il3422 NVE Corp., il3422 Datasheet - Page 10

• Manufacturer Part Number: il3422
• Description: High Speed Rs-485 And Rs-422 Isolated Transceivers
• Manufacturer: NVE Corp.

Fail-Safe Operation:
“Fail-safe operation” is defined here as the forcing of a logic high state on the “R” output in response to an open-circuit condition
between the “A” and “B” lines of the bus, or when no drivers are active on the bus.
Proper biasing can ensure fail-safe operation, that is a known state when there are no active drivers on the bus. IL3000-Series
Isolated Transceivers include internal pull-up and pull-down resistors of approximately 30 kΩ in the receiver section (R
figure below). These internal resistors are designed to ensure failsafe operation but only if there are no termination resistors. The
entire V
the required 200 mV with up to four RS-485/RS-422 worst-case Unit Loads of 12 kΩ. Many designs operating below 1 Mbps or
less than 1,000 feet are unterminated. Termination resistors may not be necessary for very low data rates and very short cable runs
because reflections have time to settle before data sampling, which occurs at the middle of the bit interval.
In busses with low-impedance termination resistors, however, the differential voltage across the conductor pair will be close to
zero with no active drivers. In this case the state of the bus is indeterminate, and the idle bus will be susceptible to noise. For
example, with 120 Ω termination resistors (R
safe biasing resistors the internal pull-up and pull-down resistors will produce a voltage between inputs “A” and “B” of only about
5 mV. This is not nearly enough to ensure a known state. External fail-safe biasing resistors (R
ensure fail-safe operation with a terminated bus. Resistors should be 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 and RS-485 standards.
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
NVE Corporation
Internal Only
Internal Only
R
560 Ω
510 Ω
FS-EXT
DD
R
will appear between inputs “A” and “B” if there is no loading and no termination resistors, and there will be more than
11409 Valley View Road, Eden Prairie, MN 55344-3617
120 Ω
120 Ω
120 Ω
None
R
T
Four unit loads (12 kΩ ea.)
Four unit loads (12 kΩ ea.)
Four unit loads (12 kΩ ea.)
32 unit loads (12 kΩ ea.)
GND
V
DD
Loading
5 V
T
A
B
) on each end of the cable, and four Unit Loads (12 kΩ each), without external fail-
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Nominal V
(inactive)
238 mV
254 mV
247 mV
10
5 mV
A-B
Operation?
Fail-Safe
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Yes
Yes
Yes
No
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FS-EXT
IL3485/IL3422
) at one end of the bus can
©2008 NVE Corporation
FS-INT
; see