Dear, we have a problem whith some ST485BDR. When the 120 ohm load resistor is applied, the pin 1 goes to low state, lighting the green RX LED

[RMele.1]

This board perform 4 channels UART to RS485/422 conversion, using a FT4232 device and four ST485BDR devices. The issue is present in some channels in aleatory way. When I replace the failed ST485BDR device by the MAX485ESA the issued chanel start to work fine.  

Best regards

Ruben H. Meleca

[Winfred LU]

ST485 has an internal failsafe circuit useful when the differential input is floating, which avoids undefined state of the receiver that stays in a stable logic level.

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 below. The two resistors Ra (pull-up) and Rc (pull-down) should be 500Ohm while Rb (load) is 130Ohm.