diff --git a/OpticalChannelTester/App.cpp b/OpticalChannelTester/App.cpp
index 089de8f..5e82e6c 100644
--- a/OpticalChannelTester/App.cpp
+++ b/OpticalChannelTester/App.cpp
@@ -47,7 +47,9 @@ void App::finishInitialization(bool factoryReset) {
   initialized_ = true;
   if (!receiver_.begin()) { Log::event("BOOT", "FATAL: capture peripheral init failed"); finish(false, FailReason::UNSUPPORTED); return; }
   Log::printf("BOOT", "capture initialized: %s", receiver_.highRateBackend() ? "RMT DMA" : "RMT ping-pong");
-  printConfiguration(); showIdle();
+  printConfiguration();
+  if (static_cast<Role>(settings_.role) == Role::SLAVE) armSlave();
+  else showIdle();
 }
 
 void App::update() {
@@ -71,11 +73,26 @@ void App::update() {
   if (state_ == AppState::IDLE || state_ == AppState::FINISHED) {
     if (modeEvent == ButtonEvent::SHORT) {
       settings_.role = (settings_.role + 1U) % 3U; const bool saved = store_.save(settings_);
-      params_ = store_.params(settings_); showIdle();
+      params_ = store_.params(settings_);
+      if (static_cast<Role>(settings_.role) == Role::SLAVE) armSlave();
+      else showIdle();
       Log::printf("ACTION", "role changed to %s, NVS=%s", roleName(static_cast<Role>(settings_.role)), saved ? "OK" : "FAILED");
     } else if (modeEvent == ButtonEvent::LONG) {
       state_ = AppState::MENU; menuItem_ = 0; Log::event("ACTION", "settings menu entered"); showMenu();
     } else if (startEvent == ButtonEvent::SHORT) { Log::event("ACTION", "test start requested"); startTest(); }
+    else if (state_ == AppState::FINISHED && static_cast<Role>(settings_.role) == Role::SLAVE &&
+             now >= slaveRearmAtMs_) armSlave();
+    return;
+  }
+  if (state_ == AppState::SLAVE_READY && modeEvent != ButtonEvent::NONE) {
+    radio_.end(); havePeer_ = false;
+    if (modeEvent == ButtonEvent::SHORT) {
+      settings_.role = static_cast<uint8_t>(Role::SOLO); const bool saved = store_.save(settings_);
+      params_ = store_.params(settings_); state_ = AppState::IDLE; showIdle();
+      Log::printf("ACTION", "role changed to SOLO, NVS=%s", saved ? "OK" : "FAILED");
+    } else if (modeEvent == ButtonEvent::LONG) {
+      state_ = AppState::MENU; menuItem_ = 0; showMenu();
+    }
     return;
   }
   if (state_ == AppState::MENU) {
@@ -85,7 +102,9 @@ void App::update() {
     else if (modeEvent == ButtonEvent::LONG) {
       sanitizeRange(); const bool saved = store_.save(settings_); params_ = store_.params(settings_);
       Log::printf("ACTION", "settings menu saved and closed, NVS=%s", saved ? "OK" : "FAILED");
-      state_ = AppState::IDLE; printConfiguration(); showIdle();
+      state_ = AppState::IDLE; printConfiguration();
+      if (static_cast<Role>(settings_.role) == Role::SLAVE) armSlave();
+      else showIdle();
     } else if (startEvent == ButtonEvent::SHORT) changeMenu(+1);
     else if (startEvent == ButtonEvent::LONG || startEvent == ButtonEvent::REPEAT) changeMenu(-1);
     return;
@@ -153,7 +172,8 @@ void App::showMenu() {
 
 void App::startTest() {
   params_ = store_.params(settings_); stageCount_ = frequencyPointCount(params_.startHz, params_.endHz, params_.stepHz);
-  stageIndex_ = 0; pendingReason_ = FailReason::NONE;
+  stageIndex_ = 0; requestedHz_ = 0; pendingReason_ = FailReason::NONE;
+  havePeer_ = false; lastHeartbeatMs_ = 0; lastPeerSeenMs_ = 0;
   if (!stageCount_) { finish(false, FailReason::UNSUPPORTED); return; }
   Log::printf("TEST", "starting role=%s stages=%lu", roleName(static_cast<Role>(settings_.role)), stageCount_);
   if (SERIAL_MINIMAL_LOG) {
@@ -175,6 +195,23 @@ void App::startTest() {
   else { state_ = AppState::SLAVE_READY; Log::event("TEST", "Slave armed and waiting for Master"); display_.show("SLAVE READY", "WAIT MASTER"); }
 }
 
