I don't have the pinouts handy, but a web search should pull them up.
Find the pinouts for both the A1200 power connector and the ATX power connector. Now just match up voltage to voltage, ground to ground and power good to power good. The ATX has 2 pins that need bridged to start it up, the pinout will show it.
Pretty easy and straight forward. I've converted PC power supplies for A500, 3000 and 4000 with no problems.
