🚀 go-pugleaf

-Greetings,
-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?

I helped build a DTMF decoder for a school project last quarter. I also
currently have a homebuilt 68010 computer with a rs232 port. 

Price should be between $20-$25 depending on what you use.

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

Silicon Systems Incorporated. Tustin, Ca. (714) 731-7110.

I happen to have their data book in my office. Let's see... Description...

SSI-202 - The SSI 202 (and 203) are complete DTMF receivers detecting a
selectable group of 12 or 16 standard digits. No front end prefiltering
is needed. The only externally required components are an inexpensive
3.58-MHz telvision "colorburst" crystal (available from Radio Shack
among others - Ed.) and a bias resistor... Both are CMOS, use 5V and
packaged in a 18 pin DIP

SSI-204 - 14 Bit version of the 202. Uses same external components.
It's missing some of the more esoteric pins like Inhibit last row decoding,
Binary coded 2 of 8, Clear Data available, early detection. Real
big loss ;-)

Both have 3 state outputs and a data available output for direct connection
to a microcomputer.

-
-2) How should I (or SHOULD I) control baud rate; eg; dipswitch and/or 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... Let's see

(300 b/s) * (1 char / 10 bits) = 30 char / s 
So 1 char every 33 ms. Ok.

Now according to the data sheet both the time to detect the tone and then
the following pause is 40 ms apiece. So with 80 ms to do 1 digit 300 bps
should be plenty fast enough.

-
-3) I need to decode all <16> characters (0-9, A-D, # & *).
No problem. Both decoders do the job.

-
-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.

The data sheet claims to be able to detect with S/N of 12db. That's really
really noisy. They warn that if you have signal above 28kHz it can alias
back into the audio range and that it needs to be filtered.

-
-Permit me to add that this project is what we call a "G-Job" - it's for my own
-personal pleasure / experimentation.  If anyone has any suggestions, comments
-or schematics on this rainy day project I'd sure appreciate hearing from you.  
-Best bet is Internet E-mail.

Two ideas immediatly spring to mind.

No micro
--------
                      ____-------              ------  -----            
              |------|____|     |              |    |  |MAX|             
 audio in ----| DTMF |____|EPROM|-8 data lines-|UART|--|232|--ascii serial out
              |------|____|     |              |    |  |   |             
                 |        -------              ------  -----            
                 |                                |                            
                 |-Data valid from DTMF to UART send                

Rather quick and dirty. Whenever the DTMF decoder receives a digit the
EPROM changes it to ascii, the data available of the decoder does a
write to the UART which sends the converted value out the serial port.
By using a MAX 232 tranceiver you can do the whole circuit with 5V supply.

Micro
-----
Replace the EPROM and UART with a EPROM microcontroller like a 68701
or an 8051. Of course then you can program it on the fly with different
baud rates and different conversions by sending data back out the serial
port. Or put an encoder in the circuit for two way communication. Or
any number of rather nifty things.

Another part you may want to look at is the SSI 2090C. All the 
functionality of the 202 plus an encoder and a simplistic ring
detection system.

-
-Many Thanks in Advance,

Sure enough. Let me know if anything comes of it.

-
-Pete Ferris, N5KBD
-pff@thumper.bellcore.com

BAJ

-- 
Another random extraction from the mental bit stream of...
Byron A. Jeff
Georgia Tech, Atlanta GA 30332
Internet:	byron@pyr.gatech.edu  uucp:	...!gatech!pyr!byron