> That is totally determinate. If you want to be able to nominate one or other
> with certainty it seems a sound idea and costs 2 resistors.
>
> If you want them to decide amongst themselves you could have a mutual
> arbitration session where the first one to start up asserts a line to claim
> mastery.
>
>     IF RX high
>         I am slave
>     ELSE
>         Assert TX high
>         Wait a short period
>         IF RX low
>             I am master
>         ELSE
>             dead heat - start again
>         END
>    END
>
> This would work well except when there was a dead heat which would be rare
> and could involve a pseudo random backoff and retry. Random generator would
> need to be dependent on a seed that was not the same for both processors.
>
> After deciding there could be some handshake to test that both had decided
> correctly.
>
> The two resistors are sounding attractive :-)

Sorry if I'm being dense but I don't see how this answers
my question.  How can we guarantee that the master doesn't
start transmitting (hence changing the state of the pulled
up Rx line on the slave) before the slave reads that it is
pulled up?

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