Replacing MicroVAX II PSU With a Modern PSU

Jon Elson elson at
Wed Nov 25 10:36:53 CST 2015

On 11/25/2015 01:34 AM, Robert Jarratt wrote:
>> -----Original Message-----
>> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Jon Elson
>> Sent: 25 November 2015 02:05
>> To: General at; Discussion at and Off-
>> Topic Posts
>> Subject: Re: Replacing MicroVAX II PSU With a Modern PSU
>> On 11/24/2015 05:52 PM, Mark Wickens wrote:
>>> I haven't looked into this at all and I suspect it's probably quite
>>> tricky indeed. I did look around a while back drew a blank.
>> I built up a Micro-VAX II system out of boards, backplanes and assorted junk.  I
>> made my own power supply.  The power supply is actually easy, just +5 and
>> +12, with a touch of
>> -12 for serial I/O.
>> The only tricky thing is the power-OK and reset logic, which really wasn't all
>> that tricky, either.
>> I had a power and thermal safety control panel on it repurposed from the
>> original 3rd party IBM mainframe memory box that would shut it down if there
>> was an abnormal voltage or cooling failure.
>> Jon
> Thanks Jon. I think standard PC PSUs only do +5 and +12, so how did you do -12?
I did not use PC power supplies, I used a couple industrial 
power supplies.  I built this system in 1986.
But, generic PC power supplies DO have a little bit of -12 V 
for serial cards.  The standard PCI connector
has a -12 V pin.  PCIe seems to have dropped the -12V supply.
> For P OK and DC OK I found this:, but just for test purposes I assume I could connect the +5 direct to P OK and DC OK.
BPOK needs to pulse low after power is stable, BDCOK should 
be high when power is OK.  Or, at least, that's what I got 
from the drawing you linked to.


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