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This figure shows biasing applied to the termination network. Forcing the wires to the idle condition when no driver is active is the most common reason to need biasing on the network. Why is bias important in an RS485/RS422 network? Why is termination important in an RS485/RS422 network? Ever tried to build a network with devices from different vendors? RS-485 typically uses a linear network with bidirectional signal wires: There are typically many devices along a RS-485 shared cable. Half of the conductors in a 50 way cable are ground return wires. In addition there is no good way to add terminations resistors at the ends of a "star" network. TODO: add connectors and pins from manufacturers such as NI, B&B, ADAM, etc. Volunteers? 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. There is also a 4-wire RS485 (sometimes mistakenly called RS422 by several vendors). While RS422 is comparable to RS485, it is limited to unidirectional data traffic, and is terminated only on the end of the line opposite the transmitter.

To insure that a receiver stays in an inactive state, when no data signal is present, bias is generally added to a network at one or more locations. However the bias resistors effect the total termination resistance. Other receivers on the network that do not have this shifted receive threshold may require that external bias resistors added to the network. 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. 0.2V the receivers output switches to a 1 and when the voltage goes below -0.2V the receivers output switches to a 0. (or vice versa if there is inversion on the receiver) Since the last bit from a UART will be the stop bit (1), then the transmitter is turned off (the differential voltage goes to 0V, but not having gone less than -0.2V), this should leave the receiver with a 1 being output to the receiving UART.

Normally you will want to use a pull-up voltage that matches the driver's supply voltage. When it was able to receive more data, the receiver would use the handshaking lines to signal the transmitter that it was OK to send more data. 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. The RS485 standard only specifies electrical characteristics of the driver and the receiver, it does not specify or recommend any protocol. There is no hardware handshaking in the RS-485 standard and in most cases it is no longer needed. With galvanic isolation the damage is generally limited to only one leg of the network, except in extreme cases of very high voltage (induced by lightening for example). Unloaded driver output differential voltage can be as high as ±6V. Converters in general can be used to change the electrical characteristice of one communications standard into another, to take advantage of the best properties of the alternate standard selected. The RS485 standard (TIA-485) specifies differential data transmission over a terminated twisted pair..

RS485 is popular for inexpensive local networks, multidrop communication links and long haul data transfer over distances of up to 4,000 feet. One of the pairs is dedicated to PC-to-peripheral communication. 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. Alas, a long 3-conductor cable intended for RS-232 can not be switched to full-duplex RS-485, which requires 5 conductors. The LocalTalk network uses RS-485-compatible differential signaling on standard 4-wire RJ11 telephone connectors and cable. LocalTalk only connects to the next-outermost pair of wires (the "outer pair" on a standard 4-wire cable). Many people recommend wiring things up on a prototype with Category 5 cable connected as point-to-point full-duplex RS-485. This cable is usually 24AWG. Category-5 cable will generally work well in most instances even though its characteristic impedance is 100 Ohms.