Replacing MicroVAX II PSU With a Modern PSU

[Robert Jarratt]

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Jon Elson
> Sent: 25 November 2015 16:37
> To: General at classiccmp.org; Discussion at classiccmp.org:On-Topic and Off-
> Topic Posts
> Subject: Re: Replacing MicroVAX II PSU With a Modern PSU
> 
> On 11/25/2015 01:34 AM, Robert Jarratt wrote:
> >
> >> -----Original Message-----
> >> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Jon
> >> Elson
> >> Sent: 25 November 2015 02:05
> >> To: General at classiccmp.org; Discussion at classiccmp.org:On-Topic 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.

Thanks, that should do the job then.

> > For P OK and DC OK I found this:
> http://home.windstream.net/engdahl/powerup_reset_circuit.htm, 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.
> 
> Jon

My reading of the Qbus spec is that P OK and DC OK are active high. That diagram tells me that both control signals are driven the same way because the two circuits are identical except for the timing caps. When the power is OK the output of the inverters is low, so the transistors are off, presumably allowing the signals to float high. When the power is not OK, the inverters are high, turning on the transistors and shorting the signal to ground.

But, I am no expert, and could have interpreted those circuits all wrong. 

Regards

Rob