Board Form Factors
The size and shape of an electronic equipment chassis is dictated by a number of factors. The type of card used within the chassis plays a large role, but many of the Board configurations result in the same size case.
The component within the Electronic Enclosure that really holds the boards is the card cage. The card cage is a metal frame with card rails and attachment points so that it can be secured to the chassis and a backplane secured to it. The type and style of card format being used dictates the size of the card cage. With the size of the cage, which just a metal frame, being only slightly larger the circuit board being supported by it.
There are a number of competing board formats. Many of the board formats are only used in the industrial or embedded area. The common PC card formats are PCIe, PCI, and AGP standards. The common embedded card formats are VME, cPCI, ATCA, and PC-104, but there are many others. Reference Board Standards vs Area. Refer to the BackPlane Buses page for a complete list of possible backplane card combinations.
Note that the cards in a personal computer are all about the same size, although there are shorter versions, because they are all designed to fit in the same basic case or housing. Embedded cards are designed to fulfill a function at a particular size. So each different series of embedded boards could and does have a completely different form factor. PC mother-boards do come in different sizes [as shown by the picture to the right], but still designed to accept the same size expansion cards.
Normally at the start of a design the board format is decided upon. Once the design matures the card functions need to be partitioned and the number of cards within a chassis starts to be fixed. There is a finite number of cards that a chassis will hold, and a fixed number depending on the bus standard used.
In addition to a standard board specification there are also two additional Card Options, which include;
Air-cooled (also referred to as convection-cooled)
Conduction-cooled (sealed and military systems)
Chassis Specifications; Next topic in this section
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Design Hint; be sure that the power supply will drive all available card slots, even if the slots are unused at the start of the design. Most backplanes come with an even number of card slots.
How To Specify a Equipment Chassis; Index page
