IBM Personal Computer/AT - Clones

Clones

IBM's efforts to trademark the name AT largely failed, and most 286-based PCs were modeled after it. The label also became a standard term in reference to PCs that used the same type of power supply, case, and motherboard layout as the 5170. Even further, "AT-class" became a term describing any machine which supported the BIOS functions, 16-bit expansion slots, keyboard interface, and other defining technical features of the IBM PC AT; in the case of the expansion slots, the term is largely synonymous with "ISA" (when the latter is not applied as a retronym to XT-class machines, as in the phrase "8-bit ISA slot".) As such, most systems with 486 and Pentium CPUs, and at least some with Pentium Pro and Pentium II processors, were describable as AT-class.

As of 2011, modern PCs still maintain nearly complete backwards compatibility with the PC AT from a software perspective, but AT mechanical and electrical compatibility is extremely rare. The AT power supply pins and its connectors, the AT motherboard form factor, and the physical ISA bus slots are no longer present on modern PCs outside of specialized embedded designs. The ATX standard from Intel has completely replaced the original AT power supply and motherboard design. Modern motherboards do not have physical ISA expansion bus connectors any more, but the bus signals live on in the modern LPC bus for software compatibility. Nearly all PC BIOS ROMs, even modern UEFI based ROMs, include code which is backwards compatible with the original AT BIOS interrupt calls. Even the 0xaa55 signature in the master boot record is still required by many BIOSes to be present on an attached hard disk for it to be recognized as a valid boot device. The PS/2 successor to the AT keyboard interface still survives in the modern market, though it is increasingly being replaced by USB in new systems. The PS/2 keyboard interface is identical to the AT keyboard interface except for the connector: the AT uses a 5-pin DIN connector, while the PS/2 uses a 6-pin mini-DIN.