+bool App::armSlave() {
+  params_ = store_.params(settings_);
+  stageIndex_ = 0; stageCount_ = frequencyPointCount(params_.startHz, params_.endHz, params_.stepHz);
+  requestedHz_ = 0; session_ = 0; sequence_ = 0; havePeer_ = false;
+  lastHeartbeatMs_ = 0; lastPeerSeenMs_ = 0; retries_ = 0; slaveRearmAtMs_ = 0;
+  if (!radio_.begin()) {
+    state_ = AppState::FINISHED; pendingReason_ = FailReason::LINK_LOST;
+    slaveRearmAtMs_ = millis() + LINK_HEARTBEAT_TIMEOUT_MS;
+    display_.show("LINK FAILED", "RADIO ERROR");
+    return false;
+  }
+  radio_.flush(); state_ = AppState::SLAVE_READY;
+  Log::event("TEST", "Slave automatically armed and waiting for Master");
+  display_.show("SLAVE READY", "WAIT MASTER");
+  return true;
+}
+
 bool App::prepareStage() {
   requestedHz_ = frequencyAt(params_.startHz, params_.endHz, params_.stepHz, stageIndex_);
   const uint32_t maxHz = TARGET_IS_C3 ? C3_STRICT_MAX_HZ :
@@ -226,11 +263,12 @@ void App::stagePassed() {
 }
 
 void App::startMasterDiscovery() {
-  session_ = esp_random(); if (!session_) session_ = 1; sequence_ = 1; havePeer_ = false; radio_.flush();
+  session_ = esp_random(); if (!session_) session_ = 1;
+  sequence_ = 1; stageIndex_ = 0; requestedHz_ = 0; havePeer_ = false; radio_.flush();
   pendingPacket_ = makePacket(MessageType::DISCOVER); radio_.sendBroadcast(pendingPacket_);
-  lastSendMs_ = millis(); deadlineMs_ = millis() + LINK_DISCOVERY_TIMEOUT_MS; retries_ = 0;
+  lastSendMs_ = millis(); retries_ = 0;
   state_ = AppState::MASTER_DISCOVER; Log::printf("ESP-NOW", "discovery started session=%08lX", session_);
-  display_.show("MASTER SEARCH", "WAIT SLAVE");
+  display_.show("MASTER SEARCH", "HOLD START=STOP");
 }
 
 ProtocolPacket App::makePacket(MessageType type) const {
@@ -245,10 +283,27 @@ ProtocolPacket App::makePacket(MessageType type) const {
 
 void App::sendCurrent(MessageType type) {
   ++sequence_; pendingPacket_ = makePacket(type);
-  const bool ok = radio_.sendTo(peer_, pendingPacket_); lastSendMs_ = millis();
+  const bool ok = radio_.sendBroadcast(pendingPacket_); lastSendMs_ = millis();
   if (!ok) Log::printf("ESP-NOW", "sendCurrent %s FAILED", messageName(type));
 }
 
