Regarding Juliusz's problem, what is the application?

If you want to have an internet link to your friend's house I presume
the distance is a bit too far for reliable RS232. You could simply
buffer the signals to RS422/485 levels (5V differential) and run
internet's SLIP (Serial Line Interface Protocol).

If you want to halve cable cost by running in half-duplex mode, then can
SLIP cope with this? If not you'll have to write software that can.
If it's a one-off, it is not worth the human time to do.

Neither is re-inventing one-off control networks. It's been done for
factory control systems (called fieldbuses). The biggest problem with
them is that there are so many of them that nobody dares commit
factories to a standard incase it does not become _the_ most popular
standard.

Having spent some professional design time with these networks, I can
say that many used half-duplex RS485 as the physical layer. To do it
right to spec, you need low-capacitance twisted pair, shielded
(factories are electrically noisy) and preferably with a drain wire. The
latter you connect to the shielding cases of nodes via a 100nF/1M
parallel RC network. This shorts HF AC noise, and drains DC signals like
static electricity without providing a potentially lethal low-impedance
path. Most economically practical cables are simplytwisted pairs with
shield - no ground ref to connect. You can get round this by having say
a 100R from you buffer's isolated ground to the cable shield.

The RS485 bus needs 75R terminations at either end if you intend to run
it at worthwhile speeds. Node buffers are usually SN176 types. Buffers
are usually in  listen' mode (output hi-z). Nodes have intelligence to
enable their transmitters at approriate times. Typically this done by a
UART handshake output signal - (RTS?).

Isolating the RS422/485 side of the nodes is pretty essential in
industrial I/O networks. There are several ways to do this. Usually
isolated DC/DC converter and high-speed optocouplers (Hewlett Packard!).
DCDC modules are much (about ten times) dearer than 7805 regs, which
soon adds up, so some buses provide a 12-24V power rail for this
purpose. Which transfers cost to the cable. High-speed optocouplers are
not cheap either.

My recommendation: don't waste your life writing software for one-off
networks (unless someone pays you to!)