When Form Fits The Function, Why Change It?

There is an urban myth that the US standard railroad track width dates back to Roman chariots, where the distance between the ruts was determined by the width of the back-end of two horses. True or not, it is true that the 4 foot 8.5 inches was previously used by Stephenson’s railway in the UK, built by the same labourers (known as ‘navigators’ or ‘navvies’) who had previously dug the canals. This gauge still accounts for 60% of the installed track worldwide and the dimension can be traced back further to the ruts left by sleds and carts that pre-date the Romans. Indeed some of the most ancient tracks are still visible on the island of Malta, where Bronze Age carts appear to have settled at a track width of between 4ft 7in and 4ft 9in.

Clearly some standards are enduring, with critical parameters influenced by physical constraints. With vast reductions in the size of circuitry over the years, based on Moore’s Law (if you subscribe to that), one thing hasn’t changed much; the size of the human body. So although smart phones now eclipse the first attempts at a laptop, they still have to interface with the human form; fingers determine key size, and eyes need screens large enough to view.

On a more practical level standards prevail in the office and the home to provide choice and stability. For example, 600mm width standards for kitchen equipment allow us to mix and match units, while offices have standard door sizes and ceiling heights, which in turn influences the size of furniture and equipment. This is the type of standardisation that ensures maximum compatibility and longevity of a product range.

Packaging

In the world of electronic packaging where well-established standards exist, newcomers are normally driven by step changes, such as the adoption of architectures like ATCA. Here the driving need was for large real-estate boards in a tightly packaged form factor for high volume telecom applications. At the other end of the scale, PC104 was conceived as an architecture without a backplane, just a stack of cards; innovative, but not very convenient.

There also remains the option of designing bespoke packaging, typically undertaken by large manufacturers, looking to define a brand image, who can afford the complex injection moulding and special coatings for EMC shielding. Prime examples are test instruments and medical equipment.

However the most enduring standard is still the Eurocard form factor. Although easily dismissed in new designs as only relevant to existing standard bus architectures, such as VME and cPCI, its contemporary reputation belies its heritage.

Basic Eurocard dimensions revolve around a modular approach; the smallest board is some 100mm (3.937in) high and the same in depth. The preferred board size of 100mm x 160mm commonly forms the basis of a ‘3U’ module, and typically increases in steps of 133.35mm (5.25in) vertically, and 60mm in depth. The height of any module is based on a vertical pitch of 1.75in (44.45mm).

This is the first of the mixed measurements; 100mm is a metric size, however the integer vertical steps are designated as ‘U’. This 1.75in supposedly dates back to the British Post Office Type 3000 relay (used in telephone and railway signalling applications), the 1.75in being the minimum vertical distance between the horizontally stacked trays. So all these metric cards fit on an imperial vertical grid and finish up in a 19in rack — again, a supposedly legacy product, this time from a Ferranti standard cabinet and certainly around at the time of the Argus computer in the late 50s (although others may lay claim to that fame).

Cards are aligned according to a ‘Horizontal Pitch’, abbreviated to ‘HP’, corresponding to an integer value of 0.2in (5.08mm), so each card or module will be 4, 6HP or nHP wide. It would be easy to imagine that this must lead to confusing dimensions, but more than 25 years ago these figures were laid down in agreed specifications that makes the adoption of this packaging standard easier to implement than might first be assumed.

Challenges

##IMAGE_3_R##There have been challenges along the way; an early standard was to implement the DIN 41612 connector onto the Eurocard. This connector had pins on a 0.1in pitch and although the connector dimensions were given in metric format the so-called ‘hard metric’ standard did not emerge until a connector proposal for a standard under consideration (the ill-fated Futurebus+).

It led to a truly metric connector (2mm pitch), and metric mounting holes and board spacing also changed to a 20 or 25mm pitch. The packaging companies accommodated this but markets determine the winners; the original standards prevailed.

So, all these years later we are left with an enduring legacy, a set of dimensions tried and tested, ratified by international bodies and available from multiple sources. So much so that a straw poll taken of the packaging companies showed the Eurocard format is still dominant by a large margin. It may seem daunting at first, what with Us, HPs and a number of other buzzwords, but the crucial factor is when you design your solution on a Eurocard format, you will always have a source of packaging, indeed multiple sources.

Endurance

These standards are now getting to the point of being a quarter of a century old; that would normally imply being well beyond their sell-by date, however in the world of packaging they are as relevant now as they ever were. Take the latest standard for bus architectures, VPX. This is the latest in a string of bus standards that has called upon these packaging standards. With serial architectures running at Gbit rates, these state-of-the-art backplane and connectors are at the cutting edge of processing architectures, but can trace their heritage back to the first bus systems, with wire-wrapped interconnect, where the connectors were often mounted on rails with a simple power bus, preceding the use even of printed circuit backplanes.

So, who do we have to thank for these standards? There are of course the recognisable standards groups that ensure correct policing of standards, The IEC (International Electrotechnical Commission) ratified the use of the DIN41612 (Deutsches Institut für Normung) connector found in many bus architectures and in 1984 ratified the “Dimensions of Mechanical Structures”, standardising the sub-rack dimensions for 19” enclosures. This was further developed in 1987 by the IEEE (Institute of Electrical and Electronics Engineers) to define the interoperability of dimensions and tolerances, under IEEE 1101. This has been further developed to cover the facility for rear plug-up, in the form of rear transition modules and enhanced EMC control.

Other industry groups, such as VITA (VMEbus International Trade Association) had been working on standards since 1982, continuing to drive these standards forward. Subsequently groups such as cPCI added 2mm connectors and the standards continued to evolve. At the heart of all these groups were the real heroes; members of commercial companies who sat on the groups. It was the expertise of many companies — competitors in many cases — that worked together to give the industry the framework that has allowed the Eurocard form factor to stand up as the preferred packaging. Verotec members served on all of these standards groups and were instrumental in assuring good engineering practice that has provided such an enduring set of standards and developed a complete range of products to support customers wishing to adopt these standards.

##IMAGE_5_R##Users can house boards of multiple sizes in a variety of enclosures, from two slot 1U horizontal subrack systems right up to 42U cabinets, with accessories such as modular power supplies in a wide range of sizes and power output. Mechanical enhancements such as injectors and ejectors were devised to ensure easy insertion and removal of boards, latches to provide positive retention and coded keying to make certain that only the correct board is inserted in a certain slot; it’s all there, as standard.

At the smaller end, a design may be accommodated on a 100mm x 160mm board but big users of the standards, such as the huge physics experiments seen around the world, typically adopt the 9U x 400mm as a preferred size. This gives the designer a range of more than 10:1 in terms of board real-estate capacity. Where else can we see such an accommodating standard? Manufacturers such as Verotec have further kept these ranges alive offering services to produce branded parts for users such as custom front panels and corporate colour paint schemes. A designer will need to look a long way to find such flexibility off the shelf anywhere else, if it can be found at all.

As these standards enter their next quarter century, there are other contenders that a designer has available to consider, those derived from PC based systems, small form factor mezzanines and so on. But for anyone who wants to put a system together that guarantees flexibility, a high degree of future proofing, is aesthetically pleasing and has a ready source of supply with no set-up costs, then the Eurocard form factor must be one of the favourites. It is not ready for the recycling bin just yet.