APPLICATION NOTE

1

AN207

ADVANTAGES OF ENHANCED RECEIVER FAILSAFE FEATURES

IN RS-485/RS-422 SERIAL TRANSCEIVERS

1.0 INTRODUCTION

RS-485/RS-422 receiver failsafe keeps the data bus in a known state when there are no active drivers on the

bus. The Exar RS-485/RS-422 transceivers offer two different types of receiver failsafe: standard and

enhanced. This application note will discuss how the enhanced receiver failsafe feature can help customers

with failsafe problems while terminated.

2.0 STANDARD RECEIVER FAILSAFE

RS-485/RS-422 devices such as Exar’s SP491 offer a standard receiver failsafe. The standard receiver

failsafe feature for this device will provide a logic high at the receiver output when the receiver inputs are open

circuit or floating. For example, when the data cable is disconnected, the receiver output will keep the line in a

known high state. Standard receiver failsafe has an internal weak biasing network that will force the open

inputs to >200mV thus resulting in a logic high output. Figure 1 shows the receiver configuration for failsafe

logic high output.

Some applications operating at high speeds incorporate a termination resistor at the ends of the bus to

prevent signal reflections. The termination resistor value should match the characteristic impedance of the data

cable. In a majority of applications, the termination resistor is typically 120 ohms. When a termination resistor is

connected between receiver inputs "A" and "B" the standard receiver failsafe feature will not operate properly

(see Figure 2).