Here is a picture of signals between motherboard and rtc clock. RTC uses Intel timing protocol. Pulse duration is about 1µs and time between pulses are about 24µs. AS means Address Select. It functions like clock signal. When DS is low then motherboard wants to read rtc and when RW is low motherboard wants to write to rtc memory. Data protocol is same as generic RAM memory. All this is valid only if MOT pin is connected low. Otherwise Motorola timing is used and it is completely different.
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.