AN-207 Datasheet, PDF (3/6 Pages) Exar Corporation – ADVANTAGES OF ENHANCED RECEIVER FAILSAFE FEATURES

APPLICATION NOTE
AN207
This feature is very useful for terminated applications in that no external biasing network is required for proper
failsafe operation. Many of Exar’s original RS-485 transceivers with standard failsafe may be upgraded to a pin
compatible design using SP1486, SP3080-3088, SP3495-3497, XR3170-3178 and XR5486-5488 products.
The SP3086E is pin compatible to SP491. The SP3086E receiver offers an enhanced failsafe feature and has
a negative input threshold (see figure 5).
FIGURE 5. ENHANCED RECEIVER FAILSAFE SENSITIVITY
When the connected driver is disabled while the receiver input is terminated the potential difference at receiver
input will be 0V. Since the enhanced receiver failsafe device has a negative threshold the output will still be
logic high when the differential voltage is 0V. Figures 6, 7 and 8 show the three different conditions for
enhanced receiver failsafe logic high output.
FIGURE 6. OPEN INPUT
3