After I posted the "RS485" message I realized that another requirement is that the PC also must have a RS485 port or a RS232 to RS485 adapter.

An adapter could be made with a Maxim MAX-233 & a RS485 transceiver with the RTS line from the PC enabling the RS485 output driver.
You would need a +5 volt power supply to drive the above.
There is not enough power available from a RS232 port to drive a long line RS485 buss.

Also I forgot about the termination that is recommended for long lines. This termination should be on the last slave on the line and is as follows:
A 100 ohm resister between +sig & -sig.
A 1K ohm resister from +sig to +5 volts.
A 1K ohm resister from -sig to ground.
This also forces a "1" "idle" level on the buss when all RS485 ports on the line have their output drivers disabled. This is required by the UARTs on the PC & slave devices.

Sorry for the omission.