Expansion Card - History

History

The first commercial microcomputer to feature expansion slots was the Micral N, in 1973. The first to establish a de facto standard was the Altair 8800, developed 1974-1975. Initially, bus implementations were proprietary (such as the Apple II and Macintosh), but by the late 1970s manufacturers of Intel 8080/Zilog Z80-based computers running CP/M had settled around the S-100 standard. IBM introduced the Industry Standard Architecture (ISA) bus, with the IBM PC in 1981; it was then called the PC bus. The IBM XT, introduced in 1983, used the same bus (with slight exception). XT (a.k.a. 8-bit ISA) was replaced with ISA (a.k.a. 16-bit ISA), originally known as AT bus, in 1984. 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 closed 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 replacement for ISA. The standard (now at version 3.0) is found on PC motherboards to this day. The PCI standard supports 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.

The USB format has become a de facto expansion bus standard especially for laptop computers. All the functions of add-in card slots can currently be duplicated by USB, including Video, networking, storage and audio. USB 2.0 is currently part of the ExpressCard interface and USB 3.0 is part of the ExpressCard 2.0 standard.

FireWire or IEEE 1394 is a serial expansion bus originally promoted by Apple Inc. Often used for storage and video cameras, it has application for networking, video, and audio. It has seen some adoption amongst PC manufacturers also.

After the S-100 bus, this article mentions only buses used on IBM-compatible/Windows-Intel PCs. Most other computer lines that were not IBM compatible, including those from Apple Inc.(Apple II, Macintosh), Tandy, Commodore, Amiga, and Atari, offered their own expansion buses. Apple used a proprietary system with seven 50-pin-slots for Apple II peripheral cards, then later used the NuBus for its Macintosh series until 1995, at which time they switched to a standard PCI Bus. Generally PCI expansion cards will function on any CPU platform if there is a software driver for that type. PCI video cards and other cards that contain a BIOS are problematic, although video cards conforming to VESA Standards may be used for secondary monitors. DEC Alpha, IBM PowerPC, and NEC MIPS workstations used PCI bus connectors.

Even many video game consoles, such as the Sega Genesis, included expansion buses; at least in the case of the Genesis, the expansion bus was proprietary, and in fact the cartridge slots of many cartridge based consoles (not including the Atari 2600) would qualify as expansion buses, as they exposed both read and write capabilities of the system's internal bus. However, the expansion modules attached to these interfaces, though functionally the same as expansion cards, are not technically expansion cards, due to their physical form.

For their 1000 EX and 1000 HX models, Tandy Computer designed the PLUS expansion interface, an adaptation of the XT-bus supporting cards of a smaller form factor. Because it is electrically compatible with the XT bus (a.k.a. 8-bit ISA or XT-ISA), a passive adapter can be made to connect XT cards to a PLUS expansion connector. Another feature of PLUS cards is that they are stackable. Another bus that offered stackable expansion modules was the "sidecar" bus used by the IBM PCjr. This may have been electrically the same as or similar to the XT bus; it most certainly had some similarities since both essentially exposed the 8088 CPU's address and data buses, with some buffering and latching, the addition of interrupts and DMA provided by Intel add-on chips, and a few system fault detection lines (Power Good, Memory Check, I/O Channel Check). Again, PCjr sidecars are not technically expansion cards, but expansion modules, with the only difference being that the sidecar is an expansion card enclosed in a plastic box (with holes exposing the connectors).