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From: dale@wucs.UUCP (Dale Frye)
Newsgroups: net.micro
Subject: Re: RS232 Voltages
Message-ID: <1065@wucs.UUCP>
Date: Mon, 29-Jul-85 11:51:32 EDT
Article-I.D.: wucs.1065
Posted: Mon Jul 29 11:51:32 1985
Date-Received: Thu, 1-Aug-85 07:45:41 EDT
References: <55@ssc-vax.UUCP>
Distribution: net
Organization: Washington U. in St. Louis, CS Dept.
Lines: 21

In article <55@ssc-vax.UUCP>, Finney@ssc-vax.UUCP (Ken Finney) writes:
> The official voltages levels are +12 and -12, giving
> an awful lot of noise margin.  To save money, microcomputers
> years ago started using +5 and 0.  I have seen a lot of
> peripherals that clip the incoming voltages to +4.7 and
> 0.7 using a zener and a resistor.  These peripherals work
> whether you are using the official voltages or not.
>
The 1489 line receiver (or is it 1488, I can never remember, the other one
is the driver) uses -1 and -3 as the threshold voltages however this can
be adjusted by a resistor to a bias pin (adjusted up).

Military specs are +- 6 volts.

Question: Why does DEC have their receivers load the line. I've seen DEC
equipment pull a 12v signal down to 3 or 4 volts. Can't they design something
that needs less than 10ma. WARNING!!!: Don't use a passive (i.e. with no power
source) breakout box on DEC equipment. Between the the drain of the box and 
DEC, things have a habit of not working.

Dale Frye @ Washington University in St. Louis