Can't find a very good comparison, but the main reason current loops are not used a lot is because they consume a lot of power. For sparse communication with idle=off this doesn't matter. Now for RS485, apparently RTS is supposed to be used as an enable signal. http://electronics.stackexchange.com/questions/23988/automatic-rts-control-for-rs-232-to-rs-485-converters FTDI chips have a direction pin. Or use a monostable trigger on the TX start bit. EDIT: disadvantage of current loop is that it's single duplex (or needs transformer coupling).