As the RS-485 is a differential standard the minimum potential difference between outputs is 1.5V or (-)1.5V and the maximum +/-6V. In most cases it is not related to 0V or GND; it is as I said difference in potential between the outputs, for example, if this is the 75176 which is supplied of 5V A output can be 1V and B output can be 4V, then the difference is (-)4V, what is within the RS-485 standard, but as you can see there is no negative voltage in the circuit.
As there is always voltage drop in the transmission cables the receiver has to more sensitive then the minimum transmitting differential voltage. For the RS-485 it is 200mV. So if potential difference between A and B (or input+ and input-) is ≥200mV the receiver sees logic 1, if the B input has higher potential than A by at least 200mV then the receiver sees logic 0. Compare it with the attached truth table for the 75176..
Regards,
IanP