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Getting the best out of the VME backplane

Edited by the Engineeringtalk editorial team Sep 12, 2000

We have now reached a point where today's 2mm Hard Metric connectors can provide a future-proof standard, according to Alan Simpson of Selectronix

Since its inception, the Futurebus interface for backplane applications has undergone several evolutions.

We have now reached a point where today's 2mm Hard Metric connectors can provide a future-proof standard as we see computer, telecommunications and related designs move towards the gigahertz clock range.

The three main 2mm grid connector designs are Futurebus, HDM, and Hard Metric (HM).

They consist of multi-row pin headers on the backplane and right-angle female connectors on the daughter card.

The most noticeable difference between the three styles is in the terminal area design.

The HDM type has exposed terminals on the daughter card connector whilst the Futurebus and Hard Metric styles feature encapsulated pins.

The real benefit of the Hard Metric design comes in real estate.

The 7 row (5+2) HM backplane connector is not only 3mm narrower than the equivalent Futurebus version, but also slimmer than the 6 row HDM connector.

So the HM maximises the space on the backplane between the connectors and allows greater component density.

What is even more important is the mating distance of the Hard Metric design.

This is measured from the rear edge of the daughter card to the front surface of the backplane when the daughter card is fully seated.

In the Futurebus design this dimension is 10mm.

Hard Metric connectors have a mating distance of 12.5mm.

Why is this important? Because it matches the mating distance of both the well-established DIN41612 connector as well as the 2.5mm IEC61076-4-100 standard.

The most advanced Hard Metric designs use a compliant contact with a C-shaped configuration which not only provides good mechanical stability but also eliminates PCB through-hole damage.

Three standard backplane plug configurations are common.

All incorporate seven rows of contacts - five for signal and two for shielding.

Type A features 110 signal and 44 shield contacts in a 50mm external width.

The guide and coding mechanisms are located in the centre of the connector.

Type B is open-ended, also 50mm wide and uses 125 signal plus 50 shielding contacts.

Type C has the guide and coding mechanism at one end and provides 55 signal with 22 shielding contacts in an external width of 25mm.

Varying lengths of contacts can be specified for the shield rows (z and f) to facilitate the required performance.

Daughter card mounted female connectors are, of course, available in complementary package sizes and can be equipped with shielding plates.

Shielding plates can be fitted in pairs, an upper and a lower, which mate with alternate contacts of rows Z and F of the corresponding backplane plug connector, or singly to comply with the Compact PCI standard for HM connectors.

Compliance with IEC917 (DIN43355) and IEB1076-4-101 ensures that the Hard Metric design matches that of the Eurocard package (IEC273 or IEEE1101/1101.10) and allows designers within those industry sectors the flexibility to combine the popular 96 way DIN41612 connector with the newer HM interface.

This, of course, supports legacy architectures whilst at the same time taking advantage of the newer features which require the higher density of a 2mm grid connector.

This is the reason that the VME64 Extensions committee chose the Hard Metric connector system for the PO/JO interface.