I'm running the network at 38400 baud but whet I came up with this 
solution I have run tests with two RS232-RS485 convertors built on a 
testboard between two computers, on 500m of UTP cable and it worked 
at 115200 baud without loosing a single byte. With a turn on/off time 
of 4ns, Q1 toggles the RE/DE pin fast enough to keep the bit within 
accepted tolerance even for 115200. If cable capacitance is your 
concern, choose the two line biasing resistors close to minimum 
accepted value. At 38400 I even didn't had to terminate the line. Do 
the math on your cable parameters at 1200m which if I recall right is 
the maximum length of a 485 line.
The resistors on A and B line are for holding the line steady in 1 
logic to increase the noise immunity of the network (if I recall 
right, the 485 specs are not to fresh in my memory).
After all it's a simple schematics to run a test on breadboard and 
see for your self. Take care on the power line decoupling and GND 
routing though.

And remember, always have fun :)

My best,
Mircea Chiriciuc

At 09:35 PM 6/16/2007, you wrote:
>The resistors won't pull the line to the '1' state as quickly
>as the driver would

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