Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!samsung!usc!apple!vsi1!daver!wombat!george
From: george@wombat.UUCP (George Scolaro)
Newsgroups: comp.sys.handhelds
Subject: Re: Easy HP infra-red PC receiver.
Message-ID: <843@wombat.UUCP>
Date: 7 Jan 90 18:12:02 GMT
References: <5916@sdcc6.ucsd.edu> <10169@microsoft.UUCP>
Reply-To: george@wombat.UUCP (George Scolaro)
Organization: Assn. for the prevention of Polar Bears and Kangaroos
Lines: 46

In article <10169@microsoft.UUCP> alonzo@microsoft.UUCP (Alonzo GARIEPY) writes:
>In article <5916@sdcc6.ucsd.edu> pa1409@sdcc13.ucsd.edu (Steven Haehnichen) writes:
>| I recently ran across a gadget at Radio Shack that seemed too easy
>| to be true, and still might be.  It's their "GP1U52X Infrared
>| Receiver/Demodulator"  (Catalog # 276-137). 
>| 
>
>Chances are that this device will not work with the HP-28.  Modulation 
>involves transmission of information on top of a carrier frequency.  
>The frequency of the carrier used by the HP-28 is different than that 
>used by remote controls.  Unless this device is smart enough to detect 
>the carrier and switch (very unlikely), it won't properly detect the 
>signal (although I am sure a great deal of noise will be generated).  
>The other problem is that the signal from the HP-28 is very dependent 
>on timing.  You need more than just pulse detection, and you need very 
>accurate timing information.

Is the HP28(s) similar to the HP42S? I think it must be since the same
printer can be used with both? Anyhow, a friend and I have just put
together a trivial little circuit based on the Motorola MC3373, and
8 pin minidip IR receiver. We wrote a small (hundred or so line 
assembler program) and have it decoding the IR coming from the HP42S.
At his stage the code is 68000 on a homebrew system (the handiest
hardware platform we had), but it should be real easy to convert
to any other platform. The output of the circuit is a single bit
signal, 1 for IR on, 0 for IR off. The signal timing looks pretty
easy to decode on any system which can do a timed software loop with
+-30% accuracy. Of course the deluxe method is to use a micro to turn
the stuff into real RS232.

The unit as it stands decodes the IR and the software re-converts the
data into 8 bit binary, which for the 42S is pretty well ascii, except
for some special characters which we handle via a conversion table.
The circuit came pretty well straight out of the motorola master
selection guide in the remote control circuits section. Its basically
a few dollars worth of bits.

Thanks for the decoding of the IR data to Eric Toonen, who posted it
via gmuvax2.gne.edu. In fact the info that he posted was for the 28S
and I used it for the 42S, hence my idea that the two are compatible.

regards,
-- 
George Scolaro
george@wombat
(try {pyramid|sun|vsi1|killer} !daver!wombat!george) [37 20 51 N / 122 03 07 W]