David Drummond wrote in a message to Roy J. Tellason:

 DD>> How do you achieve the -12V? You'd have to set up your 12 volters
 DD>> to produce 24V overall, with the centre tap as zero....

 RJT> You'd achieve it by doing it the way they do it now.  

 DD> They filter etc AC, this can be configured to swing negative of a
 DD> centre tap each cycle. DC just stays positive.

 RJT> With a transformer,  and appropriate rectifiers and filtering and 
 RJT> regulator...

 DD> Last time I looked, transformers and rectifiers didn't do much on
 DD> DC.... Sounds like you need an inverter..

You'd need one anyway,  the lower voltages used are used at current levels
that are substantial enough to make any sort of a linear regulator *very*
inefficient -- you'd waste more power as heat than would be delivered at
the output.

 RJT> There would be no difference on the output side -- it's only the 
 RJT> input side where things would be very different.

 RJT> How much stuff uses that -12 anyway?

 DD> The RS232 port.

And I can't think of much else,  either.  It's possible to make ports that
don't need a -12 supply,  using chips like the MAX232 if I'm remembering
that number right.  Dunno what if anything uses the -5 these days either...

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