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Does RS-485 2-wire mode actually require only 2 wires?

forums.ni.com › t5 › Instrument-Control-GPIB-Serial › Does-RS-485-2-wire-mode-actually-require-only-2-wires › td-p › 3756774

Hi Ricky, The selected mode should correspond to the interface you want together with the connections. They are all linked. In RS-422 mode (which is 4-wire) the 4-wire attribute mist be selected with no short between RX & TX. In RS-485 mode (which is 2-wire) the 2-wire Auto attribute must be selected with the short between RX & TX. In RS-422 mode the transmitter is always enabled (single master configuration) in RS-485 the transceiver must (automatically) switch between transmit and receive mode. RS-422 can be used as a Full-duplex interface i.e. transmit and receive at the same time RS-485 can only be used in Half-Duplex i.e. either transmit data or receive data. You mentioned 'protocol'. Where we talked about is only hardware. As we say the first layer, the hardware layer. A communication protocol is 1 or more levels higher and is related to software. e.g. a ModBus protocol. From the hardware perspective a line is used to transport data. Any data. A protocol defines the rules how to communicate. Commands, formats, etc. This can be done through any hardware layer. RS-422, RS-485, RS-232, ethernet, smoke, etc. Terminators: RS-485 is a very robust interface. It will work also in harsh environments. When I connect two (or more) device together on my workbench I don't care about terminators, twisted pair cable, RS-485 cable, Ground connection,etc. Just using a few fires and it works. If it doesn't I have to keep in mind that I used a sh*t connection. If we are installing an interface at the customer site it has to work for many years and we want to be sure that it will always work. So that is why we use the terminators, Daisy chain wiring and good RS-485 cable. RS-485 (and RS-422) are balanced lines. If you have an electronic background you know what that is. Balanced lines must be terminated to have a guaranteed energy transport. To be sure that the lines are forced in a idle state fail-safe terminators are use at one side. See attached picture. Vcc and Gnd are the power supply connection of the RS-485 transceiver. At the outside it is not always possible to wire it this way. Look in the manual to see if your master has terminitors. If you are not able to create a fail-safe terminator use the next step. A passive terminator as connected to the end of the line. Kees

Windmill Software

windmill.co.uk › rs485.html

Understanding RS485: Wiring, Connection, Monitoring Software

RS485 and RS422 drive lines in a differential fashion: two wires are required for each signal. If you wish to transmit four signals then eight wires are needed. The figure below shows a single RS485 / RS422 signal being transmitted. To transmit a logic 1, line B is high and line A is low.

Chipkin

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RS485 Cables: Why you need 3 wires for 2 (two) wire - Chipkin

RS485 is a 2 wire network if done right. Profibus is a typical example of a true 2 wire RS485 system. The secret to trouble free RS485 is to use isolated power supplies to run the RS485 transeiver.

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