With no driver connected, the differential voltage on the wires will depend on the termination resistor and biasing. The termination resistor should be equal to the characteristic impedance of the cable. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. The receiver's input impedance is specified in terms of a "unit load" where a unit load is specified as input current in mA at a voltage referenced to ground. In the following example the desired result is to get 0.2V of bias across Rterm, and Rterm is 120Ω. It should also be noted that this example ignores any current from or to the drivers and receivers. Since RS-485 is half-duplex, RS485 standard it is difficult for the receiver to tell the transmitter to "shut up" when it can't get a byte into the incoming data. A third interface is defined as a transceiver, which has both a generator and receiver. It also defines three generator interface points (signal lines); A, B and C. The data is transmitted on A and B. C is a ground reference.

Because a mark (logic 1) condition is traditionally represented (e.g. in RS-232) with a negative voltage and space (logic 0) represented with a positive one, A may be considered the non-inverting signal and B as inverting. The RS-485 standard includes the voltage levels of the two wires when a binary 0 or 1 is on the two RS-485 wires, but specifically excludes the logic function of the generator or receiver. The exact voltage level that a logic device considers ON or OFF varies by logic type, but when the voltage is high (usually but not always approaching the IC's supply voltage), the output is on and a binary 1 is on the wire, and when the voltage is approaching 0 the output is off and, a binary 0 is on the wire. With Modbus, BACnet and Profibus, A/B labeling refers A as the negative green wire and B as the positive red wire, in the definition of the D-sub connector and M12 circular connector, as can be seen in Profibus guides. How much of each depends on the wire. It depends on many factors. Since the driver and receiver are differential circuits, the input and output voltages are specified as differential, but these voltages are also referenced to a circuit common.

None of these protocols are part of the RS-485 standard, and implementation is up to the engineer writing the software. RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus. It shows a diagram of the driver and receiver with two wires connecting them, and a third point "C" that is called a common. The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. And since a 120 Ω termination resistor can cause enough signal loss with an extremely long 120 Ω cable to stop the network from functioning, a 500 Ω or even a 1kΩ termination resistor may improve the signal quality enough without causing too much signal loss for the network to function. Low data rates are primarily limited by the DC resistance of the cable (the effects of the DC resistance of the cable are made worse if a termination resistor is used) and high data rates are limited by the AC effects of the cable on signal quality.

In reality, if you are using relatively modern equipment (say 1990 or later) and a low baud rate, line lengths of 1,000 feet (300 m) or more are possible. In this figure the driver has two leads and connects via a stub to the transmission line at points "A" and "B". There are two other devices in this figure (receiver and transciever) that are connected to the transmission line the same way. Figure 1 of the RS-485 standard is a diagram (not shown for copyright concerns) which shows the "balanced interconnecting cable" as a transmission line of two wires. Figure 1 also shows the output signaling waveforms, redrawn here in color for clarity. In the RS-485 standard, one wire is labeled "A" and the other is labeled "B", and the wires are twisted together (a "twisted pair"). Its annex also states "It is desirable for proper operation over the interchange cable that the DC wire resistance not exceed 25 Ω per conductor." Any cable that meets the capacitance requirement can be used. This allows RS-485 to transmit over longer distances than RS-232.