid replacement of I/O assemblies without interrupting the system's numerous telephone or network connections (standardisation proposal IEEE standard P1101.11).
Externally, the system is identifiable from its 'Eurocard format' based on standard IEC60297-4 and the extension for Eurocard mechanical systems, IEEE1101.1 and IEEE1101.10. This tried-and-tested design, combined with high-quality connectors, ensures a high level of modularity and fail-safe operation under tough everyday conditions. In principle, the system is based on familiar VMEbus technology, which explains why the two types of computer have the same mechanical system. CPCI has certain advantages with 32-bit applications, thanks to its 3U size (VME is 6 U). The lower build height with smaller cards produces very compact systems, combining a high level of stability with resistance to shocks and vibrations.
CPCI makes the "Peripheral Component Interconnect (PCI)" suitable for industrial use and facilitate cross-system communications, eg with Multibus II and VMEbus. The PCI bus has established itself as a non-processor-dependent bus architecture offering a number of advantages. The range of programs across the PC spectrum which are supported by CPCI is correspondingly vast. Furthermore, programmers are able to generate new programs and adaptations in familiar languages with a reasonable level of input. However, when it comes to a direct comparison of hardware costs, CPCI is slightly less favourable than VME.
A short circuit-protected ATX power supply unit (open frame) with 235W and built-in fan supplies voltage to all active components. The four output voltages are 3.3V/14A; 5V/22A; +12V/8A; -12V/1A, which are indicated by an LED display module with power switch. Possible fan failures are likewise indicated on the display.

Complete solutions

CPCI or VMEbus - there is no single system which promises the ultimate solution for all applications. Instead, it is a question of selecting the system best suited to the particular task in question, and having it professionally modified where applicable. A rational analysis of all the specific features is likely to achieve better results than a theoretical debate on the fundamental benefits of one system versus another. Specialist suppliers of microcomputer packaging systems are often able to provide invaluable assistance with decision-making, particularly when they are not tied to one particular brand, such as Rittal, who offer a full range of industrial PCs. As well as VMEbus and CompactPCI, the company also offers AT and ATX, as well as freely configurable, customised solutions.

ENQUIRY No 48A

Werner Sonnabend is Head of the Electronics Division at Rittal, Herborn, Germany. www.rittal.de