Specifications

Chapter 2 - System Configuration RS-422 And RS-485 Applications Ebook
Page 33 of 137 Manual Documentation Number: <DocNumber>
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mV) so that when a driver does start sending serial data the start bit
transition (from logic 1 to logic 0) can be detected.
However, since all drivers are tri-stated nothing is holding the
communication line in the idle state. Without anything driving the
network, the state of the line is unknown--essentially floating. If V
AB
is
less than -200 mV the logic level is indeterminate (and internally, at
the output of the receivers will typically be the value of the last bit
received).
In order to maintain the proper idle state, bias resistors must be
included in the communications outputs to force the data lines to the
idle state voltage. Bias resistors are nothing more than a pull-up
resistor on the DATA B(+) line (typically to 5 volts) and a pull-down
resistor (to ground) on the DATA A(-) line. The following diagram
illustrates how bias resistors are connected on a transceiver in a two-
wire configuration.
Figure 23: Bias Resistors
The value of the required bias resistance is dependent on termination
resistance in a system and number of nodes connected. The goal is to
generate enough DC bias current in the network to maintain a voltage
across the data lines (when no drivers are enabled) that will be
interpreted as a mark or logic 1 state. The EIA/TIA-485 standard