imsai 2 - test-drb@ccmp.vtda.org

17 Aug 1999

I'm not sure that "jumpering across" the 7805 will be healthy for the 7805.
Of course, if the input doesn't drop below the output during shutdown this
may not be a concern.  However, if there's a significant off-board load on
the input switching supply, and significant capacitance (bypass, for
example) on the board, it's likely to cause problems.

Dick

-----Original Message-----
From: CLASSICCMP(a)trailing-edge.com &lt;CLASSICCMP(a)trailing-edge.com&gt;
To: Discussion re-collecting of classic computers
&lt;classiccmp(a)u.washington.edu&gt;
Date: Tuesday, August 17, 1999 4:20 PM
Subject: Re: imsai 2

...
  What puzzles me
is why the IMSAI folks decided to use a switching power
supply when the box and everything else already supported the needs of the
S-100 with the previously available and now quite inexpensive unregulated
supplies of yesteryear. 
Probably an issue of economy, using PC-clone power supplies at about
$25 each vs using a custom-wound transformer at $200 or so a pop.

>  One of the main benefits of the S-100 was that it
>had on-board regulation, so that if you didn't need a given supply, you
>didn't have to bring it on board and regulate it, dissipating power as you
>went.  If the new box is capable of running the original boards, it must
>provide the raw 8 and +/- 16-volt supplies.  Where's the benefit in having
a
...
 >switching regulator sitting in the back of the box?
 I suppose it creates a
...
  market for a
power distribution module to put +5 and +/- 12 on each board
which needs it rather than using the on-board regulators, but that opens
another can of worms.  What's the "right" way to distribute it without
tampering with a historically correct board? 
According to what I read from the web page, there's a PC-clone switching
power supply followed by a "boost" switching circuit to get back to
+8 and +/- 16.  That's not completely unreasonable, but it sounds a
little bit fishy to me in the sketchiness of the details.

Several S-100 manufacturers who were still at it in the mid-80's had
regulated +5V on the S-100 backplane.  Those who wanted to use their
older S-100 cards in such a machine just jumpered across the 7805's and
viola!

--
 Tim Shoppa                        Email: shoppa(a)trailing-edge.com
 Trailing Edge Technology          WWW:   http://www.trailing-edge.com/
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