Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!usc!zaphod.mps.ohio-state.edu!mips!daver!wombat!george
From: george@wombat.UUCP (George Scolaro)
Newsgroups: sci.electronics
Subject: Re: Help Needed: TTL<->RS232 without -12vdc
Keywords: RS232, TTL, level shifting
Message-ID: <1164@wombat.UUCP>
Date: 26 Aug 90 18:13:42 GMT
References: <7574@scolex.sco.COM> <1990Aug25.232127.4269@hayes.fai.alaska.edu> <28835@netnews.upenn.edu>
Reply-To: george@wombat.UUCP (George Scolaro)
Organization: Assn. for the prevention of Polar Bears and Kangaroos
Lines: 32

In article <7574@scolex.sco.COM>, deanr@sco.COM (Dean Reece) writes...
>
>Hi, I need some help with RS232 drivers/receivers
>I'm building a project that needs a standard RS232 port, but only has
>available supplies of +5, +12, and +24 volts.  I thought that I
>might be able to use the 0(GND), +12, and +24 kinda like the +/- 12v supply
>usually called for.  I couldn't think of any trivial way to do this,

One method, which I have used successfully is to use a chip like the LT1054
(made my Linear Technology, T.I. and probably others). It is an 8 pin
minidip, add 2 caps and wire it for inversion. Feed in +12 and out comes -12
(less a small drop). It will supply up to 100ma. 

In reasonable volumes, the LT1054 + 14c88 + 14c89 comes in at about the same
cost as a MAX232. The main point though is that the MAX232 does not have
enough drive (+12/-12) to transmit RS232 on long cables at high baud rate.
On a test using 1000ft of 6 wire flat telephone cable we were <9600 baud to
get reliable data transmission with the max232, but with the 14c88/14c89
combo we were up at 38400 baud. Observing the output levels on the end of
the cable using a max232 as the driver showed the signal looking very
'rounded' and not reaching +/-3V. The 14c88/14c89 though, still exceeded
+/-3V at 38400.

As an alternative to the max232 (if you need 3 tx and 3 rx) is Motorola's
recent MC145407, which has +5V only operation (with external caps). It
has sufficient drive to run itself and one MC145406 (3 tx and 3 rx). These
are CMOS parts.

regards,
-- 
George Scolaro
george@wombat.bungi.com                [37 20 51 N / 122 03 07 W]