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I am currently trying to use an arduino to send an RS485 via an SP3078EEN RS485 transciever. I have my arduino code set the DE pin high, wait 1ms, then write and flush a data buffer to the serial which is connected to the DI pin of the transciever. Post-this it sets the DE pin low again. This is almost all working as intended except for the fact that I am getting an extra 0x00 byte after my data as read by my oscilloscope and PC (measured via an RS485 to USB converter).

Below I have the important portion of my code as well as the oscilloscope measurements of Di (Driver Input), A (RS485 Positive Output) and DE (Driver Enable). I also have provided a simplified schematic of my circuit. The only thing I can see that might be the cause of this is the seemingly capacitive discharge on the "A" signal which occurs when I switch DE from high to low. I don't know why this is happening or how to fix it. I have tried putting pullup / pulldown resistors on RO, RE and DI to no avail. I have also tried increasing the pre-write delay and also adding a post-write delay, also to no avail.

If anyone can help me figure this out it would be greatly appreciated.

Oscilloscope Oscilloscope

Code:

void setup() {
    Serial0.begin(19200, SERIAL_8E1);
    Serial1.begin(19200, SERIAL_8E1);
}

void loop() {

    // Setting the buffer to a static value for debugging purposes
    byte buffer1 = { 0x01, 0x03, 0x00, 0x00, 0x00, 0x04, 0xC4, 0x0B };

    // Set the DE_1 pin high to enable writing to the RS485 bus
    digitalWriteFast(DE_1_PIN, HIGH);

    delay(1);

    // Write the buffer to the RS485 bus
    Serial1.write(buffer1, 8); 
    Serial1.flush();
    
    // Disable writing
    digitalWriteFast(DE_1_PIN, LOW);

    delay(1000);
}

Super-Simplified Schematic Simplified Schematic

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  • Welcome to SE/Arduino! Please take the tour to learn how this site works, and read "How to Ask". -- Beside the fact that your source and the record don't match in data, your schematic does not show the obligatory pull-up and pull-down resistors on the bus. Please edit your question to add whether you have them or not. Commented Oct 1, 2024 at 5:29
  • Anyway, your issue is not actually Arduino related. It could be closed soon for this reason. A better place would be SE/EE. Commented Oct 1, 2024 at 5:31

1 Answer 1

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Your TX/RX pins are reversed on the schematic: TX should go to DI, and RX to RO. I guess this is an error on the schematic only, not on the actual circuit.

Regarding the extra 0x00 byte after the message... I cannot see it on the scope trace. I see the A signal falling when it stops being driven, and this looks very much like a start bit followed by a stream of zeros. However, there is no matching stop bit. Thus, this shouldn't be interpreted as a transmitted byte, but rather discarded as a framing error. More appropriately, this is a “break” condition on the line.

I see two options for you:

On the software side, you should probably configure your receivers to not ignore framing errors, but instead discard mis-framed bytes. On the hardware side, you probably already have some termination resistors at the ends of the line. You can replace those by voltage dividers between GND and +5V in order to make the undriven A line idle at a potential higher than the undriven B line. This way the undriven bus idles at logic 1, like a serial line is supposed to. You should be able to find documentation on the Internet showing how to compute the appropriate resistor values.

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