Pinouts
-------
Pin # Function
----- --------
1 +12V
2 +5V
3 +5V
4 +5V
5 +5V
6 +5V
7 Ground
8 Ground
9 Ground
10 Ground
11 Ground
12 Ground
13 N/C
14 -12V
15 /PFW
The Power Fail circuitry in the Macintosh II power supply senses a level
transition from the logic board, initiated by pressing either the button
on the back of the machine or the reset key on the ADB keyboard. If this
transition does not occur, the power-up sequence does not start. If a
steady voltage is applied to the Power Fail signal line into the power
supply, the sequence will not start. The power supply must see the
transition to begin the power-up cycle. A modified power supply that
would power up with a voltage applied steadily, instead of level-shifted,
would work in this case.
When the Reset key is pressed, pin 4 (Ground) of the ADB port is
connected to pin 2 (PwrOn*) of the ADB port through a 1N914 diode. This
applies a ground to the input of a CMOS chip on the logic board, which
turns on a transistor and applies approximately +6VDC to the /PFW signal
line (Power Fail Warning*), pin 15. This level shift on the PFW* signal
line initiates the power-up sequence in the power supply. Once the power
supply comes up, +5VDC is applied through another diode to the same line
to keep the power supply up.
A power-fail-type circuit is necessary in the current power supply,
because there is a thermal sensor on the Macintosh II logic board, which
shuts down the system in the event of too-high temperatures. The same
circuit shuts off the power supply in response to the Shut Down menu
command.
There is also logic in the power supply that generates a Power Fail
Warning signal to the system in the event of an AC input voltage failure.
Additionally, if AC power is removed from the system, the power fail
circuit pulls /PFW low at least 2 ms before the DC outputs fail. There is
further information on the operational restrictions of the power fail
circuitry in "Designing Cards and Drivers for the Macintosh II and
Macintosh SE" from Addison Wesley.