AN-207 Datasheet, PDF (1/6 Pages) Exar Corporation – ADVANTAGES OF ENHANCED RECEIVER FAILSAFE FEATURES

APPLICATION NOTE
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.
FIGURE 1. OPEN INPUT RECEIVER FAILSAFE
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).
FIGURE 2. TERMINATED RECEIVER INPUT
1