I am looking for how I can setup a null-modem (RS422) connection between two computers to share internet.
I have been looking at pppd using what this says: http://www.tldp.org/HOWTO/PPP-HOWTO/index.html but I can't get it to work. Some things must have changed in 18 years...
I am running Debian 9/Ubuntu-mate 18.04/Raspbian Stretch.
So, where do I start?

Ps. Minicom works fine using zmodem and xmodem on the link.

Hi MrSVCD & welcome to LQ.

A few things sure have changed. pppd is basically dead; People no longer use dialup modems. RS422 is niche use only. Cables would be awkward to source. People are into wifi, bluetooth usb or ethernet. The documents on pppd always leave you knowing less about the subject than you did before. I have tried making sense of them. He just goes from one tangent to another without giving a grounding in the basics. PCs don't usually have serial ports. Nothing PC related does the voltages (+/- 6V) anyhow. I wouldn't fancy any twisted pair line for 1500 Metres, and certainly not at 10 Mbit!

A simple mechanical way to connect 2 boxes is by ethernet cards. It can also be done over usb. You can stick a wifi hub somewhere to join them up if you have wifi. Hey, even a smart phone set up as a wifi hotspot can connect things. Welcome to the 21st century

If you have some special application that needs RS422, explain what your needs are.

My last two desktops does have serial port headers, but no external connectors.
1km at 64kbps would be plenty speed to send/receive control signals to ip based things in a greenhouse or other low bandwidth situations where radio signals are not an option.

Theoretical limits for RS-422 is 4000ft and 100kbps so what you purpose is reasonable and I agree with business_kid that trying to make sense of the documentation is not easy.

I would think you would want to create a bridge between two LAN networks. I found a howto using two Raspberry Pis so I would think you could adapt it for debian or Ubuntu.

https://www.raspberrypi.org/forums/v...c.php?t=149927