+void App::updateHeartbeat() {
+  if (!havePeer_ || state_ == AppState::FINISHED) return;
+  const uint32_t now = millis();
+  if (now - lastPeerSeenMs_ >= LINK_HEARTBEAT_TIMEOUT_MS) {
+    Log::event("ESP-NOW", "peer heartbeat timeout");
+    finish(false, FailReason::LINK_LOST);
+    return;
+  }
+  if (static_cast<Role>(settings_.role) == Role::MASTER &&
+      now - lastHeartbeatMs_ >= LINK_HEARTBEAT_INTERVAL_MS) {
+    ProtocolPacket heartbeat = makePacket(MessageType::HEARTBEAT);
+    heartbeat.sequence = sequence_;
+    radio_.sendBroadcast(heartbeat);
+    lastHeartbeatMs_ = now;
+  }
+}
+
 bool App::packetForCurrent(const ProtocolPacket &p) const {
   return p.session == session_ && p.stage == stageIndex_;
 }
@@ -257,29 +312,35 @@ void App::handleRadio() {
   ReceivedPacket r;
   while (radio_.receive(r)) {
     const MessageType type = static_cast<MessageType>(r.packet.type);
-    if (state_ != AppState::SLAVE_MEASURE)
+    if (type != MessageType::HEARTBEAT && type != MessageType::HEARTBEAT_ACK &&
+        state_ != AppState::SLAVE_MEASURE)
       Log::printf("ESP-NOW", "RX %s session=%08lX stage=%u seq=%u",
         messageName(type), r.packet.session, r.packet.stage, r.packet.sequence);
-    if (state_ == AppState::SLAVE_READY && type == MessageType::DISCOVER) {
+    if ((state_ == AppState::SLAVE_READY || state_ == AppState::SLAVE_WAIT_START) &&
+        type == MessageType::DISCOVER && (!havePeer_ || !memcmp(peer_, r.mac, 6))) {
       memcpy(peer_, r.mac, 6); havePeer_ = true; session_ = r.packet.session; stageIndex_ = 0; sequence_ = r.packet.sequence;
-      ProtocolPacket ack = makePacket(MessageType::DISCOVER_ACK); ack.sequence = r.packet.sequence; radio_.sendTo(peer_, ack);
-      state_ = AppState::SLAVE_WAIT_START; display_.show("SLAVE LINKED", "WAIT PREPARE"); continue;
+      lastPeerSeenMs_ = millis();
+      ProtocolPacket ack = makePacket(MessageType::DISCOVER_ACK); ack.sequence = r.packet.sequence; radio_.sendBroadcast(ack);
+      state_ = AppState::SLAVE_WAIT_START; display_.show("MASTER SEEN", "ACK SENT"); continue;
     }
     if (state_ == AppState::MASTER_DISCOVER && type == MessageType::DISCOVER_ACK && r.packet.session == session_) {
-      memcpy(peer_, r.mac, 6); havePeer_ = true; requestedHz_ = frequencyAt(params_.startHz, params_.endHz, params_.stepHz, stageIndex_);
+      memcpy(peer_, r.mac, 6); havePeer_ = true; lastPeerSeenMs_ = lastHeartbeatMs_ = millis();
+      requestedHz_ = frequencyAt(params_.startHz, params_.endHz, params_.stepHz, stageIndex_);
       sendCurrent(MessageType::PREPARE); state_ = AppState::MASTER_WAIT_READY; retries_ = 0; deadlineMs_ = millis() + LINK_REPLY_TIMEOUT_MS;
       char mac[20]; Radio::macText(peer_, mac, sizeof(mac)); Log::printf("ESP-NOW", "Slave selected %s", mac); continue;
     }
-    if (state_ == AppState::SLAVE_WAIT_START && type == MessageType::DISCOVER &&
-        r.packet.session == session_ && !memcmp(peer_, r.mac, 6)) {
-      ProtocolPacket ack = makePacket(MessageType::DISCOVER_ACK);
-      ack.sequence = r.packet.sequence; radio_.sendTo(peer_, ack); continue;
+    if (havePeer_ && !memcmp(peer_, r.mac, 6) && r.packet.session == session_) lastPeerSeenMs_ = millis();
+    if (havePeer_ && !memcmp(peer_, r.mac, 6) && r.packet.session == session_ &&
+        type == MessageType::HEARTBEAT) {
+      ProtocolPacket ack = makePacket(MessageType::HEARTBEAT_ACK);
+      ack.sequence = r.packet.sequence; radio_.sendBroadcast(ack); continue;
     }
+    if (type == MessageType::HEARTBEAT_ACK) continue;
     if (havePeer_ && !memcmp(peer_, r.mac, 6) && type == MessageType::RESULT &&
         r.packet.session == session_ && r.packet.stage < stageIndex_) {
       ProtocolPacket ack = {}; ack.type = static_cast<uint8_t>(MessageType::ACK);
       ack.session = session_; ack.stage = r.packet.stage; ack.sequence = r.packet.sequence;
-      radio_.sendTo(peer_, ack); continue; // idempotent ACK for a retried old result
+      radio_.sendBroadcast(ack); continue; // idempotent ACK for a retried old result
     }
     if (!havePeer_ || memcmp(peer_, r.mac, 6) || !packetForCurrent(r.packet)) continue;
     if (type == MessageType::ABORT) { finish(false, FailReason::ABORTED); continue; }
@@ -288,12 +349,13 @@ void App::handleRadio() {
       sendCurrent(MessageType::START_STAGE); state_ = AppState::MASTER_WAIT_RESULT; retries_ = 0; deadlineMs_ = millis() +
         params_.testTimeMs * params_.repeats + LINK_REPLY_TIMEOUT_MS + (1000UL * PWM_SETTLE_CYCLES / actual_.actualHz) + 20;
     } else if (state_ == AppState::MASTER_WAIT_RESULT && type == MessageType::RESULT) {
-      ProtocolPacket ack = makePacket(MessageType::ACK); ack.sequence = r.packet.sequence; radio_.sendTo(peer_, ack); pwm_.stop();
+      ProtocolPacket ack = makePacket(MessageType::ACK); ack.sequence = r.packet.sequence; radio_.sendBroadcast(ack); pwm_.stop();
       if (!r.packet.passed) finish(false, static_cast<FailReason>(r.packet.reason)); else stagePassed();
     } else if (state_ == AppState::SLAVE_WAIT_START && type == MessageType::PREPARE) {
       params_.testTimeMs = r.packet.testTimeMs; params_.repeats = r.packet.repeats;
       params_.accuracyPct = r.packet.accuracyX100 / 100.0f; requestedHz_ = r.packet.requestedHz;
-      ProtocolPacket ready = makePacket(MessageType::READY); ready.sequence = r.packet.sequence; radio_.sendTo(peer_, ready);
+      ProtocolPacket ready = makePacket(MessageType::READY); ready.sequence = r.packet.sequence; radio_.sendBroadcast(ready);
+      display_.show("SLAVE LINKED", "MASTER ONLINE");
     } else if (state_ == AppState::SLAVE_WAIT_START && type == MessageType::START_STAGE) {
       actual_.actualHz = r.packet.actualHz; actual_.actualDutyPct = r.packet.actualDutyX100 / 100.0f;
       if (!startLocalMeasurement(actual_.actualHz, actual_.actualDutyPct)) { finish(false, FailReason::UNSUPPORTED); continue; }
@@ -308,22 +370,25 @@ void App::handleRadio() {
 void App::updateMaster() {
   const uint32_t now = millis();
   if (state_ == AppState::MASTER_DISCOVER) {
-    if (now >= deadlineMs_) { finish(false, FailReason::LINK_LOST); return; }
     if (now - lastSendMs_ >= LINK_RETRY_INTERVAL_MS) {
-      Log::printf("ESP-NOW", "DISCOVER retry=%u", retries_ + 1); radio_.sendBroadcast(pendingPacket_);
-      lastSendMs_ = now; ++retries_;
+      Log::event("ESP-NOW", "DISCOVER retry"); radio_.sendBroadcast(pendingPacket_);
+      lastSendMs_ = now;
     }
     return;
   }
+  updateHeartbeat();
+  if (state_ == AppState::FINISHED) return;
   if (now < deadlineMs_) return;
   if (retries_ >= LINK_PACKET_RETRIES) { finish(false, FailReason::LINK_LOST); return; }
   Log::printf("ESP-NOW", "%s retry=%u", messageName(static_cast<MessageType>(pendingPacket_.type)), retries_ + 1);
-  radio_.sendTo(peer_, pendingPacket_); ++retries_;
+  radio_.sendBroadcast(pendingPacket_); ++retries_;
   deadlineMs_ = now + (state_ == AppState::MASTER_WAIT_RESULT ?
     params_.testTimeMs * params_.repeats + LINK_REPLY_TIMEOUT_MS : LINK_REPLY_TIMEOUT_MS);
 }
 
