APPLICATION NOTE

3

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).

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.