Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!uunet!sco!deanr
From: deanr@sco.COM (Dean Reece)
Newsgroups: sci.electronics
Subject: Help Needed: TTL<->RS232 without -12vdc
Keywords: RS232, TTL, level shifting
Message-ID: <7574@scolex.sco.COM>
Date: 25 Aug 90 00:01:30 GMT
Sender: news@sco.COM
Reply-To: deanr@sco.COM (Dean Reece)
Followup-To: sci.electronics
Organization: The Santa Cruz Operation, Inc.
Lines: 25


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,
however.  If I can't get it really simple, I'll probably just use a
MAX232 and not bother, but I've got lots of 1488/9 chips to use up,
and I prefer to save my MAX232 for 5V only projects.  Note that all
the logic uses +0 as GND and are TTL or HCT compatible devices.

Schemes I've thought of (not simple enough to replace MAX232):

Tie the 1488(xmitter) GND to +12, +/- go to +24/+0.  Feed the TTL XMIT
line to a 7407 high voltage (30v) driver.  The 7407 output is pulled up
to +24 via a 1k resistor.  This would then supply the signal to the 1488.
-or-
Connect the 1488 supplies as above.  Use an opto-isolator to shift levels.
This would work for the receiver as well.

Thanks for your thoughts and suggestions (TIA)  -deanr
 ______________________________________________________________________
| Dean Reece     Member Technical Staff |"The flames are all long gone |
| The Santa Cruz Operation 408/458-1422 | but the pain lingers on"     |
|___________deanr@sco.com_______________|___________________Pink_Floyd_|