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In article <1441@thumper.bellcore.com>, pff@thumper.bellcore.com (Peter Ferris) writes:
> Excuse me if this is something that's been asked 583 times already, but I'm
> attempting to build a DTMF (Touch-Tone) --> RS-232 converter.  I've seen the
> units in the magazines for $90, but it seems to me that's about $75 too much!

> Does anybody have any experience in this area?

	You rang? :-)

> 1)  I guess the choices seem to be SSI-202 (Radio Shack = c.$13.00)  or the
> SSI-204 IC's.  Who makes these (Samsung?), and what is the difference between
> them?

	They are both made by Silicon Systems.  Both of these IC's are almost
identical, but the SSI-202 is a somewhat better quality device with a few
more control lines (which may be unnecessary for your application, though).

> 2) How should I (or SHOULD I) control baud rate; eg; dipswitch software?
> Or would it be simpler / cheaper / "better"  to decide on a "fixed" baud rate
> (would 300 baud be fast enough for a very rapid burst?)?

	Well, if *I* were going to build this device, I would use an 8571
(or 8031 with external eprom) single-chip microprocessor with built-in
uart; the programming is trivial and can be done in less that 50 lines of
assembly language.  The microprocessor could provide two-way serial
communication to set options via software command.  You won't have any
convenient software method of setting options without some intelligence.

	However, you could probably get away with a simple uart with
internal baud rate generator for hardware "programmed" operation.  You
can use the valid tone detect pair output (DV pin) to strobe the uart
transmitter.

	For fixed baud rate, 300 baud is fine.  No DTMF system will EVER
dial that fast; typical autodialing speed is 10 digits/sec.


> 3) I need to decode all <16> characters (0-9, A-D, # & *).

	No problem.

> 4) The circuit should be somewhat "forgiving" in that the DTMF will be decoded
> from a variety of audio sources, ie; scanner, handie-talkie, vcr, and other
> audio sources - but the audio could have static or other artifacts in it.

	That's a tough call.  One basically gets what one pays for, and
these ciruits are not THAT forgiving - but they may be fine for your
application.  If you want forgiving circuits, you will have to spent more
$$$ and go to a quality device like those made by Tel-Tone or Mitel.
Tel-Tone is on the Net, so maybe someone will give you more specific
advice about their products.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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