RS-485 -----> converter -----> USB --------> Linux

RS232 to usb to linux should be trivial. Can you translate 485 to 232?

Sounds like you will learn a lot by doing it. Have fun.

Use an Ethernet/serial(RS-485) converter and it will work from any operating system (and any operating system version) supporting socket (TCP) or even only UDP connections.

Be careful with serial port numeration issues if you have two identical adapters.

While conversion from RS-232 to RS-422 would be trivial, just make the voltage translation. For 2 wire RS-485 the situation is more complicated, since you need the data direction control (basically transmit enable).

For big and heavy operating systems like Linux or Windows, turning on and off the RS-485 with explicit commands by the host over USB may be far too unreliable (unpredictable latencies), so the USB converter itself must generate the transmit enable signal.

Especially the transmit disabling is critical since if the transmitter is on too long after the frame, the other part may start the reply and cause a bus conflict.

Get a USB dongle that either includes RS-485 hardware support or an RS-232 USB dongle that supports automatic half-duplex RTS toggle under Linux (that way it will work properly with an RS-232RS485 converter). Alternatively, you can sometimes find RS-485 converters that handle line direction automatically and don't require RTS to be toggled -- but converters like that are hard to find and usually expensive.

I do a lot of serial programming on Linux, and if I were you instead of C I'd use Python and either PosixSerial or pyserial, but it's your funeral.

PosixSerial is a very thin wrapper around the Posix/Linux API that has been tested only on Linux. pyserial has much better documentation and is portable across Linux, Windows and MacOS (the Linux support in pyserial is based on PosixSerial). PosixSerial supports only Python

If you really want to use C, then get a copy of "Serial Programming Guide for POSIX Operating Systems" by Michael Sweet. There are old copies available for free

It looks like you have to pay for more recent versions.

Do NOT pay any attention to the TLDP serial programming HOWTO:

It has many errors and omissions.

FWIW, I just updated PosixSerial so it works with Python 3.2

There's USB to RS485 adapters out there. I've been out of the game for too long to know who the players are any more, but a quick web search should be helpful. Black Box used to be one of the go-to places; they had everything, albeit it steep prices.

Please check the galvanic isolation issue, if the RS-485 network is larger than one room.

Another reason to go with an Ethernet attached serial port: you get galvanic isolation for free (unless the designer was foolish enough to connect signal ground to earth ground inside the 'converter').

pyserial is how I do these things too. Python is often easier for this sort of thing, especially when playing around and learning. And if I wanted to do it in C, I'd start by looking at the pyserial implementation for clues.

FTDI make nice USB-to-UART chips, and nice cables with the chips neatly embedded - they are always my first choice for something like this.

Learn a little about udev rules if you want to use more than one cable in a system - it can help you get predictable names for the different cables (I have rules that give fixed name links to cables on different ports of a USB hub - so I refer to /dev/ttySerial_hub3 rather than /dev/ttyUSB2, etc. You can also fix names based on serial numbers or other features of the device).