il3585etr13 NVE Corp., il3585etr13 Datasheet - Page 7

• Manufacturer Part Number: il3585etr13
• Description: Profibus-compatible Isolated Rs-485 Interface
• 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
internal resistors are designed to ensure failsafe operation but only if there are no termination resistors. The entire V
“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 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
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
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 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
Internal Only
Internal Only
R
560 Ω
510 Ω
NVE Corporation
FS-EXT
R
T
) on each end of the cable, and four Unit Loads (12 kΩ each), without external fail-safe biasing resistors the internal pull-up and pull-
120 Ω
120 Ω
120 Ω
None
11409 Valley View Road
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.)
FS-EXT
GND
V
) at one end of the bus can ensure fail-safe operation with a terminated bus. Resistors should be
DD
Loading
5 V
Eden Prairie, MN 55344-3617 USA
A
B
Nominal V
(inactive)
238 mV
254 mV
247 mV
7
5 mV
Telephone: [phone]
A-B
Operation?
Fail-Safe
Yes
Yes
Yes
No
Fax [phone]
FS-INT
DD
; see figure below). These
Internet: www.isoloop.com
will appear between inputs
IL3685