ATX Power Supply
The power supply I chose for my PDP-11 was the Athena Power AP-U1ATX70 700W power supply. I wanted a small form factor unit that could be attached to the card cage and not be too bulky. The +5 VDC output on this particular unit is rated at 25 amps, which should be just enough to handle a full card cage. This unit replaced a smaller 350W power supply that struggled to handle half the system configuration. The new unit runs much cooler and quieter.
The old power supply came with an adapter plug that I modified to fit the backplane power terminals. This provides a convenient disconnect point for the power supply.
The power supply is designed from an older ATX12V specification, with a 20-pin connector and a -5 VDC output. It was designed to go in a 1 RU server. The following table describes how I have it wired to the system.
| Pin | Signal | Color | Use | Connection |
|---|---|---|---|---|
| 1 | +3.3VDC | Orange | +3.3 VDC Supply | Not used. |
| 2 | +3.3VDC | Orange | +3.3 VDC Supply | Not used. |
| 3 | COM | Black | Ground | Not used. |
| 4 | +5VDC | Red | +5 VDC Supply | Connected to backplane +5V terminal. |
| 5 | COM | Black | Ground | Not used. |
| 6 | +5VDC | Red | +5 VDC Supply | Connected to backplane +5V terminal. |
| 7 | COM | Black | Ground | Connected to the front panel control board. |
| 8 | PWR_OK | Gray | "Power Good" Signal | When PS_ON# is activated, this signal becomes active 100-500ms after all voltages are above the under-voltage threshold. Connected to the front panel control board. |
| 9 | +5VSB | Purple | +5 VDC Standby Supply | This output becomes active when AC power is applied, and can deliver a couple of amps. Connected to the front panel control board to power its logic. It also supplies the signal to the backplane BPOK H line. |
| 10 | +12VDC | Yellow | +12 VDC Supply | Connected to the backplane +12V terminal. |
| 11 | +3.3VDC | Orange | +3.3 VDC Supply | Not used. |
| 12 | -12VDC | Blue | -12 VDC Supply | Connected to the backplane -12V terminal. |
| 13 | COM | Black | Ground | Connected to the first backplane GND terminal. |
| 14 | PS_ON# | Green | Remote Power Control | This input has a pull-up and debounce circuit. Simply grounding this input turns the power supply on. Connected to the DC ON/OFF switch of front panel control board. |
| 15 | COM | Black | Ground | Connected to the first backplane GND terminal. |
| 16 | COM | Black | Ground | Connected to the second backplane GND terminal. |
| 17 | COM | Black | Ground | Connected to the second backplane GND terminal. |
| 18 | -5VDC | White | -5 VDC Supply | Not used. |
| 19 | +5VDC | Red | +5 VDC Supply | Connected to the backplane +5V terminal. |
| 20 | +5VDC | Red | +5 VDC Supply | Connected to backplane +5V terminal. |
The photo below is of the backplane and card cage, with the old power supply installed. I plan to update the photos of the new configuration soon. The power harness is tie-wrapped to the cable management bars on the back of the cable guide. The green connector terminates the power supply control signals for the front panel board.
The power supply is tie-wrapped to the card cage. Not an elegant solution, but it does the job. Someday I may fabricate a mounting bracket that will allow the power supply to be properly attached to the card cage.
The power cord plugs into the front. Not ideal, but the rest of the cables exit from the front, as well.
References
- ATX / ATX12V Power Supply Design Guide, Version 1.1, August 2000, Intel Corporation
Additional Resources
- LSI-11, PDP-11/03 User's Manual, 3rd Edition, May 1976 (EK-LSI11-TM-003)
