Daughterboard

In computing, the expansion card, expansion board, adapter card or accessory card is a printed circuit board that can be inserted into an electrical connector, or expansion slot, on a computer motherboard, backplane or riser card to add functionality to a computer system via the expansion bus. In computing, the expansion card, expansion board, adapter card or accessory card is a printed circuit board that can be inserted into an electrical connector, or expansion slot, on a computer motherboard, backplane or riser card to add functionality to a computer system via the expansion bus. An expansion bus is a computer bus which moves information between the internal hardware of a computer system (including the CPU and RAM) and peripheral devices. It is a collection of wires and protocols that allows for the expansion of a computer. Even vacuum-tube based computers had modular construction, but individual functions for peripheral devices filled a cabinet, not just a printed circuit board. Processor, memory and I/O cards became feasible with the development of integrated circuits. Expansion cards allowed a processor system to be adapted to the needs of the user, allowing variations in the type of devices connected, additions to memory, or optional features to the central processor (such as a floating point unit). Minicomputers, starting with the PDP-8, were made of multiple cards, all powered by and communicating through a passive backplane. The first commercial microcomputer to feature expansion slots was the Micral N, in 1973. The first company to establish a de facto standard was the Altair 8800, developed 1974-1975, which later became a multi-manufacturer standard, the S-100 bus. Many of these computers were also passive backplane designs, where all elements of the computer, (processor, memory, and I/O) plugged into a card cage which passively distributed signals and power between the cards. Proprietary bus implementations for systems such as the Apple II co-existed with multi-manufacturer standards. IBM introduced what would retroactively be called the Industry Standard Architecture (ISA) bus with the IBM PC in 1981. At that time, the technology was called the PC bus. The IBM XT, introduced in 1983, used the same bus (with slight exception). The 8-bit PC and XT bus was extended with the introduction of the IBM AT in 1984. This used a second connector for extending the address and data bus over the XT, but was backward compatible; 8-bit cards were still usable in the AT 16-bit slots. Industry Standard Architecture (ISA) became the designation for the IBM AT bus after other types were developed. Users of the ISA bus had to have in-depth knowledge of the hardware they were adding to properly connect the devices, since memory addresses, I/O port addresses, and DMA channels had to be configured by switches or jumpers on the card to match the settings in driver software. IBM's MCA bus, developed for the PS/2 in 1987, was a competitor to ISA, also their design, but fell out of favor due to the ISA's industry-wide acceptance and IBM's licensing of MCA. EISA, the 32-bit extended version of ISA championed by Compaq, was used on some PC motherboards until 1997, when Microsoft declared it a 'legacy' subsystem in the PC 97 industry white-paper. Proprietary local buses (q.v. Compaq) and then the VESA Local Bus Standard, were late 1980s expansion buses that were tied but not exclusive to the 80386 and 80486 CPU bus. The PC/104 bus is an embedded bus that copies the ISA bus. Intel launched their PCI bus chipsets along with the P5-based Pentium CPUs in 1993. The PCI bus was introduced in 1991 as a replacement for ISA. The standard (now at version 3.0) is found on PC motherboards to this day. The PCI standard supports bus bridging: as many as ten daisy chained PCI buses have been tested. Cardbus, using the PCMCIA connector, is a PCI format that attaches peripherals to the Host PCI Bus via PCI to PCI Bridge. Cardbus is being supplanted by ExpressCard format. Intel introduced the AGP bus in 1997 as a dedicated video acceleration solution. AGP devices are logically attached to the PCI bus over a PCI-to-PCI bridge. Though termed a bus, AGP usually supports only a single card at a time (Legacy BIOS support issues). From 2005 PCI-Express has been replacing both PCI and AGP. This standard, approved in 2004, implements the logical PCI protocol over a serial communication interface. PC/104(-Plus) or Mini PCI are often added for expansion on small form factor boards such as Mini-ITX.