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ISO3082: Signal Output Anomaly On Rx pin

Part Number: ISO3082


We are facing an issue related to ISO3082. When data is transmitted, Rx pin goes low for no obvious reason.

Please refer to circuit diagram as shown in Fig. 1.

Fig. 2 shows signals on RE and DE pins during data transfer operation.

Blue: DE

Yellow: RE

Fig. 3 shows transmitted data and signal at Rx pin during data operation.

Yellow: Rx pin

Blue: Transmitted Data

Please let us know for any modifications in the circuit or any changes required to avoid this problem.

Note: We tried this using separated isolated circuit as well and our application end with different sets of ICs. Still the problem persists repetitively.

  • Hi Hrishikesh,

    Thank you for reaching out to us!

    There is inconsistency between the schematic and  waveform that puzzles us.

    On the schematic the DE and REBAR are tied together but in the waveform there are separate signals for DE and REBAR.

    Could you please provide the following items for clarification?

    1. Please provide a waveform that plots  REBAR and R output. Please show the voltage per division.

    2. Please provide the schematic that clearly shows the wiring and the value of the pull-up resistor.


  • Hello Alfred

    Thank you for responding..

    Please find a revised schematic diagram attached herewith:

    Controller is 3.3V operated and supply (VCC) is 5V.

    Also wave forms as requested by you are attached as follows:

    A) REBAR (Yellow) and R (Blue)

    B) REBAR (Yellow) and D (Blue)

    C) D (Yellow) and R (Blue)

    We do not want any signal on R pin while transmitting the signal. In datasheet it is mentioned that R will be fail-safe high when REBAR is high. However, we got this issue.
    Please guide us in this matter. 

  • Hi Hrishikesh,

    Thank you for providing the requested information.

    According to the datasheet, "When RE is logic high or left open, the receiver output is high-impedance and the magnitude and polarity of VID are irrelevant. Internal biasing of the receiver inputs causes the output to go failsafe-high when the transceiver is disconnected from the bus (open-circuit), the bus lines are shorted (short-circuit), or the bus is not actively driven (idle bus)."

    Hence, Setting REBAR to logic high does not put the output (R) to failsafe-high but put it in the high-impedance state. The output (R) is only in the failsafe-high when the bus (A and B) is in open-circuit, short-circuit, or idle.

    When you set the REBAR to high, the output (R) is in the high-impedance state. Only the pull-up resistor is pulling up the output (R). The 1kohm resistor has too much resistance and hence the output (R) did not reach the Vcc1. Please replace the 1kohm resistor with a 470ohm resistor for good pull-up ability.


  • Hi Hrishikesh,

    The ISO3082 on the left side has both Vcc1 and Vcc2 connected to 5V.
    When REBAR is in logic low, and output (R) is in the high impedance state, the current flows from the 5V source through the pull-up resistor. The controller is powered by a 3.3V source. Since current flows from high voltage point to low voltage point, this creates a path from the 5V source of the ISO3082 to the 3.3V source of the microcontroller through the IO diodes. Please look at the purple line in the below figure that illustrates the current path from 5V source to 3V source. This explains why the voltage of the output (R) is around 3.3V in the first waveform you provided.

    One approach to prevent the current from flowing from 5V to 3.3V source is to disconnect Vcc1 and Vcc2. Then, connect Vcc1 with 3.3V source and Vcc2 with 5V source.


  • Thank you Alfred.. This resolved my issue..