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ISO1430: ISO1430DW IC has a loop back fault

Vijay Balaji
Prodigy 130 points

Part Number: ISO1430

Hi,

I am communicating between a microcontroller and a computer using an RS485 IC (ISO1430DW), which periodically transmits a value (such as the letter "A") to pin 6(Tx) and receives it back at pin 3 (Rx).

How Can the Issue Be Fixed?

Thanks & Regards,

Vijay

  • 0
    TI__Expert 3496 points

    Hi Vijay,

    Thanks for reaching out. 
    ISO1430 is only an isolated buffer and it doesn't transmit any value periodically. The periodic transmission should either be from the master or peripheral devices.
    Please provide us the schematic and waveforms on side 1 and side 2 to confirm the connections of ISO1430.


    Thanks

    Vikas J

  • 0 in reply to Vikas Jeevannavar
    Prodigy 130 points

    Hi Vikas

    I will mention the Schematic below.

    Thanks and Regards,

    Vijay

  • 0 in reply to Vijay Balaji
    TI__Expert 3496 points

    Hi Vijay,

    The schematic looks fine. The periodic transmission is not due to ISO1430. It might be microcontroller or the computer. Please bypass the ISO1430 or probe TX and RX to narrow down the source of the transmission.

    Thanks

    Vikas J

  • 0 in reply to Vikas Jeevannavar
    Prodigy 130 points
    Vikas Jeevannavar said:

    Hi Vijay,

    The schematic looks fine. The periodic transmission is not due to ISO1430. It might be microcontroller or the computer. Please bypass the ISO1430 or probe TX and RX to narrow down the source of the transmission.

    Thanks

    Hi Vikas,

    I'll examine it. for your assistance.

    Thanks & Regards,

    Vijay

  • 0 in reply to Vikas Jeevannavar
    Prodigy 130 points

    Hi Vikas,

    I'll examine the entire process. The transmit value is returned to the Receiver Pin (Rx).
    Shown in the screenshot below. Rx is the green line, and Tx is the yellow line Read and write enable is the Blue line.

    thanks and Regards,

    Vijay

  • 0 in reply to Vijay Balaji
    TI__Expert 3496 points

    Hi Vijay,

    Thanks for providing the screenshots. It is not very clear on the exact issue faced. Please elaborate the issue so that we can narrow down to the problem. The waveform has data on TX (yellow). It looks like the receiver is disabled as DE and REn are shorted. Since the receiver is disabled, the output RX (green) is tri-stated and is slowly going to 0V.

    Thanks

    Vikas J

  • 0 in reply to Vikas Jeevannavar
    Prodigy 130 points

    Hi Vikas,

    My issue is that the Receiver side is receiving the Transmit value.
    I'll walk you through the process:
    RS485 is connected to the slave device and microcontroller. 
    I get into problems when I receive commands from slave devices.
    1. The DE and REn pins are both shorted. 
    Rx is Green, Tx is Yellow, and DE and REn are Blue.
    2. For receiving some commands in the Rx pin (such as the command "R!"), DE and REn pins are set to low.
    3. Send some bytes in the Tx pin when you receive the "R!" command (for example, send the value "RDTD 1 14:28:32 05.03.23!" in the Tx pin with the pattern "RDTD_!_HH:MM:SS_DD.MM.YY!").
    4. Before transmission, the DE and REn pins are set to High.

    Thanks and Regards

    Vijay

  • 0 in reply to Vijay Balaji
    TI__Guru 57445 points

    Hi Vijay,

    Thank you for walking us through the data receive and transmit process and also briefly describing the issue that you are facing.

    From the steps you listed out, I understood that RXD pin receives commands like "R!" and when such commands are received, I understood that the MCU transmits some data on TXD pin in the format you have specified.

    This is understood, but I didn't see you mention what is the issue and when exactly it happens. I know you mentioned RXD is seeing the same information that is being transmitted on TXD. Did you mean to say that when you transmit the data like "RDTD 1 14:28:32 05.03.23!" on TXD, do you see all this information also show up on RXD?

    Since ISO1430 is a half-duplex device, everything that is transmitted on TXD will show up on RXD unless the receiver is disabled by making RE\ HIGH continuously during the time when data is being transmitted. If you are receiving the transmitted data on RXD, then it is possible that the code may be momentarily enabling the receiver every time data is being transmitted. You might want to check this and verify in the code.

    From the waveform you have shared, RXD (Green) waveform is clearly going to 0V when data is being transmitted. It doesn't look like RXD is receiving the data and the waveform confirms that. Can you please check your code to see if the transmitted data is accidentally being also pushed to the receive registers?

    The best way to debug hardware and software issues is rule out the hardware issue first by testing the hardware separately. i.e., you can apply a square wave to TXD and monitor RXD and A/B pins all at the same time to verify if ISO1430 is working as expected. Since ISO1430 is only a transceiver, it simply passes the information between the two sides based on what channels are enabled. It doesn't have any intelligence to manipulate the data in any other way.

    Please do help answer the questions I have listed above and perform the experiments I have suggested to isolate the issue. Please let me know if you have any questions, thanks.


    Regards,
    Koteshwar Rao