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In the alternative, ground one end of the shield and connect the other end to ground through a bi-directional transient protector (from a few volts to a few hundred volts depending on the situation). SC, G or reference, the common signal reference ground used by the receiver to measure the A and B voltages. This way, the lines will be biased to known voltages and nodes will not interpret the noise from undriven lines as actual data; without biasing resistors, the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered. There are two other devices in this figure (receiver and transciever) that are connected to the transmission line the same way. The RS-485 standard calls for a differential signaling scheme that will operate over a much longer line length than RS-232 can. 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. The bias resistance needs to be taken into account when selecting the termination resistor. The first time is in the total load limit stating that the total load limit of the network including fail-safe provisions, should be no less than 54Ω. This implies that the fail-safe provisions are resistance external to the receiver.

For long cables, heavier gauge wire may be needed to reduce resistance. When the driver increases the voltage on one of the wires, it simultaneously decreases the voltage on the other wire resulting in a differential voltage between the two wires. How much of each depends on the wire. From a hardware point of view, full-duplex RS-485 has some major advantages over RS-232 -- it can communicate over much longer distances at higher speeds. Determining which fault has occurred would take additional hardware. Automatic Converters take care of the timing problems and allow fast communications without software intervention. Modern processors are fast enough that handshaking is rarely needed. When there is no data activity on an RS485 network (or in many instances RS422 networks, other than point-to-point), the communications lines are "floating" and, thus susceptible to external noise or interference. Low-level RS485/BitBus Network implementation This class provides simple low level access to the underlying RS485 hardware implemented on port 4 of the Lego NXT. Low level write to the RS485 hardware port. There is no hardware handshaking in the RS-485 standard and in most cases it is no longer needed.

PURPOSE AND SCOPE OF COVERAGE - This standard is applicable to the interconnection of data terminal equipment (DTE) and data circuit-terminal equipment (DCE) employing serial binary data interchange. PURPOSE AND SCOPE OF COVERAGE - This standard is applicable to the interconnection of data processing terminal equipment and data communication equipment. The RS-485 standard is obsolete and has been superseded by TIA-485, but many if not most engineers and applications guides continue to use the RS designation even though it has officially changed. Typically Telecom applications are more I/O intensive than standard controllers. These characteristics make RS-485 useful in industrial control systems and similar applications. One reason is that, large networks accumulate distributed electrical noise which can make communications unreliable. The combination of the cable impedances and/or termination resistors will load the network and can make communications unreliable. Care must be taken that an SC connection, especially over long cable runs, does not result in an attempt to connect disparate grounds together - it is wise to add some current limiting to the SC connection.

In general RS485 is designed for multi-drop, "daisy-chain" operation over a single twisted pair cable with a nominal characteristic impedance of 120 Ohms. Worse is the fact that annex A states that this graph is a conservative guide based on empirical data of 24AWG telephone cable (POTS). This graph is from annex A of RS-422. For example: An RS-422 network has an always active driver and receiver. It calculates and displays the measured parameters and calculated active and reactive powers. It also monitors and display engine operating parameters (Oil Pressure, Engine Temperature, Battery Voltage, Speed etc). An Auto Start controller is an enhances Manual controller which uses a digital input to control the Start / Stop of the engine. Typical UART communications with an RS-422 device, uses both a transmitter and receiver on a network of two balanced pairs of wires for a total of four wires. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). It is possible to connect several RS485 circuits in parallel if the distances are below about 200 feet per leg @ 9600bps. At greater distances and higher data rates, the cable impedances add up and load the network.

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