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'1200 Baud MODEM'
1997\05\14@142428 by lowell

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I've got a friend I'm helping out who wants to implement a 1200 Buad
MODEM in a PIC.

Has anyone achieved this?  I think I have heard of people getting 300
Baud MODEMs to work.

He seems to think we can take something like a PIC16C65 and load the PWM
unit with several different sine wave table lookups with the right phase
relationships to represent the zeroes and ones.

He thinks we just have to make a 1200 Hz sine wave with a small bit of
variability to get things to work on the TX side.

Is it really this simple?  Does anyone know where I can find a copy of
the BEL whatever spec for this on the net?

On the RX side, I'm not sure who to do this simply.  Does anyone know
that tricks?  Can we reshape the sine wave into a square wave(easy) and
us interrupts to figure out phase relationships?

What are the complications of doing this in a PIC?

If 1200 Baud is out of the question, does anyone know where I can find
the data to straightfowardly implement 300 Buad?  Microchip doesn't have
any app-notes on this published.

Regards, Bob Lowell


'1200 Baud MODEM'
1997\07\04@151105 by Jeff King
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At 11:00 AM 5/14/97 -0700, Bob or Diana Lowell wrote:
>I've got a friend I'm helping out who wants to implement a 1200 Buad
>MODEM in a PIC.
>
>Has anyone achieved this?  I think I have heard of people getting 300
>Baud MODEMs to work.

Bob Clements (don't know the URL) is doing 1200 baud Bell 202 tones
out of a PIC for a GPS based project he calls "MIC-E". Look at
http://www.tapr.org for more details. It however is just the
transmit tones so no receive.

You didn't specify what modem standard you are looking for. The
so called standard for telephones is Bell 212 which is DPSK. BTW,
there are many cheap single chip solutions out there so not sure
what your point is trying to code this on a PIC.

Regards,

------------------------------------
| Jeff King      Aero Data Systems |
| spam_OUTjeffTakeThisOuTspammich.com  P.O. Box 510895   |
| (810)471-1787  Livonia, MI 48151 |
|F(810)471-0279  United States     |
------------------------------------

1997\07\04@162233 by BG Packet Gateway

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At 15:06 04.7.1997 -0400, you wrote:
>At 11:00 AM 5/14/97 -0700, Bob or Diana Lowell wrote:
>>I've got a friend I'm helping out who wants to implement a 1200 Buad
>>MODEM in a PIC.
>>
>>Has anyone achieved this?  I think I have heard of people getting 300
>>Baud MODEMs to work.
>
>Bob Clements (don't know the URL) is doing 1200 baud Bell 202 tones
>out of a PIC for a GPS based project he calls "MIC-E". Look at
>http://www.tapr.org for more details. It however is just the
>transmit tones so no receive.
>

I just finish 1200 AFSK Packet Radio Modem, based on PIC16C65 and
PIC12C508, 62256,  MAX232 and LM393. To PC conn is 4800/9600/19200 (X)KISS.
More details will be available in the middle of august.

Chris - lz4ic

1997\07\05@125847 by Mats Monier

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At 23:21 1997-07-04 +0300, you wrote:
{Quote hidden}

like a miniature cordless phone (modemlink)

1997\07\05@224305 by Sean Breheny

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>>>>I've got a friend I'm helping out who wants to implement a 1200 Buad
>>>>MODEM in a PIC.
>>>>
>>>>Has anyone achieved this?  I think I have heard of people getting 300
>>>>Baud MODEMs to work.

>>hi i have a projekt that needs some knowledge about building small tranceivers
>like a miniature cordless phone (modemlink)
>
>

I built a 150 baud modem for a pic out of one IC, an LMC567. This little
chip is only 8 pins, and can detect a tone by simply pulling a  pin low when
the tone is detected. I'm not sure what its maximum throughput rate is, but
there are other chips (made by national semiconductor and others) similar to
this that should definately be able to go up to 1200 baud. I used an FSK
modulation system, using a 1N4002 diode in a crystal oscilator at 49.860 MHz
to shift the frequency between  two frequencies, one for mark, the other for
space. The diode was driven by a transistor which in turn was driven by a
PIC pin. The diode acted as a varactor, i.e. its capacitance in the reversed
biased state depended upon the reverse bias voltage, which was switched
between two levels by the transistor. On the receiver end, there was a
direct conversion receiver consisting of an RF preamp, an NE602
mixer/oscilator, and a single transistor audio amp, the output of which went
directly to the LMC567, which then switched hi/low a pin on the PIC. When I
got at least a 70mv audio signal out of the audio amp of the receiver, the
system worked very well.
The system was for a keyless entry system.

Good Luck,
Sean

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