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Path: utzoo!linus!philabs!cmcl2!seismo!hao!noao!terak!jb
From: jb@terak.UUCP (John Blalock)
Newsgroups: net.micro
Subject: Re: servo computer products
Message-ID: <645@terak.UUCP>
Date: Tue, 23-Jul-85 11:58:11 EDT
Article-I.D.: terak.645
Posted: Tue Jul 23 11:58:11 1985
Date-Received: Thu, 25-Jul-85 23:20:21 EDT
References: <732@abnji.UUCP> <7000006@uicsl>
Organization: Terak Corporation, Scottsdale, AZ, USA
Lines: 24

> Does anybody know how to get an RS232 interface off a board with
> only a +5 power supply? I thought -12 was needed...

Both +12v and -12v are needed if you want true RS232 levels.  The RS232 spec
defines a space (logic 0) as +3 to +15v and a mark (logic 1) as -3 to -15v.
However, most RS232 equipment doesn't need these levels to work properly.
If the equipment you are driving uses a 1489 line receiver (and most do these
days) you can get away with standard TTL levels.  The 1489 logic low threshold
is specified to be between +0.75 and +1.25 volts.  For short runs you can drive
the "RS232" line with a 7404 (remember you need to invert the logic levels).
For somewhat longer lines, I recommend using a 7405 or 7406 open collector
inverter with a on-board pull up resistor of ~2K.  Neither will give you the
capability to drive lines as long as standard RS232 drivers can drive, nor will
you have the noise immunity of standard RS232.  But to drive a < 10-foot line to
a local terminal or printer I've never experienced any problems using TTL
voltage levels except with equipment that didn't use the 1489 line receivers..

John Blalock, W7AAY

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