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