Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!mcgill-vision!bloom-beacon!mintaka!ogicse!ucsd!ucbvax!hplabs!hp-pcd!hpcvra!prestonb
From: prestonb@hpcvra.CV.HP.COM (Preston Brown)
Newsgroups: comp.sys.handhelds
Subject: Re: 48sx Serial Connector
Message-ID: <21580023@hpcvra.CV.HP.COM>
Date: 13 Mar 90 18:58:56 GMT
References: <1990Mar12.141924.9175@hellgate.utah.edu>
Organization: Hewlett-Packard Co., Corvallis, OR, USA
Lines: 17

>Just by the way, the RS-232 of the HP-48SX doesn't quite meet the RS-232
>standard specifications.  A "mark" is supposed to be between 5 and 25 volts,
>but the calculator only puts out about 4.1 volts for the signal.  It would
>have required some extra hardware for the calculator to do otherwise, since
>it runs off of 4.5v worth of batteries.  However, the RS-232 standard also
>specifies that the receiver should be able to detect signals in the range 
>of 3 to 25 volts, so if the cable isn't too long, there should be no
>problem.

The I/O is specified to output a minimum of +/-3V into a 3K ohm load.  
This voltage is regulated and is not straight from the batteries.  
Very few users would want to run the serial port over long distances 
and using +/- 3V increases the battery life.

Actually, the signal ground "SGND" is the system VDD.

Preston