My understanding is that it was for driver circuit compatibility with older current-loop TTY devices. Early Teletype et. al. terminals were designed to run ove

My understanding is that it was for driver circuit compatibility with older current-loop TTY devices. Early Teletype et. al. terminals were designed to run over potentially long wires with high resistive losses[1], and to be received by 1960's electronics with very low impedances (the ancestral devices were mechanical switches!). So the idea was you'd just switch a big DC supply onto the lines and figure it would be enough to get 20 mA at the other end, so that it could see it. When RS-232 came along, they deliberately specified a very wide band of allowed voltages so that in many cases manufacturers could just cable the current loop device in parallel with a resistor and be compliant. Don't quote me though, I don't remember where I read this. [1] So... yeah, it's basically solving the same problem as differential signaling but at a different scale.