IL3522 Datasheet, PDF (7/10 Pages) List of Unclassifed Manufacturers – Isolated RS485/RS422 Interface

IL3522
selected so that under worst-case power supply and resistor tolerances there is at least 200 mV across the conductor pair with no active drivers to
meet the input sensitivity specification of the RS-422 standard.
Using the same value for pull-up and pull-down biasing resistors maintains balance for positive- and negative going transitions. Lower-value
resistors increase inactive noise immunity at the expense of quiescent power consumption. Note that each Unit Load on the bus adds a worst-case
loading of 12 kΩ across the conductor pair, and 32 Unit Loads add 375 Ω worst-case loading. The more loads on the bus, the lower the required
values of the biasing resistors.
In the example with two 120 Ω termination resistors and four Unit Loads, 560 Ω external biasing resistors provide more than 200 mV between
“A” and “B” with adequate margin for power supply variations and resistor tolerances. This ensures a known state when there are no active
drivers. Other illustrative examples are shown in the table below.
Fail-Safe Biasing
5V
V_DD
A
R
B
GND
R_FS-EXT
Internal Only
Internal Only
560 Ω
510 Ω
R_T
None
120 Ω
120 Ω
120 Ω
Loading
Four unit loads (12 kΩ ea.)
Four unit loads (12 kΩ ea.)
Four unit loads (12 kΩ ea.)
32 unit loads (12 kΩ ea.)
Nominal V_A-B
(inactive)
238 mV
5 mV
254 mV
247 mV
Fail-Safe
Operation?
Yes
No
Yes
Yes
7
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