Transceiver Modes on NI RS-485 Serial Cards


This article provides information about the different transceiver modes on the NI RS-485 serial cards. It also explains the tri-state condition for an RS-485 Port.

Transceiver Modes

NI Serial 485 boards have 4 different transceiver modes:


In this mode, transmitters and receivers are always enabled. There are two different pairs of transmission and reception lines which allow for simultaneous bi-directional communication. The transmission lines (TX+ and TX-) are always enabled for send bytes (i.e., they will always 'drive' the lines). Likewise, the reception lines (RX+ and RX-) are always ready to accept bytes.

2-Wire DTR Controlled With Echo

In this mode, the receiver is always enabled and the transmitter is enabled or disabled under data terminal ready (DTR) control. To be able to use this mode, you must manually control the DTR line in your application software. For an example, refer to Controlling DTR RTS Lines on a Serial Port.

If you are unsure about controlling the DTR line, please use the 2 Wire Auto mode described below.

2-Wire DTR Controlled

In this mode, the receiver is automatically disabled when the transmitter is enabled. This avoids the echo of the information being transmitted by the same port, so you do not receive the bytes that you just transmitted.

If you are unsure about controlling the DTR line, please use the 2 Wire Auto mode described below.

2-Wire Auto

In this mode, the transmitter is enabled automatically when needed and disabled as soon as transmission is finished. The receiver is disabled whenever the transmitter is enabled to avoid the echo. This is the recommended mode of operation for 2-wire data transmission since you do not have to enable or disable the transmitters manually.

When in any of the 2-wire modes, the transmitter is normally tri-stated and the board is in a listening state, waiting for information to be received. Therefore, the tri-state and read state are the same. This behavior is compatible with RS-422 and RS-485.


Additional Resources

For diagrams of the 2-wire and 4-wire topologies, refer to the Serial Quick Reference Guide.

For information about wiring these topologies, refer to the following links or the product manual for your specific device.

For information about setting the transceiver mode in software, refer to Setting the Transceiver Wire Mode on Ports for NI Serial Boards.

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