Often, though evidently not in this case, people use RS422, which is differential, so you can get megabits per second or kilometers, though not both. Shared-bu

Often, though evidently not in this case, people use RS422, which is differential, so you can get megabits per second or kilometers, though not both. Shared-bus RS422 is RS485, like LocalTalk or DMX512. The voltages are actually lower than the ±12V normally used by RS232. Converting back and forth between RS232 and RS422 is easy and cheap. https://www.ti.com/lit/an/slla070d/slla070d.pdf https://www.ti.com/lit/an/slla070d/slla070d.pdf is a TI appnote with an overview.