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RS485 is not a protocol but a Physical Layer.
It is basically a Differential line transmission like Ethernet.
There is a TX+ & TX- along with RX+ and RX-.
For conversion to the RS232 you would require to convert this diffential signal to a Single ended signal. There are many Transceivers available for this from Maxim, TI and ST. You can search for them in the websites.
Not much of a problem to connect a 485 data i/o chip to an rs232 ic. You effectivly need to convert rs232 levels to normal logic voltages. The real issue is the fact that 485 uses the same pair of lines for both reception and transmission, where 232 has one for each way. If you need bidirectional communication, then you need logic to control the direction pins on the 485 chip. If you have a full serial interface like on a pc, you may be able to use one of the other signals like RTS to control direction. I think you can buy commercial adapters to do this.
The hard part is where to get power for this. RS485 is a 5v powered differential 50 Ohm line driver RS232 is two [or more] single ended +/- 9v signals. Ok use a RS485 transciever chip to convert to/from TTL levels plus a RS232 I/O with at least 1 in & 2 out you need one with a built in charge pump to get +/- 9v. just tie the TTL outputs to inputs for Rx/Tx simple. Now convert RS232 request to send (RTS) to TTL and use it to control the RS485 direction pin note you may need to invert it, also tie RTS to CTS the clear to send RS232 line. It should work if your RS232 supports the status lines [many don't] AND you don't Rx/Tx at the same time.
RS485 operates in both 2 and 4 wire mode. 2 wire is half duplex as GrandAlf describes, 4 wire allows full duplex (send and recieve at the same time).
What interface do you require for you interface? Then design to that. I use Maxim chips for conversion, but that's just what I find is easily obtainable, they provide interface chips for full and half duplex.
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