Specifications

Chapter 2 - System Configuration RS-422 And RS-485 Applications Ebook
Page 36 of 137 Manual Documentation Number: <DocNumber>
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Example 2: Biasing when termination resistors are not used
In this example, ten RS-485 transceiver nodes are connected together
on a daisy-chain network. No termination resistors are connected. 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 .
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 + 1200 + 470
= 2.24k .