This is a response to a Post or request from 11-30-96.
Pretty old but just uncovered it in the process on cleaning up
the E-Mail DB.
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They are interfaced to a device know as a DTMF encoder.
DTMF stands for Dual Tone Multi Frequency.

You have a single tone at each row, and at each column.
The two tones are combined and produce a unique Multi
Frequency tone.

You can buy chips that have the straight row/col inputs
and a Multi Frequency output.

The reverse version of this chip is also availible, or
you can use a single chip which is a Tranceiver.  I has
both the DTMF Encoder and the DTMF Decoder in one package.

My favorite is the Silicon Systems' 75T2090 which is easily
interfaced to micro-controllers w/o any latches or other
interfacing parts needed.


At 09:32 PM 11/30/96 +0100, you wrote:
>Hi all,
>
>any info regarding above mentioned point is highly appreciated.
>
>Thanks
>-mrm
>