ISL3150E-59E INTERSIL [Intersil Corporation], ISL3150E-59E Datasheet

ISL3150E-59E

Manufacturer Part Number

ISL3150E-59E

Description

Add a Loss of Signal (LOS) Indicator to Your RS-485 or RS-422 Transceiver

Manufacturer

INTERSIL [Intersil Corporation]

Datasheet

1.ISL3150E-59E.pdf (3 pages)

Current page: 1 of 3
Download datasheet (497Kb)

Add a Loss of Signal (LOS) Indicator to Your RS-485 or

RS-422 Transceiver

Introduction

A desirable feature on any network may be a Loss of Signal

(LOS) indicator. Such an indicator alerts a monitoring controller

that a potentially serious network error has occurred, thereby

allowing for notification and/or intervention. In an RS-485/

RS-422 network, a LOS may occur if a connector disconnects

from a board, if a cable fails, or if the network driver fails or

loses power. When this occurs, bus activity ceases - as shown

in Figure 1 - and an LOS detector would assert its output. This

application note discusses the RS-485/RS-422 receiver (Rx)

operation, explains how the Rx interprets the differential bus

voltages, and introduces a method for generating the desired

LOS indicator.

RS-485 Receiver (Rx) Basics

The RS-485/RS-422 Rx is a differential circuit (comparator)

that compares voltages at the noninverting and inverting

inputs (A and B), and outputs a corresponding logic level. Per

the Standards, if A-B (i.e., the differential input voltage) is

greater than +200mV, then the Rx outputs a logic high; if

A-B < -200mV, then the Rx outputs a logic low. Any differential

voltage between -200mV and +200mV is undefined, and the

Rx might output a logic high, a logic low, or may even oscillate.

February 1, 2011

AN1593.0

-1

-3

-5

FIGURE 1. TYPICAL LOS OPERATION

LOS

TIME (1µs/DIV)

BUS

CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.

1-888-INTERSIL or 1-888-468-3774

Intersil (and design) is a trademark owned by Intersil Corporation or one of its subsidiaries.

Rx Failsafe

In RS-485/RS-422 terminology, “failsafe” is used to indicate

Rx functionality that drives the Rx output to a known state

when common bus faults occur. The logic high state typically

represents the idle (no transmission in progress) state, so

driving the Rx output to a logic high under fault conditions is

the usual failsafe implementation. Without the failsafe

function, an Rx output’s inadvertant logic low, or oscillation,

might be interpreted as a message start bit. This could cause a

processor to waste valuable time attempting to service

phantom messages.

The most common form of failsafe is failsafe open, with

“open” referring to the condition where the Rx inputs are

floating. This can occur when a network bus is cut, or if the bus

connector separates from the networked device containing the

Rx. Most RS-485 Rx available today incorporate this function,

and it is typically accomplished via an internal pull-up resistor

on the noninverting (A) input. The pull-up is sized to ensure that

if the Rx inputs float, then the Rx input circuitry creates enough

of a positive offset so that the Rx comparator interprets the

input condition as a logic high.

The “Full-Failsafe” RS-485 Receiver

An improved differential Rx includes all the features

mentioned previously, plus the addition of circuitry to ensure

that the Rx is failsafe if the differential input voltage (V

(failsafe “shorted”). This condition may occur due to a cable

error (e.g., crimped), a connector error (shorted pins), or in

normally operating networks where multiple drivers operate

on a terminated bus (multi-point network). Multi-point

operation requires that all drivers be tri-statable, and there are

periods of time when all drivers are simultaneously tri-stated.

When this occurs, the differential termination resistor(s)

causes the bus voltage to collapse to the V

mentioned before, V

Rx with only failsafe open functionality may generate

erroneous start bits.

The full-failsafe Rx solves this problem by ensuring that the

output remains in the idle state. Full-failsafe functionality is

accomplished by designing the Rx so that the minimum input

high level is slightly negative (-10mV to -25mV), rather than the

+200mV used for standard RS-485 and RS-422 Rx. Note that

this new threshold definition remains RS-422 and RS-485

compliant.

=0V condition is recognized as a logic high input, so the Rx

All other trademarks mentioned are the property of their respective owners.

=0V is an undefined RS-485 level, so an

Application Note 1593

=0V condition. As

Author: Jeff Lies

) is 0V

Related parts for ISL3150E-59E

ISL3150E-59E Summary of contents

Page 1

Add a Loss of Signal (LOS) Indicator to Your RS-485 or RS-422 Transceiver Introduction A desirable feature on any network may be a Loss of Signal (LOS) indicator. Such an indicator alerts a monitoring controller that a potentially serious network ...

Page 2

Using Full-Failsafe Receivers for LOS Detection This full-failsafe feature can be exploited to detect the LOS bus condition. The advantage of using full failsafe receivers for LOS detection is that the V =0V condition is now a valid input ID ...

Page 3

... TABLE 1. INTERSIL RS-422/RS-485 FULL-FAILSAFE RECEIVER FAMILIES PART NUMBER TYPE ISL83080E-88E Transceivers ISL83070E-78E Transceivers ISL3150E-59E Transceivers ISL3170E-79E Transceivers ISL32470E-78E ±60V Fault Protected ISL32490E-98E Transceivers ISL3280E-84E Receiver Only ...

Related keywords

ISL3150E-59E datasheet
ISL3150E-59E data sheet
ISL3150E-59E pdf datasheet
ISL3150E-59E component
ISL3150E-59E part
ISL3150E-59E distributor
ISL3150E-59E RoHS
ISL3150E-59E datasheet download