Hardware and radio connection

/*
 * ============================================================================
 *  PC <-> PC Full-Duplex Bridge via Bluetooth SPP + RS-232
 * ============================================================================
 *  Architecture:
 *  [PC] <-BT-> [ESP32] <-UART2 TTL (IO16/IO17)-> [HW-707/MAX3232] <-RS232-> ...
 *
 *  Logic:
 *   - Whatever arrives over Bluetooth (i.e. from the computer) is written to Serial2
 *     (i.e. forwarded to the MAX3232 -> RS232 cable -> the other ESP32)
 *   - Whatever arrives on Serial2 (i.e. from the RS232 cable / the other ESP32) is
 *     written to Bluetooth (i.e. forwarded to the computer)
 *
 *  The exact same sketch is uploaded to both ESP32s (only BT_DEVICE_NAME
 *  changes, so you can tell them apart when pairing).
 *
 *  Pinout ESP32 (UART2 default):
 *    IO17 (TX2) -> RXD on the HW-707
 *    IO16 (RX2) -> TXD on the HW-707
 *    GND        -> GND on the HW-707
 *    3V3        -> VCC on the HW-707 (the module is powered at 3.3V).
 *
 *  Board: "ESP32 Dev Module" (ESP32-WROOM-32)
 *  Requires: Arduino-ESP32 core (BluetoothSerial comes built in).
 * ============================================================================
 */

#include "BluetoothSerial.h"

// ----------------------------------------------------------------------------
// Settings
// ----------------------------------------------------------------------------

// Change the name on each ESP32 so you can tell them apart when pairing
// e.g. "ESP32_BRIDGE_A" on the first, "ESP32_BRIDGE_B" on the second.
#define BT_DEVICE_NAME "IRIS 1"

// If you send "a lot" of data quickly and see data loss or stalls, the
// first suspect is this: BT SPP is much faster than the
// RS-232 serial line, so if your setup allows it (cable,
// MAX3232, and the other end), raise it, e.g. to 38400 or 115200 - less
// bottleneck = fewer losses. It must be the SAME on both ESP32s.
#define RS232_BAUD     9600
#define RXD2_PIN       16   // IO16 -> RX2 (receives from HW-707 TXD)
#define TXD2_PIN       17   // IO17 -> TX2 (sends to HW-707 RXD)

// Size of the intermediate buffer used to move data per loop() "cycle"
#define CHUNK_SIZE     256

// Larger internal buffers so that bursts of data are not lost while
// Serial2 (slower, 9600 baud) catches up and sends them out.
#define UART2_RX_BUFFER_SIZE  4096
#define UART2_TX_BUFFER_SIZE  4096

#if !defined(CONFIG_BT_ENABLED) || !defined(CONFIG_BLUEDROID_ENABLED)
#error Bluetooth is not enabled! Please run `make menuconfig` to enable it
#endif

BluetoothSerial SerialBT;

// ----------------------------------------------------------------------------
// Setup
// ----------------------------------------------------------------------------
void setup() {
  // Serial (USB) only for debugging/monitoring via the Arduino IDE, optional.
  Serial.begin(115200);
  delay(200);

  // We enlarge the buffers BEFORE begin() - otherwise it has no effect.
  Serial2.setRxBufferSize(UART2_RX_BUFFER_SIZE);
  Serial2.setTxBufferSize(UART2_TX_BUFFER_SIZE);

  // UART2 to the MAX3232 (HW-707) -> RS232 -> the other ESP32
  Serial2.begin(RS232_BAUD, SERIAL_8N1, RXD2_PIN, TXD2_PIN);

  // Bluetooth SPP (Serial Port Profile) to the computer
  SerialBT.begin(BT_DEVICE_NAME);
  // On arduino-esp32 core >= 2.x there is also SerialBT.setRxBufferSize(size)
  // (it must go BEFORE begin()). If your IDE finds it, uncomment:
  // SerialBT.setRxBufferSize(4096);

  Serial.println();
  Serial.print("[BOOT] Bluetooth device started, name: ");
  Serial.println(BT_DEVICE_NAME);
  Serial.println("[BOOT] Serial2 (RS-232 bridge) started at " + String(RS232_BAUD) + " baud");
  Serial.println("[BOOT] Bridge ready. Pair this ESP32 from the PC's Bluetooth settings,");
  Serial.println("[BOOT] then open the resulting virtual COM port in a terminal.");
}

// ----------------------------------------------------------------------------
// Loop: bidirectional data forwarding
// ----------------------------------------------------------------------------
void loop() {

  // ---- Direction 1: Bluetooth (PC) -> Serial2 (RS-232 -> the other ESP32) ----
  if (SerialBT.available()) {
    uint8_t buf[CHUNK_SIZE];
    size_t n = 0;
    while (SerialBT.available() && n < CHUNK_SIZE) {
      buf[n++] = (uint8_t)SerialBT.read();
    }
    if (n > 0) {
      Serial2.write(buf, n);
      // Serial.write(buf, n); // <- uncomment for debug echo on the USB monitor
    }
  }

  // ---- Direction 2: Serial2 (RS-232 <- the other ESP32) -> Bluetooth (PC) ----
  if (Serial2.available()) {
    uint8_t buf[CHUNK_SIZE];
    size_t n = 0;
    while (Serial2.available() && n < CHUNK_SIZE) {
      buf[n++] = (uint8_t)Serial2.read();
    }
    if (n > 0) {
      SerialBT.write(buf, n);
      // Serial.write(buf, n); // <- uncomment for debug echo on the USB monitor
    }
  }

  // No delay() needed - available()/read() are non-blocking and
  // both directions are served in turn on every pass of loop(),
  // effectively achieving full-duplex operation.
}