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Rbias- are 1440Ω and Rterm is 120Ω there will be 0.2V of bias across the termination resistor. 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. The current could cause damage to the wire, or the RS-485 port. The annex of RS-485 states that "When employed, the shield shall be connected only to frame ground at either or both ends depending on the specific application." Shields usually have a lot more copper (and/or aluminum) than a single wire and can therefore carry a lot more current. It is commonly considered that a 12k resistance is 1 unit load, but unit load is more complex than than a single resistance. The driver's impedance (when active) is not specified, but the driver needs to be capable of driving 32 unit loads and a termination resistance as low as 60 Ω. The driver has to be capable of driving 32 unit loads. The driver has the capability of driving 10 receivers of 4k impedance, but the actual number that can be driven depend on the actual input impedance of the receivers, bit rate, wire, stub lengths, biasing and termination of the network.

The termination resistor should be equal to the characteristic impedance of the cable. The bias resistance needs to be taken into account when selecting the termination resistor. ↑ a b c "The art and science of RS-485: Termination" (PDF). ↑ "Guidelines for Proper Wiring of an RS-485 (TIA/EIA-485-A) Network". To understand how to terminate a network (the wires) you must first understand transmission lines. When the microprocessor's in the computers and peripheral's speed was increased to a blistering 1MHz, handshaking was still necessary, but the stop sending / OK to send lines would toggle less often. The processor could check the level of the buffer and only had to stop the data when the buffer was too full. 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.

The logic function of the driver and receiver are not defined, only the binary state of the differential voltages on the wires. The voltages are measured at the signal wire and are referenced to signal common. The receiver is designed to operate with voltages between 3 and 15V in magnitude (i.e. both positive and negative voltages) but must be able to handle an input of 25V without damage. The driver must output between 5V and 15V in magnitude into a load of 3kΩ to 7kΩ. The driver must not be able to output more than 100mA when shorted to any other conductor in the cable, must not be able to output more than 25V and must be able to handle an open circuit, or a short to any other conductor in the cable. The driver's output must be limited to 250mA peak output current, but may be limited to much less. This is too complicated a subject to go into depth here but a basic tutorial using oversimplified concepts may help.

A binary 1 may (or may not) be inverted by the driver before it is output. Logic IC's don't output a precision voltage. Before we discuss polarity lets take a look at logic levels and binary states. For example; EIA-449 and EIA-530 are connector standards that reference RS-422 for electrical levels. The signals are named from the standpoint of the DTE. The signals appear differentially on these two wires. The signals appear differentially on two wires of each pair. RS-485 has a single pair of wires. A single set of bias resistors could be used at one end of the network, if the resistance was halved to 720 Ω. When a "bad message" shows up on the cable, it is more difficult (but not impossible) to figure out which node(s) transmitted that message when you have a shared-medium with a dozen nodes connected to the same single cable, RS485 standard compared to a point-to-point medium with only 2 nodes connected to any particular cable. 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.