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THVD4411: Technical Inquiry: Unexpected RS232 Loopback Behavior

Ajmal A
Prodigy 20 points

Part Number: THVD4411
Other Parts Discussed in Thread: THVD4431,

Dear Support Team,

We are currently evaluating Industrial Gateway and have encountered some behavior regarding the UART interfaces that we would like to clarify.

Environment:

  • Operating System: Linux

  • Interfaces: UART ports configured for RS232 and RS485 modes

  • Test Method: Simple read/write operations to the serial ports

Observed Behavior:

During our evaluation, we conducted a simple serial communication test where we configured a port in RS232 mode and performed write and read operations. We noticed that when no external wiring (loopback connection) was present between the TX and RX pins, data written to the port was still being received by a reading process on the same port.

However, when we configured the same port in RS485 full-duplex or half-duplex mode and performed identical tests without any external wiring, we did not observe any data reception on the same port.

Our Concerns:

  1. Is the self-reception behavior observed in RS232 mode an intentional feature of the board's hardware design or the UART controller? Some UART implementations have internal loopback capabilities for diagnostic purposes.

  2. If this is an intentional feature, is there a way to enable/disable it through hardware configuration or software control?

  3. If this is not intentional, could this be due to signal coupling, ground path issues, or some other electrical phenomenon that might affect normal operation in a real-world application?

  4. Does the UART controller or the interface circuitry have any built-in echo or loopback functionality that might be enabled by default in certain modes?

Context:

Our goal is to understand whether this behavior is expected and to determine if any configuration changes are needed to ensure reliable RS232 communication in our application. Since RS485 does not exhibit similar behavior, we want to confirm that both interfaces are functioning correctly.

Any insights or documentation references you can provide regarding this behavior would be greatly appreciated.

Thank you for your assistance.

 

Best regards,

Ajmal Aboobakar

Embedded Software Developer

  • 0
    TI__Mastermind 32110 points

    Hi Ajmal,

    The device expert supporting this device is currently out of office, he will get back to you once he return to work. Please expect a respond by Friday or Monday next week at latest.

    Thanks for you patience.

  • 0
    TI__Mastermind 28712 points

    Hi Ajmal,

    Thanks for reaching out E2E, and thanks for your patience as I have been out of the office until now. 

    1. No, this device does not have a "loopback mode." Figure 7-2 shows the proper implementation when MODE1=0, MODE0=1. 

    2. Refer to 1. I would double check the MODE1 and MODE0 pin settings if you are able. I would also double check the exact part number. The THVD4431 does have loopback mode. 

    3. Yes, there is a chance. But likely there is a connection somewhere at the hardware level on the PCB. 

    4. I cannot help with your questions about the UART controller as I only support the THVD4411 transceiver. 

    Ajmal A said:
    However, when we configured the same port in RS485 full-duplex or half-duplex mode and performed identical tests without any external wiring, we did not observe any data reception on the same port.

    This is odd. When in RS-485 half duplex mode, you should see the signal loopback. This is because in half duplex mode the pins are internally shorted together. Figure 7-3 shows this in the datasheet.

    Best,

    Ethan