I actually had to figure out how to make a device that took 7e2 RS232 (yeah, 6 data bits, even parity, 2 stop bits... wtf!) work a while ago. No documentation. Figuring out what the hell was going on was interesting and involved an oscilloscope.
current loop is some real prehistoric stuff, hell I think its obsolescence predates my birth by at least a decade. Basically current over x milliamps (iirc, 16 or 20mA) was considered a mark, while no current was considered a space. 2 wires for send, 2 for receive, no ground required. With good 7500v optoisolators at each end you could achieve basically any ground imbalance you needed, but I don't believe anyone ever really used it past about 4800 baud. Need to add a few miles to your serial data link? OK, add a few miles of cable, doesn't matter if it's twisted pair or shielded, now stack more batteries in your loop supply till you hit 16mA Mark current again. Done.
RS423/485 is different iirc, isn't that differential signaling RS232 basically, with one also adding multi-device operation?