unsigned long tic_loop = 0;
const unsigned int FRAME_TIME = 25; // 20 ms -> 50 FPS

byte channels_buffer[512] = {0};
unsigned int bytes_to_write = 512;

const unsigned int START_ADDR = 10;
const unsigned int NUM_CHANNELS = 9;

byte dmx_data[] = {
    0,
    0,
    134,
    255,
    0,
    0,
    0,
    0,
    0,
};

void setup()
{
    Serial.begin(115200); // USB

    Serial1.begin(250000, SERIAL_8N2); // DMX
    tic_loop = millis();

    for (int i = 0; i < NUM_CHANNELS; ++i)
    {
        channels_buffer[START_ADDR - 1 + i] = dmx_data[i];
    }
}

void loop()
{
    // update_buffer();

    // this section gets executed at a maximum rate of around 40Hz
    if ((millis() - tic_loop) > FRAME_TIME)
    {
        tic_loop = millis();

        send_dmx_header();
        Serial1.write(channels_buffer, bytes_to_write);
    }
    delay(1);
}

void update_buffer()
{
    int n = Serial.available();

    if (n < 1)
        return;

    n -= 1;

    if (Serial.read() == n)
    {
        Serial.read(channels_buffer, n);
        bytes_to_write = n;
    }
    else
    {
        // incomplete
        while (Serial.available())
            Serial.read();
    }
}

void send_dmx_header()
{
    Serial1.flush();
    Serial1.begin(90000, SERIAL_8N2);
    while (Serial1.available())
        Serial1.read();
    // send the break as a "slow" byte
    Serial1.write(0);
    // switch back to the original baud rate
    Serial1.flush();
    Serial1.begin(250000, SERIAL_8N2);
    while (Serial1.available())
        Serial1.read();

    Serial1.write(0); // Start-Byte
}