The AT-format ( English a dvanced T echnology "advanced technology") is a form factor for housing and motherboards of IBM PC - compatible personal computers . The AT form factor is the predecessor of ATX format and the successor to the XT format (engl. E x tended t echnolog y). XT and AT formats were developed by IBM .
This format was introduced with the IBM PC AT .
A form factor is a standard of the size of the motherboard, hole spacing up to standards of the power supply. This also determines the position of the components.
There is the normal AT format and the later Baby AT format (BAT). BAT is more compact in shape, but fits into the same housing.
|Dimensions||Size in mm||Size in inches|
|AT motherboard||305 × 279… 330||12 × 11… 13|
|Baby AT motherboard||216 × 204 ... 330||8.5 × 8… 13|
|8-bit ISA plug-in card||107 x 333.5 x 12.7||4.2 x 13.13 x 0.5|
|16-bit ISA plug-in card||122 x 333.5 x 12.7||4.8 x 13.13 x 0.5|
|EISA plug-in card||127 x 333.5 x 12.7||5 x 13.13 x 0.5|
The AT-class computers have been known by the Intel 80286 - processor (or its successor, 80386 , 80486 , Pentium , ...) and by 16-bit ISA - slots for expansion cards . The 16-bit ISA bus is therefore often called the AT bus, the 8-bit ISA bus was called the XT bus.
Main boards in AT or BAT format also exist with slots for other bus systems. The usual combinations are 16-bit ISA slots + VLB slots, 16-bit ISA slots + PCI slots (i486 of the last generation and newer) and, as the last representatives of their kind, slot 1 and super socket 7 motherboards with 16-bit ISA slots + PCI slots + AGP slot. Main boards with extended ISA slots were also available in AT format .
The off switch of the AT housing was looped directly into the 120 or 230 V supply line of the PC power supply unit and not, as with the ATX format, connected to the motherboard. The computer could therefore not be switched on or off via software. Unlike later PCs, the devices did not draw any power when they were switched off. A software- controlled standby mode was still possible, but only became common with the introduction of the Advanced Power Management (APM) energy-saving standard, which was only introduced at the time of the ATX. Before that, AT computers had proprietary standby / suspend-to-RAM modes, some with their own standby button on the AT housing. The wake-up from standby / soft-off via PCI card was only introduced with the ATX standard 1.0.
In the first PCs, the power switch was located on the back or on the side of the power pack. With later housing types it was attached to the front for reasons of convenience. To do this, a cable carrying mains voltage had to be routed across the PC case and the protective conductor connected to the front of the PC case. The cables and switches used for this were neither standardized in terms of their electrical wiring nor their mechanical design, which made it difficult to replace the power supply unit.