Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!cica!ctrsol!ginosko!uunet!portal!cup.portal.com!bcase
From: bcase@cup.portal.com (Brian bcase Case)
Newsgroups: comp.sys.mac
Subject: Re: 5 Volts from Mac Serial Ports?
Message-ID: <20657@cup.portal.com>
Date: 21 Jul 89 18:17:43 GMT
References: <36400002@hpindwa.HP.COM>
Organization: The Portal System (TM)
Lines: 20

>Do any of you EE-types know
>exactly what is being done on the board that extracts 5 volts from the serial
>ports?  Or is this all proprietary?  I'd really rather not have to use an 
>external power supply!

In the MIDI interface:

The way it is done is pretty clever:  the power comes from the serial
lines by using diodes to steer the current from the wires to the power
terminals of the few chips correctly no matter what is the polarity of
the serial lines.  The key is the realization that the serial lines
are differential:  if one is high, the other is low.  Thus, two diodes
per polarity will guarantee that current is always flowing, *except*
when the polarity is changing.  A cap will smooth out the pulses.

Obviously, this scheme will only supply power to a couple ICs at most.

A friend and I figured this out by looking at the MIDI circuit board;
there are no schematics of which I know.  If you can get them, that
would make figuring it out easier.