Board / Slot Keying

Keying a board is used to allow or disallow board styles from being plugged into certain slots. A card which is keyed to a particular slot will only plug into that slot, or any other slot which accepts that key. Keying the card and chassis forces users to install cards into a particular slot or particular order. A system board keyed the same way as slot one, but not keyed to any other slot forces the installation of that board into slot one and no other.

There are three main ways a board and chassis may be keyed. The most popular way to key a system is to add a key to the bottom of the board face plate, and than key the bottom face of the card cage within the chassis. The second most popular way to key a card is to key the rear board connectors on the card and than key the connectors on the back plane to accept the card. A third way to key a system is to apply form fitting keying systems around both the board and backplane connectors. This last method is normally used only after the system is designed with out a formal keyed system. The keying was left out, but found to be required later.

Reasons for Using Keyed Backplanes
Keyed boards and backplanes force particular cards only into certain card slots. This may be required for a number of reasons; for example heat dissipation. A keyed system might be used to insure that the cards dissipating large amount of power are separated to even out the heat load in the card cage. Or so that a hot card ends up directly over a fan, or near a card that is not generating large amounts of heat.

A system might be keyed to insure that noise generating cards such as a power supply board does not end up in a slot next to a noise sensitive board such as a DAC board [Digital to Analog Converter].

Of course when User Defined I/O pins are being used [as in VME], the card that required that input would need to be in the correct slot. User Defined cabling running to the rear of the back-plane would normally be card specific so card keying would insure the correct card would receive the expected input. The use of a rear mounted transition card would also require the correct interfacing card to be in the right slot in the front of the chassis.

Some backplanes may be keyed by design, in that the specification might use different connector styles in different card slots. For example some board standards define slots for power supply cards or slot controller boards, both of which use different style connectors than normal interface cards.

Reasons Not To Use Keyed Backplanes
Sometimes keyed backplanes cause problems. It is not always obvious which circuit card should be install in which slot, a few insertion attempts may be required. A new user of the system may not even notice at first that a board is keyed, perhaps attempting to force a card into a wrong slot.
The keying hardware is a separate line item, and need to be ordered. Of course both the chassis and cards need to be keyed.

Design Hint; Not all systems use the same keying methods. Some key the cards and backplanes, others might key the front panel and card cage.

Birtcher {Conduction Cooled Keying manufacturer}

Next topic Connector Selection

How To Specify a Equipment Chassis; Index page

PC motherboard

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