Re: DTMF dead?

nico@xxxxxxxxxxx (Nico Coesel) wrote:

The sort of wiring hasn't been nailed down yet, but at this moment I
would prefer a system with 2 wires which carry both power and
communication. RS485 is also in the picture, but we need to have up to
several hundred devices on a short bus (say 20 meters / 60 feet). I
think this would be stretching RS485 too much to its limits even at
low baud rates.

I think the cost of components to apply a modulation to power lines
(presumably with several hundred regulators hung across them and a hefty
power supply at one end) and demodulating the same is going to be a lot
more than 1 or 2 extra pins on a plug and socket and an extra bit of cable.
Because you were considering DTMF it sounds like you can afford to be dog
slow (10 baud?). I think I would be looking at a single data line with 24v
(or more) logic levels, pulled up at the master with open collector/drain
drives. The receiver could probably be as crude as a high impedance voltage
divider tapping down to processor logic levels. One extra connector pin,
one LL MOSFET and 2 resistors - sounds cheap to me.

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