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From: ericksen@unc.UUCP (James P. Ericksen)
Newsgroups: net.analog
Subject: Re: Any 555 mavens or PUT weenies out there?
Message-ID: <46@unc.UUCP>
Date: Wed, 17-Apr-85 19:15:17 EST
Article-I.D.: unc.46
Posted: Wed Apr 17 19:15:17 1985
Date-Received: Fri, 19-Apr-85 00:52:47 EST
References: <469@umd5.UUCP>, <396@psivax.UUCP>
Organization: CS Dept., U. of N. Carolina at Chapel Hill
Lines: 44


>  The problem is that the only power supply available
>on the game paddle connector is +5 - and I'd rather not kluge in the Apple's
>two 12 volt supplies on the unused pins or run more wires.  I need a nominal
>amount of + and - 12 for the TTL-RS232 converters (MC1488 or MC1489) and for
>some strapping for DTR type stuff.

>> You can run an RS232 with the voltages +5 and -5.  You already have the +5.
>> In order to make the -5 you need to get an Intersil 7660 (negative voltage
>> generator).  I have heard that this part is available at such places as
>> Radio Shack.  This chip and two capacitors is all that is required to 
>> generate the -5 voltage.


Good idea.  One word of warning though ... the MC1488 RS232 driver will NOT work
with a +5 volt supply (it requires at least +9 volts, an undocumented fact i
learned the hard way).  The easiest way to drive an RS232 line with +/- 5 volt
supplies is to use a PNP transistor:


                         +5v
                          |
                          \
                          / 3.3k            +5v
                          \                  |
                          /                  |
                          |                  /
                          |      3.3k      |L
                TTL  -----------/\/\/------|
                                           |\
                                             \
                                             |
                                             |------------ RS232
                                             |
                                             /
                                             \  330 ohms
                                             /
                                             \
                                             |
                                            -5v

Jim Ericksen
Dept of Comp Sci
UNC Chapel Hill