Arduino + ESP32 + RS485 = nightmare

What's wrong now, Fatty?

Last time I used the ESP32-S3-POE-ETH-8DI-8DO module, it was happy to talk to the Bojke laser sensor. But can I get the ESP32-S3 Mini to talk to it? Can I f*ck.

Claude Code got its knickers in a good old twist trying to figure it out but finally I lost the will to live.

Here's what I had:

/**
 * ============================================================
 *  BOJKE BL-30NZ-485 Laser Displacement Sensor
 *  Waveshare ESP32-S3-POE-ETH-8DI-8DO board
 * ============================================================
 */
#include <Arduino.h>
#include <Wire.h>
// ── I2C pins (from I2C_Driver.h) ─────────────────────────────
#define I2C_SCL_PIN   41
#define I2C_SDA_PIN   42
// ── TCA9554 I2C expander (from WS_TCA9554PWR.h) ──────────────
#define TCA9554_ADDRESS     0x20
#define TCA9554_INPUT_REG   0x00
#define TCA9554_OUTPUT_REG  0x01
#define TCA9554_CONFIG_REG  0x03
// ── RS485 pins (from WS_GPIO.h) ──────────────────────────────
#define TXD1     17
#define RXD1     18
#define TXD1EN   21
// ── RS485 serial port ─────────────────────────────────────────
HardwareSerial rs485Serial(1);
// ── Modbus settings ───────────────────────────────────────────
#define MODBUS_BAUD                115200
#define MODBUS_SLAVE_ID            0x01
#define SENSOR_REG_ADDR            0x0000
#define SENSOR_REG_COUNT           0x0002
#define MODBUS_RESPONSE_TIMEOUT_MS 500
#define EXPECTED_RESPONSE_LEN      9
#define POLL_INTERVAL_MS           1000
// ─────────────────────────────────────────────────────────────
//  TCA9554 helpers
// ─────────────────────────────────────────────────────────────
void TCA9554_WriteReg(uint8_t reg, uint8_t data) {
    Wire.beginTransmission(TCA9554_ADDRESS);
    Wire.write(reg);
    Wire.write(data);
    Wire.endTransmission();
}
void TCA9554_Init(uint8_t pinMode, uint8_t pinState) {
    TCA9554_WriteReg(TCA9554_OUTPUT_REG, pinState);
    TCA9554_WriteReg(TCA9554_CONFIG_REG, pinMode);
}
// ─────────────────────────────────────────────────────────────
//  CRC-16/IBM
// ─────────────────────────────────────────────────────────────
uint16_t modbusCRC(const uint8_t *buf, uint8_t len) {
    uint16_t crc = 0xFFFF;
    for (uint8_t i = 0; i < len; i++) {
        crc ^= (uint16_t)buf[i];
        for (uint8_t j = 0; j < 8; j++)
            crc = (crc & 1) ? (crc >> 1) ^ 0xA001 : crc >> 1;
    }
    return crc;
}
// ─────────────────────────────────────────────────────────────
//  Build FC04 request
// ─────────────────────────────────────────────────────────────
void buildFC04Request(uint8_t *frame, uint8_t slaveId,
                      uint16_t regAddr, uint16_t regCount) {
    frame[0] = slaveId;
    frame[1] = 0x04;
    frame[2] = regAddr >> 8;
    frame[3] = regAddr & 0xFF;
    frame[4] = regCount >> 8;
    frame[5] = regCount & 0xFF;
    uint16_t crc = modbusCRC(frame, 6);
    frame[6] = crc & 0xFF;
    frame[7] = crc >> 8;
}
// ─────────────────────────────────────────────────────────────
//  Read sensor distance
// ─────────────────────────────────────────────────────────────
float readDistanceMM() {
    while (rs485Serial.available()) rs485Serial.read();
    uint8_t request[8];
    buildFC04Request(request, MODBUS_SLAVE_ID,
                     SENSOR_REG_ADDR, SENSOR_REG_COUNT);
    Serial.print("[TX] ");
    for (int i = 0; i < 8; i++) {
        if (request[i] < 0x10) Serial.print('0');
        Serial.print(request[i], HEX);
        Serial.print(' ');
    }
    Serial.println();
    rs485Serial.write(request, sizeof(request));
    rs485Serial.flush();
    uint8_t response[EXPECTED_RESPONSE_LEN];
    uint8_t bytesReceived = 0;
    uint32_t deadline = millis() + MODBUS_RESPONSE_TIMEOUT_MS;
    while (millis() < deadline && bytesReceived < EXPECTED_RESPONSE_LEN) {
        if (rs485Serial.available())
            response[bytesReceived++] = rs485Serial.read();
    }
    Serial.print("[RX] ");
