AN1348 - APPLICATION NOTE

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6.3 Fail Safe Termination

ST485 has an internal failsafe circuit useful when the differential input is floating, for instance, if all the

drivers on the line are in Hi-impedance state. Failsafe circuit avoids undefined state of the receiver that

stays in a stable logic level, until the input differential voltage exceeds the ±200mV. However the internal

failsafe resistors are influenced from the external termination loader (54 ÷ 120Ohm) so that an external

failsafe circuit is required if the termination resistor is present. A typical failsafe configuration is shown in

figure 9. The two resistors Ra (pull-up) and Rc (pull-down) should be 500Ohm while Rb (load) is

130Ohm.

Figure 9: Failsafe Configuration

This load reduces the maximum numbers of transceiver connected to the network and causes a DC

current consumption. Figure 10 shows a solution of how to reduce the DC current.

Figure 10: Failsafe Configuration with Low DC Current