Overview
1-2
NET485 User Guide
1.2.1 The RS485 Enable Signal
In a balanced differential system the data signals are produced by a line driver. (See the drawing below) The
line driver generates a voltage across a pair of signal wires that transmits the data signals. A balanced line
driver can have an optional input signal called an “Enable” signal. The purpose of the enable signal is to
connect the driver to its output terminals. If the enable signal is off, the driver is disconnected from the
transmission line. When a driver is disconnected from the network it is referred to as being in the “tri-state”
condition. Because there are multiple drivers (transmitters) on a RS485 network and only one transmitter
can be enabled at a time, the use of this enable control signal is required on all RS485 networks.
The following drawing shows a typical RS485 driver. Pin 3 is the transmit enable pin. Pin 2, the receive
enable pin, is tied to 3.3V which forces the receiver to always be enabled. When Pin 3 is high the transmitter
is enabled and data passes out the transmit pins. The design of the circuit allows the receiver section to read
the data that is being transmitted. When Pin 3 is low the transmitter is disabled and the output goes to a “tri-
state” condition. In this condition, the receiver section is still listening to the network.
D
R
Vcc
3.3V
DE
DI
RO
RE
G ND
8
3
4
1
2
5
7
6
3.3V
TXDA
TXDB
U2
DATA
LINE S
TRANS MIT
ENABLE
The NET485 provides the Transmit enable signal for RS485 two-wire applications. When configured for
RS485 two-wire applications, the NET485 automatically asserts the transmit enable when it is ready to
transmit data from its serial port. Once the data has been transmitted, the NET485 automatically de-asserts
the enable signal to allow other nodes to transmit their data.
The Transmit enable pin is called a Configurable Pin, meaning it can be selected to do several functions.
One of those options is the transmit enable, which is labeled “RS485 TX Enable”. The Quick Start Guide
explains how to configure the NET485 for proper RS485 operation.
WARNING: The NET485 comes from the factory already configured for RS485 operation. If you reset it to
factory defaults, you will have to reconfigure the unit for RS422/485.
NOTE: The RS485 transmit enable pin is configured in the Configurable Pins section of the Web Manager.
Note: There are different versions of the configurable pins window, depending on the version of firmware installed. Make
sure you select CP0 for older versions that label the pins CP0 to CP2. Select CP1 for versions labeled CP1-3.
This version of the configurable pins window shows CP 0-2. Web Manager 1.6.0.2.
Summary of Contents for NET485
Page 1: ...NET485 RS422 485 Serial to Ethernet Adapter Revision E October 14 2013 ...
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Page 60: ...Tables 7 2 NET485 User Guide Click Hex The hexadecimal value displays ...
Page 62: ...Tables 8 2 NET485 User Guide 8 1 Certificate of Compliance ...