
IL3585
11
NVE Corporation
11409 Valley View Road, Eden Prairie, MN 55344-3617
Phone: (952) 829-9217
Fax: (952) 829-9189
www.IsoLoop.com
NVE Corporation
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 k in the receiver section (RFS-INT; see figure below). These
internal resistors are designed to ensure failsafe operation but only if there are no termination resistors. The entire VDD will appear between inputs
“A” and “B” if there is no loading and no termination resistors, and there will be more than the required 200 mV with up to four RS-485 worst-
case Unit Loads of 12 k. 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 termination
resistors (RT) on each end of the cable, and four Unit Loads (12 k each), without external fail-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 (RFS-EXT) at one end of the bus can 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-485 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 k across the conductor pair, and 32 Unit Loads add 375 worst-case loading. The more loads on the bus, the lower the required
values of the biasing resistors.
In the example with two 120 termination resistors and four Unit Loads, 560 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
R
B
V
DD
30K
GND
A
5 V
R
FS-EXT
R
T
R
T
R
FS-EXT
R
FS-INT
R
FS-INT
Fail-Safe
RT
Loading
Operation?
None
Four unit loads (12 k ea.)
238 mV
Yes
120
Four unit loads (12 k ea.)
5 mV
No
560
120
Four unit loads (12 k ea.)
254 mV
Yes
510
120
32 unit loads (12 k ea.)
247 mV
Yes
Nominal VA-B
(inactive)
RFS-EXT
Internal Only