IL3022

10

NVE Corporation

11409 Valley View Road, Eden Prairie, MN 55344

(952) 829-9217

www.nve.com

YouTube.com/NveCorporation

iso-apps@nve.com

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 (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-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 (RT) 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-

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-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

R

B

V

DD

GND

A

5 V

Fail-Safe

RT

Loading

Operation?

None

Four unit loads (12 kΩ ea.)

238 mV

Yes

120 Ω

Four unit loads (12 kΩ ea.)

5 mV

No

560 Ω

120 Ω

Four unit loads (12 kΩ ea.)

254 mV

Yes

510 Ω

120 Ω

32 unit loads (12 kΩ ea.)

247 mV

Yes

Nominal VA-B

(inactive)

RFS-EXT

Internal Only

Internal Only