AT standard compliant power lines are +12V, +5V, -12V and -5V. Positive voltages are easy to supply with normal mains adapter, but negative ones are harder to get. Below is schematic of my power supply. It is tested and works ok. Motherboard will start normally.
Photo of the same thing. Other adapter will transform ATX connector to AT P8 and P9 connectors.
-12V is mainly used in serial communication. -5V seems to be used in display adapter board in some sort of op-amp kind of thingy. 🙂 Display is very dim without it.