    for (int i = 0; i < bytesReceived; i++) {
        if (response[i] < 0x10) Serial.print('0');
        Serial.print(response[i], HEX);
        Serial.print(' ');
    }
    Serial.println();
    if (bytesReceived < EXPECTED_RESPONSE_LEN) {
        Serial.println("[ERR] Timeout / incomplete response");
        return NAN;
    }
    if (response[0] != MODBUS_SLAVE_ID) { Serial.println("[ERR] Wrong slave ID"); return NAN; }
    if (response[1] == 0x84) { Serial.println("[ERR] Modbus exception"); return NAN; }
    if (response[1] != 0x04) { Serial.println("[ERR] Wrong function code"); return NAN; }
    uint16_t rxCRC   = (uint16_t)response[7] | ((uint16_t)response[8] << 8);
    uint16_t calcCRC = modbusCRC(response, 7);
    if (rxCRC != calcCRC) { Serial.println("[ERR] CRC mismatch"); return NAN; }
    uint16_t raw = ((uint16_t)response[5] << 8) | response[6];
    return raw * 0.001f;
}
// ─────────────────────────────────────────────────────────────
//  setup()
// ─────────────────────────────────────────────────────────────
void setup() {
    Serial.begin(115200);
    delay(2000);
    Serial.println("\n=== BOJKE BL-30NZ-485 on Waveshare ESP32-S3-POE-ETH-8DI-8DO ===");
    // I2C init
    Wire.begin(I2C_SDA_PIN, I2C_SCL_PIN);
    Serial.println("I2C initialised");
    // TCA9554 init — all outputs, all HIGH (matches Waveshare Dout_Init)
    TCA9554_Init(0x00, 0xFF);
    Serial.println("TCA9554 initialised");
    // RS485 init — exactly as Waveshare demo
    rs485Serial.begin(MODBUS_BAUD, SERIAL_8N1, RXD1, TXD1);
    if (!rs485Serial.setPins(-1, -1, -1, TXD1EN))
        Serial.println("[WARN] Failed to set TXDEN pin");
    if (!rs485Serial.setMode(UART_MODE_RS485_HALF_DUPLEX))
        Serial.println("[WARN] Failed to set RS485 half-duplex mode");
    Serial.println("RS485 initialised");
    Serial.println("================================================================\n");
}
// ─────────────────────────────────────────────────────────────
//  loop()
// ─────────────────────────────────────────────────────────────
void loop() {
    float distance = readDistanceMM();
    if (!std::isnan(distance)) {
        if (distance >= 25.0f && distance <= 35.0f) {
            Serial.print("Distance: ");
            Serial.print(distance, 3);
            Serial.println(" mm");
        } else {
            Serial.println("Distance: OUT OF RANGE");
        }
    } else {
        Serial.println("Distance: READ ERROR");
    }
    Serial.println();
    delay(POLL_INTERVAL_MS);
}

And here's the proof that when I used a USB-RS485 dongle to sniff the traffic, it was working fine:

...using these port settings:

So there's nothing wrong with the Bojke sensor. The Stupid Fat Bloke hasn't been messing about with it behind my back and blown it up.

To get up to speed with Modbus, here's a handy little intro video:

I was starting to get the feeling there was something going on here that nobody told me about. But with some digging, I found this vid discussing why Modbus doesn't work with ESP32:

Bottom line is, Arduino is a sort of Noddy / Newbie IDE for developing code to program the Arduino family and as such it is a fairly high level, cut down version of C++, where many of the functions are actually hidden from the user, to keep life "simple"(!!). Furthermore, although the ESP32 has a lot of support within the Arduino IDE, it's pushing the capabilities of some of the functions that were never intended to be used beyond the actual Arduino family. One of those might be the support for implementing RS485.

So using the Arduino IDE to program an ESP32 to use RS485 may be a bit of an oxymoron - and nobody's going to act surprised or run to help you if you run into problems. The grownups will be using "proper" IDEs such as the dedicated ESP-IDF that Espressif have developed specifically to ..... program the ESP32 family. Oh well, time to "put my big boy pants on" and move on from this time consuming fiasco....