Using 485 for QmodbusServer library

[Chai18]

Hello Everyone,

I am using the QModbusServer library for implementation of slave device. I have tested this library and receive signal from the master device. However there is an issue regarding 485. Since it is a half duplex protocol, we need to switch the mode using GPIO pin.
Using the sample application which is given here . I am using imx6ul series embedded device with a custom Yocto build .
I am receiving correct signal from the master, I am not able to respond because I am not able to switch 485 into transmit mode.
How do I modify this code for 485 communication.
Regards,

[jsulm]

@Chai18 said in Using 485 for QmodbusServer library:

because I am not able to switch 485 into transmit mode

Why not? What happens?

[Chai18]

@jsulm where in the code do I switch the GPIO pin?

[jsulm]

@Chai18 said in Using 485 for QmodbusServer library:

where in the code do I switch the GPIO pin?

I would say before you respond

[Chai18]

@jsulm Ok, let me rephrase it . QMobusRtuSerial is handling low level serial communication. The class is emitting a signal whenever it receives a frame. Is there a way to modify this behaviour.
In a nutshell where exactly in the code do I put my GPIO pin switch code.

[kuzulis]

Maybe you can override QModbusServer::processRequest() and do all work there.

[AYJF]

Hi @Chai18 Did you find any solution ? I have a similar issue but in the client side (QModbusClient) , this is my function to write data to the holding registers , but I'm all the time getting the response timeout, and master is actually handling incoming requests and send appropriate responses automatically.

void Serial::writeData(int start, const QList<quint16> &holdingRegisters) {
  if (!modbusDevice)
    return;

  startAddress = start;
  numberOfEntries = holdingRegisters.size();

  // Switch to Transmit Mode
  switchGPIO->setValue(true); //  enables TX

  QModbusDataUnit writeUnit = writeRequest();
  QModbusDataUnit::RegisterType table = writeUnit.registerType();
  for (qsizetype i = 0, total = writeUnit.valueCount(); i < total; ++i) {
    const auto addr = i + writeUnit.startAddress();
    if (table == QModbusDataUnit::Coils)
      writeUnit.setValue(i, m_coils[addr]);
    else
      writeUnit.setValue(i, holdingRegisters[addr]);
  }

  if (auto *reply = modbusDevice->sendWriteRequest(writeUnit, serverAddress)) {
    if (!reply->isFinished()) {
      QModbusDevice::connect(
          reply, &QModbusReply::finished, this, [this, reply]() {
            // Switch back to Receive Mode
            usleep(1000);                // Small delay before switching
            switchGPIO->setValue(false); // Enable RX mode

            const auto error = reply->error();
            if (error == QModbusDevice::ProtocolError) {
              qDebug() << "Write response error:" << reply->errorString()
                       << "(Modbus exception:"
                       << reply->rawResult().exceptionCode() << ")";
            } else if (error != QModbusDevice::NoError) {
              qDebug() << "Write response error:" << reply->errorString()
                       << "(code: 0x" << QString::number(error, 16) << ")";
            }
            reply->deleteLater();
          });
    } else {
      qDebug() << "Write error: " << reply->errorString();
      reply->deleteLater();
    }
  } else {
    qDebug() << "Write error: " << modbusDevice->errorString();
    switchGPIO->setValue(false); // Ensure RX mode is enabled on failure
  }
}

Thanks in advance