Hello Julius, In RS485 you need something that take control of the transmision
 becouse is half duplex (transmision and recepcion but not at the same time), so
 I think in your case is better use RS-422. In RS422 your problem is how much is
 the cable cost, becouse you need 2 times more cable than the RS485.

BTW, for convert a RS232 signal to RS485 signal you will need a MAX232 (or
 something like that) and SN75176B (there more and best from MAXIN). I recoment
 you use a optocupler between RS232 and RS485 becouse you need take care of your
 computer, I think. Well, To tell the true I have that you need if you are
 interesting contact me directly and I will send you the
schematic and If you have EAGLE the board too.

Mark Lezama.

juliusz escribis:

> Hi
>
> I am student of the faculty of Electronics in Poland and want to build RS232
 to RS485 converter in order to use it as my home LAN extension via one twisted
 pair wire to my friend's home for Internet purpose. He lives 1000 meters from
 me and I think RS485 will be god for 115200bps. It is easy to buy it but I want
 to build myself. We will use Linux (as always).
>
> Hardware is very clear for such purpose but I can't find information how to
 convert RXD and TXD ? two signals into 1 signal of the current loop. I mean I
 don't know how to create protocol between my 485 modems? Are RXD and TXD
 multiplexed (switched) every transmitted/received byte like
>
> RXD TXD RXD TXD ....
>
> Or maybe RTS/CTS signals must be used to switch between receive/transmit mode
 and PC software (like port driver) takes care of it?
> But what will happen for instance during initial negotiation when both
 computers will want to transmit or receive simultaneously? It must be simple
 null modem but based on RS485. No drivers will not be necessary.
>
> I write software in Assembly language and I have built some good controllers
 and measuring devices with AD converters, displays etc.. I use PC everyday but
 such problem made me crazy.
> I know that my problem is very funny, and basic in general but please give me
 some piece of information or URL to find it?
>
> Please help
>
> Juliusz