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SN65HVD72: Changing control DE and RE for RS485

Josh5566
Prodigy 180 points
Part Number: SN65HVD72
Other Parts Discussed in Thread: THVD1406

Hi Sir,

SN65HVD72DR

Due to the insufficient MCU GPIO to control DE and RE pins, I propose using a TX with BJT to control DE and RE.

below is my RS485 schematic for your reference. if any issue for this desige please let me know.

  • 0
    Guru 317280 points

    Not driving the bus for a high UART_TX signal will greatly reduce the noise margin.

    Consider using a transceiver with auto-direction control like the THVD1406.

  • 0
    TI__Guru 73651 points

    Hi Josh,

    My main concern with how you set this up is the device's inherent delay on enabling and disabling the drivers. The delay times will likely cause bit distortions and may stretch out your UART baud rate and in the worse case scenario, cause bit errors. 

    Below is a reference design which helps to work around this issue by setting up the time length for the amount of packets being sent using a 555 timer.

    https://www.ti.com/lit/ug/tidubw6/tidubw6.pdf?ts=1695739229670&ref_url=https%253A%252F%252Fwww.google.com%252F

    If your UART data packet is short enough, using an RS-485 transceiver with auto direction like Clemen's mentioned can also be an alternative. (Verify by checking the minimum time of tdevice_autodir parameter in the datasheet with the worst case UART packet of 0xFF plus any additional bits that could be 1's plus the stop condition.)

    -Bobby