We have some problems with the ST485BDR product we when apply the 120 Ohm load resistor

[RMele.1]

We produced a 4 channels RS232 to 485/422 converters using a FTDI4232 and four ST485BDR. When apply the 120 Ohm load resistor, some ST485BDR send the pin 1 to low state, lighting the RX LED.

When the pin 1 goes low, the RX LED remain lite. If I touch the driver area, the LED (pin 1 of the ST485BDR) change state ramdomly. This issue do not depend if the channel is or not connected a other channel. It seems that some ST485BDR have a different behavior, although all channels are connected exactly the same. When we replace a failed ST485BDR with a MAX485ESA the problem is solved

Cano anyone help me?

Regards

Ruben H. Meleca

[Winfred LU]

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

However, the internal failsafe resistors are influenced from the external termination loader so that an external failsafe circuit is required if the termination resistor is present.

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

This load reduces the maximum numbers of transceiver connected to the network and causes a DC current consumption. Below is a solution of how to reduce the DC current.

[RMele.1]

Thank you Winfred, the problem was risolved.