Last week, you learned a little bit about Eurocard packaging and the vertical chassis dimensions described by multiples of 1U or 1.75 in. We will call that the chassis Y dimension.
The other dimension (X dimension) is described in HP dimensions where 1 HP (Horizontal Pitch) is 0.2 in. or 5.08 mm, so the Eurocard pitch became 4 HP or 0.8 in. Intriguingly enough, the Eurocard depth dimension is in millimeters. I guess that is what happens when you design by committee.
Anyway, in the early 1990s, a coordinated attempt was made to move the industry to metric packaging. The idea, sponsored by major European Telcos like Siemens and Ericsson was to create an industry standard architecture that was so well thought out that everyone would quickly see the wisdom and move in that direction.
In a nutshell, the Metric Practice proponents envisioned that all dimensions would become metric in such a way that the electronic equipment world can be very compatible. Anyone with two sets of nut drivers, one metric, and one inch based, can see the value of such an effort. In general terms, a computer room will have a metric size that is easily divisible into smaller “blocks” that can be occupied by electronic equipment, other blocks for people and wiring, and other blocks for cooling infrastructure. The hope was that when all this was completely harmonized, the equipment could exactly fill the space in a metric truck, always fit through the standardized metric doors, and fit exactly over the metric grid computer floor.
Why not extend this whole thought process to electronic packaging, making metric dimensions for the X, Y, and Z dimensions within electronic systems, thus ending the chaos caused by mixing inch and metric dimensions within equipment practice?
Going further, why not extend the thinking to connectors as well? Here are some of the key dimensions proposed:
|Key Dimensions in Hard Metric Practice|
|Board pitch||N x 5 mm||N x 5.08 mm|
|Board height||N x 25 mm||N x 25.4 mm|
|Card to backplane||12.5 mm||12.5 mm|
|Chassis height||N x 100 mm||N x 44.45 mm|
|Typical chassis depth||N x 100 mm||N x 100 mm|
|Connector pitch||2.5 mm||2.54 mm|
|Tighter pitch connector||2.0 mm||TBD|
Seems like a reasonable proposal, right? The metric racks and subracks would fit into a space slightly smaller than existing equipment practice and would be more logically modular.
Connectors for the hard metric practice quickly followed with 3 major contenders. Siemens has a 2.5-mm hard metric 5 row connector system they had been using for some years. DuPont (after Berg, before becoming FCI) had the 2-mm Metral connector, and AMP (before Tyco) had invented a 2-mm HM (Hard Metric) connector. There were some rules about what hard metric meant. (Soft metric, by the way, was any metric dimension that was converted from an inch dimension, like a 2.54-mm pitch.)
The rules were interesting and opened up many new advantages, including:
- 12.5 mm dimension from edge of the daughter card to surface of the backplane
- Continuous metric pin grid across the backplane (2.0 mm and 2.5 are both OK)
- This metric grid should go in both X and Y axis
- Connector blocks must be stackable without disrupting the grid
- The first row of contacts must be at the edge of the board, or an even pitch above or below the board surface
This combination of characteristics opened up some very interesting possibilities. For example:
- Connector on the front side can run north south, on the rear east west, creating easy orthogonal architecture
- IO connectors on the rear can connect with multiple slots in the front
- All the hardware could be standardized in N x 5 mm increments, making chassis design with multiple card sizes quite easy.
The Siemens and Amp people protested against the Metral connector since it was not perceived as being hard metric. The card to backplane dimension was only 10 mm and a row of contacts was 1.5 mm above the board. DuPont might have lost this debate, but the Metral connector had already been chosen by the IEEE Futurebus committee as its standard, so in a rare case of industry collaboration and cooperation, it was agreed to amend the rules to include all 3 parties. Card-to-backplane dimension and first row of connector above board dimension was amended to be N x 0.5 mm. Done!
With so much good will toward the metric future, what happened, and where did it go?
It is all about legacy! The 19-in. racks were there and were not going away in the name of purity. The utopian view of the continuous metric grid across the world seemed to fall apart when you considered packaging, both the cardboard kind and the metal electronic packaging that never seemed to quite be just inside the line. So today, the data centers and Telco central offices remain with 19-in. racks and 1U vertical dimensioning.
Did the person who proposed the 1.75-in. dimension way back when realize the impact that silly number would have on the world? Sort of like the story about the distance between rails on the railroad track was determined by the width of the Roman chariot wheels that was determined by the width of the rear end of the horse.
Aren’t standards wonderful?