>> Do you mean that I should have the transmitter disabled
>> when transmitting a "1" and passively pull it up?
>
>That should be - passively pull the data lines to a "0" state.

Well, I cannot remember offhand, the relevant polarities, but if using a 
normal UART to send and receive on the bus, with all transmitters tristated, 
the output of the receivers should be in the idle state. Then when a device 
wants to transmit, you enable that transmitter using the enable pin driven 
by the RTS signal from the UART. AIUI the receiver chips are arranged to 
have enough bias inside them to passively pull the inputs to the idle state.

If you are not controlling the enable line on the transmitters, then you 
will not be able to do a multi-drop connection. 

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