RS485 can be either full duplex or half duplex. It makes a difference,
because whatever you're talking to at the other end must work the same
way. If that device is half duplex, then you must make your end work
half duplex as well.

It sounds like what you're wanting to do is to add an extra serial
port to a PIC that has only one USART channel. If so, you have to 'bit
bang' the extra serial port. This depends on being able to provide a
timer for measuring the bit periods.

If the line is full duplex, it will use 4 wires. If it is half duplex,
it can use either 4 or 2 wires, depending on what's at the other end.
Often, RS485 is used with a single master and multiple slaves. In this
case, the slave deevices must be able to control  their RS485 drivers
so that they only drive the bus when given permission to send by the
master. If this is the case, you will need an extra signal from the
PIC to control the RS485 driver.

Similarly, if it's half duplex, and only 2 wires, then you will need
to be able to switch the driver on only when it is your turn to
transmit. This all depends on what is on the other end of the line.

Spme RS485 systems use a 9 bit data transmission, where the 9th bit is
used as a signal to turn the line around. That will add software
complications, but the hardware works the same way.

You'll need an RS485 transciever. For example, for 4 wires, you could
use Maxim 1483. For 2 wires, Maxim 1482.  There are many others
available from Maxim and other manufacturers.

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