firmware: harden XIAO RS485 telemetry

firmware/xiao-esp32c6-sensor-node/README.md

@@ -14,6 +14,6 @@ Power both sensors from 3.3V with shared ground. Required Arduino libraries are
 The BNO086 uses the verified `0x4B` I2C address on D7/D8. If initialization fails, verify 3.3V/GND, SDA/SCL wiring, and the breakout's PS0/PS1 configuration.
 Firmware retries the fixed address during power-up and periodically retries afterward, so a slow BNO086 power-up does not require cycling the XIAO. If the BNO086 resets at runtime, rotation-vector reporting is enabled again automatically.
 
-The node sends `sensor_status` at 115200 baud every 200 ms. See `docs/SENSOR_NODE_PROTOCOL.md`. The initial firmware is transmit-only; add a dashboard receiver after bench-testing the node, RS485 polarity, and sensor axes.
+The node sends `sensor_status` at 115200 baud every 250 ms. See `docs/SENSOR_NODE_PROTOCOL.md`. It also listens between frames for the documented level-calibration commands. Add a dashboard receiver after bench-testing the node, RS485 polarity, and sensor axes.
 
 During bring-up, `USB_TELEMETRY_ENABLED` mirrors each RS485 JSON frame to the Arduino Serial Monitor at 115200 baud. Set it to `0` in `sensor_node_config.h` when continuous USB output is no longer wanted.

firmware/xiao-esp32c6-sensor-node/rs485_transport.cpp

@@ -1,4 +1,5 @@
 #include <ArduinoJson.h>
+#include <esp_system.h>
 #include "rs485_transport.h"
 #include "sensor_node_config.h"
 #include "calibration.h"
@@ -6,9 +7,28 @@
 namespace {
 String commandBuffer;
 bool discardCommandUntilNewline = false;
+uint32_t bootId = 0;
+uint32_t frameSequence = 0;
+
+void drainTransmitEcho(HardwareSerial& serial) {
+    while (serial.available()) serial.read();
+}
+
 void sendJson(HardwareSerial& serial, JsonDocument& doc) {
-    digitalWrite(RS485_DE_PIN, HIGH); delayMicroseconds(20);
+    if (doc.overflowed() || measureJson(doc) + 1 > RS485_MAX_FRAME_BYTES) {
+#if USB_TELEMETRY_ENABLED
+        Serial.println("{\"type\":\"rs485_error\",\"error\":\"frame_too_large\"}");
+#endif
+        return;
+    }
+
+    digitalWrite(RS485_DE_PIN, HIGH);
+    delayMicroseconds(RS485_DRIVER_ENABLE_SETTLE_US);
     serializeJson(doc, serial); serial.write('\n'); serial.flush();
+    // This expansion board can echo TX into RX. At flush completion the
+    // entire transmitted line has left the UART, so discard that local echo
+    // before returning the transceiver to receive mode.
+    drainTransmitEcho(serial);
     digitalWrite(RS485_DE_PIN, LOW);
 #if USB_TELEMETRY_ENABLED
     serializeJson(doc, Serial); Serial.println();
@@ -18,13 +38,17 @@ void sendJson(HardwareSerial& serial, JsonDocument& doc) {
 
 void initRs485(HardwareSerial& serial) {
     pinMode(RS485_DE_PIN, OUTPUT); digitalWrite(RS485_DE_PIN, LOW);
+    serial.setRxBufferSize(RS485_RX_BUFFER_BYTES);
     serial.begin(RS485_BAUD, SERIAL_8N1, RS485_RX_PIN, RS485_TX_PIN);
+    bootId = esp_random();
 }
 
 void publishSensorStatus(HardwareSerial& serial, const ImuState& imu, const GpsState& gps) {
     StaticJsonDocument<2048> doc;
     doc["type"] = "sensor_status"; doc["schema_version"] = SENSOR_NODE_SCHEMA_VERSION;
     doc["node_id"] = SENSOR_NODE_NAME; doc["firmware_version"] = SENSOR_NODE_FIRMWARE_VERSION;
+    doc["boot_id"] = bootId;
+    doc["frame_sequence"] = frameSequence++;
     doc["uptime_ms"] = millis();
     JsonObject i = doc.createNestedObject("imu");
     i["online"] = imu.online; i["valid"] = imu.valid && millis() - imu.lastUpdateMs <= SENSOR_STALE_MS;

firmware/xiao-esp32c6-sensor-node/sensor_node_config.h

@@ -2,7 +2,7 @@
 #include <Arduino.h>
 
 #define SENSOR_NODE_NAME "xiao_c6_sensor_node"
-#define SENSOR_NODE_FIRMWARE_VERSION "0.1.0"
+#define SENSOR_NODE_FIRMWARE_VERSION "0.2.0"
 #define SENSOR_NODE_SCHEMA_VERSION 1
 #define RS485_DE_PIN D2
 #define RS485_TX_PIN D4
@@ -18,9 +18,12 @@
 #define BNO086_RUNTIME_RETRY_MS 5000
 #define GPS_BAUD 38400
 #define RS485_BAUD 115200
+#define RS485_RX_BUFFER_BYTES 512
+#define RS485_MAX_FRAME_BYTES 1900
+#define RS485_DRIVER_ENABLE_SETTLE_US 20
 #define IMU_REPORT_INTERVAL_US 20000
 #define IMU_STABILITY_INTERVAL_US 100000
-#define TELEMETRY_INTERVAL_MS 200
+#define TELEMETRY_INTERVAL_MS 250
 #define SENSOR_STALE_MS 2000
 #define USB_TELEMETRY_ENABLED 1
 #define LEVEL_CALIBRATION_DURATION_MS 5000