Transceiver Modes on NI RS-485 Serial Cards

Publish Date: Feb 08, 2018 | 2 Ratings | 3.50 out of 5 | Print


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.

1. Introduction

NI Serial 485 boards have 4 different transceiver modes:

  • 4 Wire: In this mode, output buffers and receivers are always enabled. In this mode of operation, you have two different pairs of transmission and reception lines and so the transmission lines (TX+ and TX-) are always enabled for transmission (i.e., they will always 'drive' the lines). Likewise, the reception pair (RX+ and RX-) will always be ready to accept the bytes.
  • 2 Wire DTR Controlled With Echo: In this mode, the receivers are always enabled and the output buffers (transmitters) are enabled/disabled under DTR control. To be able to use this mode, you must manually control the DTR line in your application software. If you are unsure about controlling the DTR line, please use the 2 Wire Auto mode described below.
  • 2 Wire DTR Controlled: This mode works like the DTR with Echo mode, but the receivers are automatically disabled whenever the transmitters are enabled. This avoids the echo of the information being transmitted by the same port, and 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 receivers are 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 output buffers are normally tri-stated and the board is in a listening state, waiting for information to be received. So, tri-state and read state are the same. This behavior is compatible with RS-422 and RS-485.

For information about wiring these topologies, please refer to the Serial Quick Reference Guide, or the product manual for your specific device.


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