|The AT format is quite old. When you look at the back side of an AT motherboard, usually the only port available is for the keyboard. And the AT keyboard port is totally different than the PS/2 keyboard port. Everything else - mouse, COM ports, printer ports, integrated video, integrated sound, USB (if the board even supports it) - is connected to the board via cables & has to be somehow attached to the case.|
The AT power supply is totally different as well. It connects to the board with two side-by-side 6-pin connectors & has an on/off switch which is hard-wired. AT PSU's generally load the amperage on the +5v rail because that's where older CPUs got their power. Modern CPUs rely on the +12v rail.