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As a general rule when the voltage measured (with respect to circuit common) is "high" the binary state is considered to be a 1 and when the voltage is low the state is 0. There are exceptions to this rule (such as differential logic and negative logic), but for the purpose of this discussion we'll talk about normal logic. The driver must produce a differential voltage between 1.5 and 5V into a loaded/terminated cable. The annex also says that certain applications may cause the resistor to fail so the installation must allow access for inspection and replacement. The maximum voltage between either of the wires and circuit common must not exceed 10V absolute magnitude, and voltages up to 10V cannot cause damage to the receiver. RS-485 requires the driver and receiver to function if the common mode voltage is shifted against circuit common (see the section on voltages for more information).

Grounding of the RS-485 hardware is another contentious issue. The reason for this is that different installations have different grounding requirements. Consider three different installations. 200mV as undefined, but the IC manufacturer can put the threshold for a 0 and a 1 anywhere they want. If you are pushing the data rate above 100kbit or the line length above 1000 meters, you may want to use a better grade of wire. If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans. This difference in earth ground may be high enough to cause damage to the RS-485 devices, RS485 standard but connecting a third wire between their circuit commons/earth grounds would try to bypass the power earth common (this is often called a ground loop) causing excessive current in the third wire. There is no wire shown connecting this third point between driver and receiver. It shows a diagram of the driver and receiver with two wires connecting them, and a third point "C" that is called a common. This installation should have a third wire connecting the earth ground/circuit common from the desktop PC to the laptop's RS-485 port circuit common.

The desktop is connected to earth ground and the RS-485 port is referenced to the earth ground. RS-485 gives limits for rise and fall times as 0.3 of the unit interval. Since a receiver may have a loading of less than one, the actual number of receivers that can be connected depend on the unit load rating of the receivers, as well as the wire, bit rate, stub lengths, biasing and termination of the network. See the sections on termination and biasing for more information. A higher value termination resistor will reduce the DC losses associated with extreme line lengths, allowing for much longer line lengths at the cost of ringing on the wires. 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. A kilometer of POTS cable can cost a lot, two kilometers twice as much.

Section A.4 of the annex defines two optional grounding arrangements. The standard defines the common-mode voltage as being referenced to ground, it defines a term "ground potential difference" as the difference in the signal ground between the driver and receiver, but it does not say that this is earth ground or just a a third wire common. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals. For an off, mark or logic 1 state, the driver's A terminal is negative relative to the B terminal. This resistor will also make any length of cable, even a short length, look like the characteristic impedance of the cable (120 Ω in the model) to the driver. RS-485, like RS-422, can be made full-duplex by using four wires. A long 3-conductor cable intended for RS-232 can often be switched to half-duplex RS-485, allowing communication at higher speeds and at higher external noise levels than the same cable used with RS-232 signaling. If this is required, it can be accomplished by any means desired including RS-485 drivers on an additional network, RS-232 drivers on extra wires, TTL levels, or really anything you can dream up since it won't work with any other RS-485 network except the one you are designing.