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From: darrylo@hpnmdla.hp.com (Darryl Okahata)
Newsgroups: comp.sys.handhelds
Subject: Re: 48SX: Hardware project to receive I/R I/O
Message-ID: <1570021@hpnmdla.hp.com>
Date: 2 Apr 91 19:41:18 GMT
References: <1371@cameron.egr.duke.edu>
Organization: HP Network Measurements Div, Santa Rosa, CA
Lines: 37

In comp.sys.handhelds, akcs.jwtrav@hpcvbbs.UUCP (John Wettroth) writes:

> The HP-48 ir couldn't be simpler.  Look at it on a scope.  It is just a
> uart driving an led.  Receiving the ir over short distances can be done

     While the I/R interface on the HP-48SX is not complex, it's not
trivial either.  It's also not an "UART driving an LED".

     The output from the HP-48SX LED consists of very narrow *pulses*,
and is not directly compatible with an RS/232 interface.  You need some
fairly simple circuitry (basically, a digital one-shot) to lengthen the
pulses into something that most RS/232 interfaces can handle.

     Back in August/September 1990, Tony Duell (ard@pva.bristol.ac.uk)
of the University of Bristol, England, posted a schematic for a
bidirectional I/R <--> RS/232 interface.  I haven't tested it, but it
looks useful.  Some of the parts may be difficult to locate, but
experienced designers should have no trouble working around that.  If
anyone wants a copy, send email (I don't think this newsgroup is
archived anywhere).

     See the (unsupported) "HP-48 I/O Technical Interfacing Guide" for
more information.  ASCII versions of this short document were posted
here, and should still be available on the HP BBS.  The troff sources,
which requires the pic processor, are available via anonymous ftp from
the HP BBS (hpcvbbs.cv.hp.com -- 15.255.72.16).  I've forgotten the name
of the file, offhand.

     -- Darryl Okahata
	UUCP: {hplabs!, hpcea!, hpfcla!} hpnmd!darrylo
	Internet: darrylo%hpnmd@relay.hp.com

DISCLAIMER: this message is the author's personal opinion and does not
constitute the support, opinion or policy of Hewlett-Packard or of the
little green men that have been following him all day.  Also, the
author, while being an HP employee, has absolutely no connection with
the calculator folks.