> Bob,
>
>
> So are the diode's there to stop a HIGH slave Tx pin being dragged low by
> the other LOW
> slave Tx pins ?
>
> Why do I need a pull-up on Master Rx pin ?, When I bread boarded a test
> circuit
> I just connected Tx -> Rx
>                  Rx <- Tx
>
> and this seemed to work ok.
>
> Thanks,
>
> Russell.


To understand this you need to know that an idle serial port is sending a
HIGH condition on TX.

The resistor and diodes form what amounts to an AND gate. If any of the
slave TX pins is low then the master's RX will be low (because the diode
will conduct).

If all of the slave TX pins are high then the resistor can pull the master's
RX high and the master will see a high.

Now, if any one slave sends, the master will see it just fine (because the
other slaves are holding their TX pins high).

So, you just need an appropriate protocol.

When the master talks, all the slaves hear it. The message should include an
address so that only the correct slave acts on it and responds to it.



Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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