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AM26C32: RS422 Receiver Fail Safe Working Issue

Sagar B N
Prodigy 90 points
Part Number: AM26C32


Hello All,

We are using AM26C32IPW RS422 Receiver ICs in our Design. As per my understanding, AM26C32 part has Fail-safe circuitry for RS422 inputs - Outputs of RS422 Receiver IC is High when there are no inputs at RS422 Receiver(Open/No driver condition/No cables connected). 

In our Boards, when there are no RS422 inputs to this IC we have made some observations:

1. Some ICs outputs are low high and some are logic low.

2. In an IC, some 2 or 3 channel Outputs were low and One/Two channel output is High.

When we provide the right RS422 input signal, we get the right output. Whenever the IC inputs are open, we are facing issues. Please provide some suggestions/ideas for this issue (Can I provide an external fail-safe circuit).

We have terminated all the RS422 Reciever Inputs on our board using 100 Ohm Resistors.

  • 0
    Guru 317730 points

    The fail-safe function of this device is not very useful because it works only when the inputs are open. But the termination resistor shorts the inputs together.

    You need external fail-safe resistors that can create a 100 mV or 200 mV drop (depending on VIC) over the termination resistor.

  • 0
    TI__Guru 52602 points

    Hi Sagar,

    As Clemens mentioned adding a resistor on "A" to VCC and on "B" to GND will create a voltage divider that will create a default state during bus idle states. Setting the voltage to 200mV should be a safe level to set - at 5V vcc those external resistors are going to be ~1200 Ohms each (that gives ~200mV over the termination in idle states - it will be slightly different due to the input impedance of the receivers on the line).

    Please let me know if you have any other questions!

    Best,

    Parker Dodson