S-100 power supply voltage ranges

Andrew Lynch lynchaj at yahoo.com
Thu Mar 5 20:43:33 CST 2009


There is/was an IEEE standard for the S-100 bus; why not hew to that, 
regarding supply voltages?

If I were designing new "retro" boards today, however, I'd go with 
one of the switching regulators that can be had in TO-220 packages.  
Basically the same appearance as the veneered and generated 7805, but 
far more efficient.



Hi Chuck!  Yes, the S-100 crates I have at home all use +8V and 18V/-16V
rails from the linear power supplies.  I understand where the power standard
came from and agree the standard values would be the ideal solution.
However I am working on a low cost small S-100 backplane project and I'd
like to know what flexibility I have with regards to power supplies in order
to keep costs low.  

8V SMPSUs aren't exactly common these days and you'd need at least one of
those and another 18/-18V SMPSU for a S-100 backplane to work properly.
Practically speaking, the linear power supplies are unregulated and generate
a range of voltages probably in the 8-10V range as Allison mentioned.  I am
thinking a 9V and a pair of 15V SMPSUs might be just good enough to work if
not a long term solution.

Since my goal is to keep costs low I am trying to find less expensive
options and still have a useable system.  The intent of the project is a
small bench unit for testing and repair of S-100 boards but not a full blown
system.  I agree with Barry and Dwight to keep the voltage rails just about
3V above the 7805/7812 SCRs output for efficient regulation without
excessive heat.  However, 9V SMPSUs are really common and cheap on eBay and
could be readily converted if not optimal, at least they'd be useable for a
while without destroying anything.  Add a pair of 15V SMPSUs and for less
than $50 you'd have a usable S-100 PS system.  I don't think you could make
a S-100 linear PS for that low cost.  Maybe I could put a high current
silicon diode in line with the 9V SMPSU and drop it to like 8.4V?  That'd be
closer to the 8V standard.

I posted a schematic and PCB layout on the N8VEM wiki but whenever I post
the URL to CCTALK it never makes it to the list.  I suspect it's being spam
filtered or something.  I believe the actual useable ranges of the S-100
unregulated power supply is about 8V to 10V, 15 to 20V, and -15 to -20V.
For current capacity since the PCB is only 4 slots, I am thinking on the
order of 1V per slot should be sufficient.  There are some older S-100
boards that will exceed that limit like those massive 8K SRAM boards but if
you are working on one of those probably not all the slots would be used.

At any rate, I am swamped with my current N8VEM related projects and the
S-100 backplane project is on slow simmer for a while.

Thanks and have a nice day!

Andrew Lynch

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