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Path: utzoo!watmath!watnot!watdcsu!rsellens
From: rsellens@watdcsu.UUCP (Rick Sellens - Mech. Eng.)
Newsgroups: net.micro
Subject: Re: RS232 Voltages
Message-ID: <1557@watdcsu.UUCP>
Date: Wed, 24-Jul-85 11:37:22 EDT
Article-I.D.: watdcsu.1557
Posted: Wed Jul 24 11:37:22 1985
Date-Received: Thu, 25-Jul-85 04:18:51 EDT
References: <55@ssc-vax.UUCP>
Reply-To: rsellens@watdcsu.UUCP (Rick Sellens - Mech. Eng.)
Distribution: net
Organization: U of Waterloo, Ontario
Lines: 23
Summary: 

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.
>
>		-Ken Finney-
>		@Boeing Aerospace  

The specs I've read say that the on and off states are voltages anywhere
between -3 and -15, and +3 and +15, with anything between -3 and +3 as
"no man's land". The result is that any machine that follows the specs to
the letter will not talk to something using the ttl levels of 0 and +5 volts.

This gave me trouble interfacing a Novation J-Cat modem (ttl) to an IBM PC
serial port (RS232, to the letter). To solve the problem I had to add an 
interface circuit consisting of an inverter and a line driver to bring the 
J-Cat's signal levels to spec. Shortcuts on specifications can be a real pain!


Rick Sellens
UUCP:  watmath!watdcsu!rsellens
CSNET: rsellens%watdcsu@waterloo.csnet
ARPA:  rsellens%watdcsu%waterloo.csnet@csnet-relay.arpa