Good advice I think ...
I spent yesterday at one of the mills, there was a flood came through last week and we had to lift a huge log out of the wheel.
We also designed a continuous belt cleaning system to replace some of the fixed screens because the output has been down lately.
With all the rain here the river is very dirty and it autumn here so leaves are a huge issue right now.
The turbine, the mill has both, was ding about 7.5kW yesterday with clean screens, but it dropped to less than 4 in two hours!!
I would have to live there to keep the screens clear hence the new design.
New chain required on the wheel at the weekend, 'O' joy, 20' of 1'' simplex is heavy, I will need blocks to pull it on.
Not a thought of coding all day. 
Rob ...
I think I have worked out how to add an RTS line to the UART based serial if your interested.
If you go that way you can use hardware serial and convert, properly, to RS585.
There is an interrupt available that fires when the last bit transmitted is done, as oppose to when the buffer is empty.
If the hardware serial Lib was modified slightly so it set a pin high when the buffer was first populated and then low gain on transmit complete you would have the enable signal required by most RS485 chips.
I think this approach would be faster, less processor load, than building/finding a filly soft RS485 solution using other pins.
That said there are 485 shields available .... Down to cost and complexity I guess.
Al