The +sig twisted with -sig covered with a ground shield.
Oddly enough, sometimes often unshielded works better. I do use shielded, but in one case had to go to unshielded. The problem with shielded is the capacitive load which, for very long runs, can deterrioate the signal. When using unshielded, the "balance" (i.e. identical a and b lines) becomes critically important.

The slave micro's ground must be isolated from grounds other
than the PC's in order to avoid ground loops introducing
noise, although the differential receivers will reject much of
the common mode noise injected by moderate levels of ground
loop currents.
I find that using isolated transcievers when you have no control of grounding often is a better solution.

RS485 spec. says that it can drive 4000 ft at 100K bits/sec with 32 transceivers on the line. No, that not 32 transceivers it is 32 "unit loads" using 1/8 unit load transcievers, you can have 256 nodes.

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. These are quite critical unless you actually write a PC program to control them. If you use "standard I/O" for the serial port, you will be better off with a B&B unit. B&B know so well how important "know-how" is that they even publish the schematics and those I know that has made copies have not been succesful.

Also I forgot about the termination that is recommended for long lines. This termination should be on the last slave on the line
No, it is not "recommended for long lines" it is mandatory. You can get by without termination, but will be severely limited in the capacity of the bus. Also the termination should be at both ends of the bus.

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.
The termination you suggest will not give a correct termination, the resistors should be 2* 750 ohms and 130 ohms.

Once more, most of this can be found in 10 ways to bulletproof 485 vhich can be found through links <--- under "best serial appnotes"

Erik