Specifications

Chapter 2 - System Configuration RS-422 And RS-485 Applications Ebook
Page 34 of 137 Manual Documentation Number: <DocNumber>
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specifies that a mark is considered anything greater than -200 mV
(V
AB
). To ensure reliable operation B&B Electronics typically suggests
that you design for approximately -300 mV.
Bias resistors can be placed anywhere in the network, or can be split
among multiple nodes. The actual bias resistance is the parallel
combination of all bias resistors in a system.
Equivalent parallel resistance is calculated using the following
formulae:
Example 1: Biasing when termination resistors are used
In this example, ten RS-485 transceiver nodes are connected together
on a daisy-chain network. Two 120 termination resistors are
connected -- one at each end of the network. Each node includes two
built-in 4.7 k biasing resistors, a pull-up and a pull-down.
Question: Will this arrangement reliably maintain the idle state when
all drivers are tri-stated?
RS-485 nodes are specified to present a load impedance of 12 k . Ten
nodes in parallel present a total load of 1200 . The two 120
termination resistors present an additional 60 load (in parallel). The
result is a total load across the signal lines of 57 .
Ten nodes, each with 4.7 k biasing (pull-up and pull-down) resistors,
produce equivalent biasing resistances of 470 pull-up and 470 pull-
down. The total resistance from 5V to ground is 470 + 57 + 470 =
997 .