Computer bus architectures may have come and gone, but more than 25 years on, the humble Eurocard still remains the packaging form-factor of choice

There is an urban myth that the US standard width for railroad track can be traced back to the Roman chariot, where the distance between the ruts was determined by the width of the back-end of two horses. The truth is in fact stranger – the 4 foot 8½ inches that was adopted had previously been used by Stephenson in the UK. It was the same labourers - known as ‘navigators’ or ‘navvies’, who had previously dug the canals, and subsequently built the railways. This gauge still accounts for 60% of the installed track worldwide and 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 can still be seen on Malta. Bronze Age carts appear to have settled at a track width around 4’7” to 4’9”. “So what?” you may ask – my point is that (i) some standards are enduring and (ii) certain critical parameters are influenced by physical constraints on how the equipment is being used.

So, how does this relate to our modern electronic world? We have seen vast reductions in the size of circuitry over the years – call it Moore’s Law, if you subscribe to that, but one thing hasn’t changed much – the size of the human body. So though we now have smart phones that eclipse our first attempts at a laptop, we still have to interface with the human form. For example, our fingers determine key size, our eyes need screens large enough to view – and it doesn’t end there. We build our offices with a standard for door sizes; Building Regulations determine both that and ceiling height. This then influences what cabinets can easily be accommodated (without the need to cut a special aperture in the equipment room). Around the home we see standards prevailing – for example 600mm width standards for kitchen equipment, that allow us to mix and match units. This is the type of standardisation that ensures maximum compatibility and longevity of a product range.

In the world of electronic packaging, there are established standards – and some more recent newcomers to the scene. These newcomers are normally driven by step changes, such as the adoption of architectures such as ATCA, where there is a driving need 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 if you want to find a way of housing it. There also remains the option of designing your own packaging – typically undertaken by large manufacturers who typically want to define a brand image and can afford the vast sums necessary for complex injection moulding and special coatings for EMC shielding – just take a look at test instruments and medical equipment.

However the most enduring standard is still the Eurocard form factor. Anyone with a new design brief may dismiss this as only relevant to existing standard bus architectures such as VME and CompactPCI. However, that is doing the standard a disservice. The Eurocard form-factor has been around for a long time – far longer than the recognisable standards that define their usage. Even these standards contain a strange heritage when you start to look at the detailed measurements.

Basic Eurocard dimensions revolve around a modular approach; the smallest board some 100mm (3.937”) 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 (5.25”) vertically, and 60mm in depth. The height of any module is based on a vertical pitch of 1.75” (44.45mm) – here you will see the first of the mixed measurements. The 100mm is a metric size, however the integer vertical steps are designated as ‘U’ – this 1.75” supposedly dates back to the British Post Office Type 3000 relay (used in telephone and railway signalling applications) – the 1.75” 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 19” rack - again supposedly a legacy product, this time from a Ferranti standard cabinet, and certainly around at the time of the Argus computer in the late 50’s, although I’m sure others may lay claim to the fame.

Cards are aligned according to a ‘Horizontal Pitch’ – abbreviated to “HP”, corresponding to an integer value of 0.2” (5.08mm), so each card or module will be 4HP, 6HP etc. wide. It would be easy to imagine that this now end in a mass of confusing dimensions – however, 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,

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 0.1” pitch and although the connector dimensions were given in metric format, it was not until the proposal for a new connector for a standard under consideration (the ill-fated Futurebus+) that there was a move to go truly metric – the so-called ‘hard metric’ standards. This would mean a truly metric connector (2mm pitch), metric mounting holes, and board spacing would now change to be on a 20 or 25mm pitch. Obviously the packaging companies accommodated this, and tooled up, but as we all know, it is the market that determines what wins out – and 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 shows that Eurocard format is still dominant, and by a large margin. It may seem daunting at first, what with U’s, HP’s 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.

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 still 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, and with state-of-the-art backplane and connectors, is at the cutting edge of processing architectures. We can trace back from these latest designs, with their combination of parallel and serial architectures running at Gigabit rates, to the first bus systems, with wire-wrapped interconnect – the connectors 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 have 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 the CompactPCI added 2mm connectors – the standards continued to evolve. At the heart of all these groups were the real heroes – members of commercial companies that 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.

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 & 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.