Hardware

AT motherboards were old version of IBM, the rear connection layout were all the same for the keyboards. The IDE , 5 pin DIN, PS2, USB , floppy etc are all lined up behind, so in this case it is crowded, specially on additional cards. The power supply sucks the air from the inside and goes out, this makes the cpu and the motherboard hot. In order to remedy the situation, they developed the ATX. The IDE and other connectors where redesigned and repositioned, so the rear is widely open for additional cards and other small connectors,(PS2, USB)bigger power supply, the air goes in and blows to the CPU and mobo, so it is cooler. ATX was developed by Intel for PII system. Now it is the standard mobo design.

one of the biggest differences is soft power on. AT mainboards do not have soft power meaning that there is always power to the mainboard where on AT power off literally is off. No juice to the board.

There are 3 "general" styles of motherboards.

AT, ATX, and Combo.

AT:
Full size 5 pin DIN keyboard connector
Peripherals may not be on the motherboard. If they are on-board, header connectors are used with ribbon cables that go to the connectors that mount on the case.
Single row 12 pin power connector. 2 6 pin connectors plug into the 12 pin header from the power supply.
Power switch switches the AC main power to the power supply. When the power switch is OFF the computer is fully off.

AT systems started with the 286 CPU.

ATX:
Mini-DIN (PS/2) connectors for keyboard and mouse.
I/O connector block
Double row 20 pin power connector
Power supply is ALWAYS part way on. 5 volt "standby" power is always on the motherboard.
The power switch is a momentary push button that in effect sends a signal to the power control circuit on the motherboard. Telling the motherboard to turn the power on.

Started to show up on "high-end" 486 systems.

Combo boards:
These have the Micro AT form factor including the full size DIN keyboard connector.
Most combo boards have a header for a PS/2 mouse connector. Some also supported PS/2 keyboard connectors.
A "true" combo board has both the AT and ATX power connectors.

Chas

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