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Thread: Ac lo, dc lo

  1. #11


    Tanks a lot for the answers. It is now perfectly clear what is behind this signals .

  2. #12
    Join Date
    Jun 2012
    UK - Worcester


    On my 11/45 the AC LO and DC LO signals (note the plurality) are generated purely by the power supply units themselves and (as has been previously stated) drive the PDP-11/45 into taking immediate action (such as saving the registers into non-volatile storage) in response to the impending power failure.

    Within each of the regulator racks is a separate power control board. It is this unit that produces the AC LO and DC LO signals. Because there are two separate power supply racks - there are two sets of separate signals. Each power control board can produce separate outputs. The AC input voltage (from the step down transformer) is rectified and a very low valued electrolytic capacitor (20 uF) provides a degree of smothing. The voltage of this rail is then compared to a set of presets (I can't remember the exact voltage threshold) and it is this that is used to drive the AC LO and DC LO signals (open collector).

    The switched mode regulators themselves are separe entities fed from separate low-voltage ac windings off the transformer. These regulators are not monitored. Hence, a switched mode regulator can fail - but the DC LO signal never operates - i.e. the AC LO and DC LO signals indicate impending incoming manins power failure and not necessarily a fault within the PDP-11.



  3. #13


    basically I must have discovered something that was just a coincidence to cause the DC LO value to halve, I believe there are much better answers than mine the followed what I wrote, please consider what I wrote to be observations and not facts.
    @ BillDeg:
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  4. #14


    For people who want more about AC LO and DC LO I found in EK-BA11L-TM-001_Oct77.pdf the explanation about H777 Power supply. (Figure 3.25 on page 3.43) Details are given and schematics are explained. But nothing about component values , I will search H777 Schematics.


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