IL3085

11

NVE Corporation

11409 Valley View Road, Eden Prairie, MN 55344-3617

(952) 829-9217

www.nve.com

YouTube.com/NveCorporation

iso-apps@nve.com

Receiver Features

The receiver output “R” has tri-state capability via the active low RE input.

Driver Features

The RS-485 driver has a differential output and delivers at least 2.1 V across a 54 Ω load. Drivers feature low propagation delay skew to

maximize bit width and minimize EMI. Drivers have tri-state capability via the active-high DE input.

Receiver Data Rate, Cables and Terminations

The IL3085 is intended for networks up to 4,000 feet (1,200 m), but the maximum data rate decreases as cable length increases. Twisted pair

cable should be used in all networks since they tend to pick up noise and other electromagnetically induced voltages as common mode signals,

which are effectively rejected by the differential receiver.

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-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 (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-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 following table: