What are Ground Terminals for? Most RS-485 interfaces, but not all (especially some USB to RS-485 converters), provide a ground, or “GND” terminal. Bear in mind that the terminating resistors electrically load the bus and may decrease the maximum length of your run, number of devices and data transmission speed. As the length of the run increases or in a noisy environment, adding a 100 ohm resistor across the terminals of the devices at each end of a run (not at every device) may improve settling time and allow faster rates. On the device, if only a single pair of terminals is available (for instance, Data + and Data -), then it is most likely a half duplex device.ĭo I need a terminating resistor? At low data rates and short runs, the wire resistance (assuming 24AWG diameter wire or smaller) is often sufficient to cause settling on the bus between transmissions.

In full-duplex, a second pair is used, allowing devices to talk and listen at the same time.

In half duplex, each device must alternate using the wiring pair to talk or listen, but not both at the same time. What is the difference between half duplex and full duplex? In order to transmit you need a pair of wires: one wire to transmit and one for a return, forming a circuit path. It is better to simply extend the main run and double back. You should also avoid a long segment of wire branching off from the main run to one or more devices.

To minimize the effect of noise, use twisted pairs as in Category 3 or Category 5 cable, minimize wire lengths and avoid running parallel to power cables or near noisy devices (motors, relays, fluorescent lights). Remember that a maximum number of devices (usually 32) can be addressed on each RS-485 network and the maximum wire length is generally 4000 ft (1200 m). Wire RS-485 networks terminal-to-terminal in a daisy chain and avoid segment branches to reduce electrical reflections and improve transmission reliability

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