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A single-board computer (SBC) is a complete computer built on a single circuit board, with microprocessor(s), memory, input/output (I/O) and other features required of a functional computer. Single-board computers were made as demonstration or development systems, for educational systems, or for use as embedded computer controllers. Many types of home computer or portable computer integrated all their functions onto a single printed circuit board.
Unlike a desktop personal computer, single board computers often did not rely on expansion slots for peripheral functions or expansion. Some single-board computers are made to plug into a backplane for system expansion. Single board computers have been built using a wide range of microprocessors. Simple designs, such as built by computer hobbyists, often use static RAM and low-cost eight or 16 bit processors. Other types, such as blade servers, include all the memory and processor performance of a server computer in a compact space-saving format.
The first true single-board computer (see the May 1976 issue of Radio-Electronics) called the "dyna-micro" was based on the Intel C8080A, and also used Intel's first EPROM, the C1702A. The dyna-micro was re-branded by E&L Instruments of Derby, CT in 1976 as the "MMD-1" (Mini-Micro Designer 1) and was made famous as the example microcomputer in the very popular 8080 "BugBook" series of the time. SBCs also figured heavily in the early history of home computers, for example in the Acorn Electron and the BBC Micro. Other typical early single board computers like the KIM-1 were often shipped without enclosure, which had to be added by the owner, other examples are the Ferguson Big Board and the Nascom. With the development of PCs there was a sharp shift away from SBC, with computers being constructed from a motherboard, with functions like serial ports, disk drive controller and graphics being provided on daughterboards. The recent availability of advanced chip sets providing most of the I/O features as embedded components allows motherboard manufacturers to offer motherboards with I/O traditionally provided by daughterboards. Most PC motherboards now offer on-board support for disk drives including IDE and SATA with RAID, graphics, Ethernet, and traditional I/O such as serial and parallel ports, USB, and keyboard/mouse support. Plug-in cards are now more commonly high performance graphics cards (really graphics co-processors), high end RAID controllers, and specialized I/O cards such as data acquisition and DSP (Digital Signal Processor) boards.
Single board computers were made possible by increasing density of integrated circuits. A single-board configuration reduces a system's overall cost, by reducing the number of circuit boards required, and by eliminating connectors and bus driver circuits that would otherwise be used. By putting all the functions on one board, a smaller overall system can be obtained, for example, as in notebook computers. Connectors are a frequent source of reliability problems, so a single-board system eliminates these problems. 
Single board computers are now commonly defined across two distinct architectures: no slots and slot support.
Embedded SBCs are units providing all the required I/O with no provision for plug-in cards. Applications are typically gaming (slot machines, video poker), kiosk, and machine control. Embedded SBCs are much smaller than the ATX-type motherboard found in PCs, and provide an I/O mix more targeted to an industrial application, such as on-board digital and analog I/O, on-board bootable flash memory (eliminating the need for a disk drive), no video, etc.
The term "Single Board Computer" now generally applies to an architecture where the single board computer is plugged into a backplane to provide for I/O cards. In the case of PC104, the bus is not a backplane in the traditional sense but is a series of pin connectors allowing I/O boards to be stacked.
Single board computers are most commonly used in industrial situations where they are used in rackmount format for process control or embedded within other devices to provide control and interfacing. Because of the very high levels of integration, reduced component counts and reduced connector counts, SBCs are often smaller, lighter, more power efficient and more reliable than comparable multi-board computers.
The primary advantage of an ATX motherboard as compared to an SBC is cost. Motherboards are manufactured by the millions for the consumer and office markets allowing tremendous economies of scale. Single Board Computers, on the other hand, are in a specialized market niche and are manufactured in much smaller numbers with the resultant higher cost. Motherboards and SBCs now offer similar levels of feature integration meaning that a motherboard failure in either standard will require equivalent replacement.
The primary advantage of a PICMG Single Board Computer is the availability of backplanes offering virtually any slot configuration including legacy ISA support. Motherboards tend to the latest slot technology such that PCI slots are becoming legacy support with PCI Express becoming the standard. In addition, motherboards offer, at most, 7 slots while backplanes can offer up to 20 slots. In a backplane 12.3" wide, similar in size to an ATX motherboard at 12", a backplane with a Single Board Computer can offer 12 slots for I/O cards with virtually any mix of slot types.
One common variety of single board computer uses standardized form factors intended for use in a backplane enclosure. Some of these types are CompactPCI, PXI, VMEbus, VXI, and PICMG. 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 have connectors which allow a stack of circuit boards, each containing expansion hardware, to be assembled without 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. 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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