Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!husc6!sunfs3!kent
From: kent@sunfs3.camex.uucp (Kent Borg)
Newsgroups: comp.sys.mac
Subject: Re: 5 Volts from Mac Serial Ports?
Message-ID: <449@sunfs3.camex.uucp>
Date: 24 Jul 89 15:40:21 GMT
References: <36400002@hpindwa.HP.COM> <20657@cup.portal.com>
Reply-To: kent@sunfs3.UUCP (Kent Borg)
Organization: Camex, Inc., Boston, Mass USA
Lines: 32

In article <20657@cup.portal.com> bcase@cup.portal.com (Brian bcase Case) writes:
>>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!
>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.

I think it might be too clever.

First, don't expect to get much power out of the serial port no matter
how you do it.  

Second, do it the way (I think) MacRecorder does: Pull a few mA's off
of pin #1.  Tell the SCC to assert *DTR (an excersise left to the
reader).  This gets inverted and sent through a few dozen ohms of rf
filtering to become Output Handshake (See Inside Macintosh Vol. IV, p.
248).  

Viola!  No filtering or rectifying required.  Now, maybe you can get a
bit more power by rectifying the data lines, but if you only need a
little power for a couple CMOS chips, try the simpler way first.

Kent Borg
kent@lloyd.uucp
or
...!husc6!lloyd!kent