Path: utzoo!mnetor!uunet!husc6!bloom-beacon!mit-eddie!uw-beaver!tektronix!reed!kamath
From: kamath@reed.UUCP (Sean Kamath)
Newsgroups: comp.sys.apple
Subject: Re: GS Serial Port
Message-ID: <9001@reed.UUCP>
Date: 23 Apr 88 21:51:09 GMT
References: <8804192242.AA27374@wpi.local> <8945@reed.UUCP>
Reply-To: kamath@reed.UUCP (Sean Kamath)
Organization: Reed College, Portland OR
Lines: 43
Keywords: RS232

In article <8945@reed.UUCP> douglas@reed.UUCP (P Douglas Reeder) writes:
>STANDARD RS-232 (which no one seems to follow) calls for balanced pair
>transmission (data pulses are sent out as positive pulses on the + lines
>and negative on the - lines).  The signal is recieved with a differential
>amplifier.
> [one line deleted.  I thought I could get rid of more, but. . .]
>This setup is neccesary when you run cables on the order of 100m.  Since
>microcomputer cables are seldom more than a few feet, everyone cheats on 
>the standard and there's seldom a problem.
>    If you need transmission over longer distances than 232 can do, there are
>other standards with different rquirements.
>
>-- 
>Doug Reeder                           USENET: ...!tektronix!reed!douglas
>10 Cyclopedia Square             from BITNET: douglas@reed.UUCP
>Terminus City                     from  ARPA: tektronix!reed!douglas@berkley
>Terminus,The Foundation               Box 502 Reed College,Portland,OR 97202


A very nice discription of rs-422, Doug!  *not* rs-232.

RS-232 is *level* sensitive.  I.e. between 2.5 and 15 volts is "true" (I
think.  I don't have my handy book here. . .), and from -2.5 to -15 V is a
"false".  It dies off much faster than rs-422.  Most micros use +/- 5V, and
so they go about 100 ft.  Or less.

between 2.5 and -2.5, it's undefined.  Realy!  Though almost all UARTs, and
hence most serial communications devices, below ~1.2V is "False".

A *REALLY* good book to get about rs-232 and 422, as well as serial
programming with UARTs and stuff (it's written in C, but the first part is a
"history" or serial communications) is:  _A Programmer GUide to Serial
Communications_ by someone.  It from Howard Sams, and costs about $24, just
like all computer books.  Get it at B.Dalton's software etc.

I suspect, Doug, that you have been tainted by all the Macs here at Reed :-)

Sean Kamath
-- 
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