APPLICATION NOTE

2

AN207

This has been a common problem with many standard failsafe receiver devices. The termination resistor

causes a potential difference of 0V because the internal biasing circuit is not strong enough to overcome the

termination load. The receiver sensitivity, as per RS-485 specifications, is +/-200mV (see Figure 3).

A potential difference of 0V is within the indeterminate range of sensitivity. This may result in an unknown state.

Under this condition, many may find the receiver output state on some devices to be good while others bad. In

fact, some devices with enough noise present may produce an oscillating clock pulse at output. One solution to

this terminated failsafe problem is to include an external biasing network that will provide a >+200mV potential

difference across the termination resistor at receiver input. This external biasing network is a simple pull-up

resistor from "A" input to Vcc and a pull-down resistor from "B" input to GND (See figure 4). The values of the

pull-up/down resistors are selected to provide at least 200mV across "A" and "B" inputs.

3.0 ENHANCED RECEIVER FAILSAFE

Exar has developed several new families of RS-485/RS-422 transceivers, SP1486, SP3080-3088, SP3495-

3497, XR3170-3178 and XR5486-5488 with an enhanced receiver failsafe feature. This enhanced receiver

failsafe provides failsafe for three different conditions: open input, shorted input and un-driven terminated lines.