Types, Standards
Currently the most common variety of Single Board Computer in use is of a specific form factor similar to other full-size plug-in cards and is intended to be used in a backplane. Some architectures are dependent entirely on single-board computers, such as CompactPCI, PXI, VMEbus, VXI, PICMG architecture, etc. SBCs have been built around various internal processing structures including the Intel Architecture, Multiprocessing architectures, and lower power processing systems like RISC and SPARC. In the Intel PC world, the intelligence and interface/control circuitry is placed on a plug-in board that is then inserted into a passive (or active) backplane. The end result is similar to having a system built with a motherboard, except that the backplane determines the slot configuration. Backplanes are available with a mix of slots (ISA, PCI, PCIX, PCI-Express, etc.), usually totaling 20 or fewer, meaning it will fit in a 19" rackmount enclosure (17" wide chassis).
Some single-board computers also exist as form factors that stack like building blocks, and do not have the form of a traditional backplane. Examples of stacking SBC form factors include PC/104, PC/104-Plus, PCI-104, EPIC, and EBX; these systems are commonly available for use in embedded control systems.
PICMG provides standards for the backplane interface: PICMG 1.0, 1.1 and 1.2 provide for ISA and PCI support with 1.2 adding PCIX support. PICMG 1.3 provides for PCI-Express support. Single Board Computers meeting the PICMG 1.3 specification are referred to as a System Host Board (SHB).
Stack-type SBCs often have memory provided on plug-cards such as SIMMs and DIMMs, however they can still be regarded as SBCs because although the memory modules are technically additional circuit boards, they have no extra functionality beyond providing memory and are basically just carriers for the RAM chips. Hard drive circuit boards are also not counted for determining if a computer is an SBC or not for two reasons, firstly because the HDD is regarded as a single block storage unit, and secondly because the SBC may not require a hard drive at all as most can be booted from their network connections.
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