firmware: prevent RS485 echo starvation

docs/CHANGELOG.md

@@ -4,6 +4,7 @@
 
 - Added standalone XIAO ESP32-C6 BNO086/MAX-M10S sensor-node firmware and a versioned RS485 telemetry contract without changing Cargo `/api/v1` or JBD/NimBLE behavior.
 - Added XIAO-side persistent level calibration with stability checking and RS485 start/clear commands; no dashboard behavior changed.
+- Prevented echoed or oversized RS485 frames from starving the XIAO sensor loop.
 
 ## Unreleased
 

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

@@ -5,6 +5,7 @@
 
 namespace {
 String commandBuffer;
+bool discardCommandUntilNewline = false;
 void sendJson(HardwareSerial& serial, JsonDocument& doc) {
     digitalWrite(RS485_DE_PIN, HIGH); delayMicroseconds(20);
     serializeJson(doc, serial); serial.write('\n'); serial.flush();
@@ -63,12 +64,25 @@ void publishCalibrationResult(HardwareSerial& serial, const String& result) {
 }
 
 void pollRs485Commands(HardwareSerial& serial, const ImuState& imu) {
-    while (serial.available()) {
+    // Bound RX work so an electrically echoed or noisy bus cannot starve sensors.
+    int bytesRemaining = RS485_COMMAND_MAX_LENGTH * 2;
+    while (serial.available() && bytesRemaining-- > 0) {
         char c = serial.read();
         if (c == '\r') continue;
         if (c != '\n') {
-            if (commandBuffer.length() < RS485_COMMAND_MAX_LENGTH) commandBuffer += c;
-            else commandBuffer = "";
+            if (discardCommandUntilNewline) continue;
+            if (commandBuffer.length() < RS485_COMMAND_MAX_LENGTH) {
+                commandBuffer += c;
+            } else {
+                commandBuffer = "";
+                discardCommandUntilNewline = true;
+            }
+            continue;
+        }
+
+        if (discardCommandUntilNewline) {
+            discardCommandUntilNewline = false;
+            commandBuffer = "";
             continue;
         }
 
@@ -77,6 +91,8 @@ void pollRs485Commands(HardwareSerial& serial, const ImuState& imu) {
         commandBuffer = "";
         if (error) { publishCalibrationResult(serial, "invalid_json"); continue; }
         String type = request["type"] | "";
+        // Some RS485 boards echo transmitted frames into RX. Ignore our own output.
+        if (type == "sensor_status" || type == "calibration_response") continue;
         if (type == "calibrate_level") {
             publishCalibrationResult(serial, startLevelCalibration(imu) ? "started" : "imu_unavailable");
         } else if (type == "clear_level_calibration") {

tests/test_xiao_sensor_node_contract.py

@@ -56,6 +56,12 @@ def test_level_calibration_is_persistent_and_non_blocking():
     assert "updateLevelCalibration" in loop
     assert "delay(5000)" not in calibration
 
+def test_rs485_echo_cannot_starve_sensor_loop():
+    transport = source("rs485_transport.cpp")
+    assert "bytesRemaining" in transport
+    assert "discardCommandUntilNewline" in transport
+    assert 'type == "sensor_status"' in transport
+
 def test_node_does_not_take_cargo_or_ble_authority():
     combined = "\n".join(p.read_text() for p in NODE.glob("*.*"))
     assert "NimBLE" not in combined and "JBD" not in combined and "server.on(" not in combined