AlphaServer CS20 (and likely DS20L) power supply pinout and docs for replacement

[Tobias Nygren <tnn%NetBSD.org@localhost>]

I'll drop this note here for archival purposes, in case there are other
people out there who have dead CS20 machines. This document can be used
as a reference to build an ATX power supply adapter.

Overview:

The power supply is a single rail 12V supply rated at 40A peak.
The clamp meter says idle current is approximately 13A after POST.
Contained inside is an ADM9240A environmental monitoring chip on SMBUS.

Pins marked with (*) are mandatory for successful POST.
All others are optional and affect SRM "show power" output only.

I/O connector pinout (fixed width font):

          +--+--+
    G_LED | 1| 2| R_LED
    F_LED | 3| 4| Y_LED
   GND(*) | 5| 6| N/C
SMBUS SCL | 7| 8| SMBUS SDA
  PWR_OK? | 9|10| SENSE
   GND(*) |11|12| N/C
      N/C |13|14| TACH
PWR_SW(*) |15|16| PS_ON(*)
          +--+--+

1 RG LED green anode, driven by MB
2 RG LED red anode, driven by MB
3 Red (fault?) LED anode, driven by MB
4 Yellow LED anode
5 ground(*)
6 not connected
7 SMBUS SCL
8 SMBUS SDA
9 PWR_OK? (Unused. Leave N/C if using ATX power supply)
10 Sense return signal? (Leave N/C if using ATX power supply)
11 ground(*)
12 not connected
13 not connected
14 I/O fan tachometer signal: 266Hz, 50% duty cycle, open collector. 5V pullup provided by MB
15 PWR_SW(*) physical switch on PSU. Momentarily short this pin to ground on motherboard to start server. 5V pullup by MB.
16 PS_ON(*) MB pulls low to turn on power.

Note: a ceramic botch cap is soldered between pin 10 and 11 on the motherboard (probably for sense stability?)

Power connector pinout (fixed width font):

       +--+--+
  +12V |8 | 1| +12V
  +12V |9 | 2| +12V
  +12V |10| 3| +12V
 +5VSB |11| 4| GND
   GND |12| 5| GND
   GND |13| 6| GND
   GND |14| 7| GND
       +--+--+

-Tobias