 void App::updateSlave() {
+  updateHeartbeat();
+  if (state_ == AppState::FINISHED) return;
   if (state_ == AppState::SLAVE_MEASURE) {
     const MeasureState ms = measurement_.update();
     if (ms != MeasureState::PASS && ms != MeasureState::FAIL) return;
@@ -332,13 +397,13 @@ void App::updateSlave() {
     pendingPacket_.passed = ms == MeasureState::PASS && measurement_.reason() == FailReason::NONE;
     pendingPacket_.reason = static_cast<uint8_t>(measurement_.reason()); pendingPacket_.periods = measurement_.stats().periods;
     pendingPacket_.minPeriodTicks = measurement_.stats().minPeriod; pendingPacket_.maxPeriodTicks = measurement_.stats().maxPeriod;
-    pendingPacket_.sequence = ++sequence_; radio_.sendTo(peer_, pendingPacket_);
+    pendingPacket_.sequence = ++sequence_; radio_.sendBroadcast(pendingPacket_);
     Log::printf("TEST", "Slave result prepared: %s reason=%s periods=%lu",
       pendingPacket_.passed ? "PASS" : "FAIL", failName(static_cast<FailReason>(pendingPacket_.reason)), pendingPacket_.periods);
     state_ = AppState::SLAVE_WAIT_ACK; retries_ = 0; deadlineMs_ = millis() + LINK_REPLY_TIMEOUT_MS;
   } else if (state_ == AppState::SLAVE_WAIT_ACK && millis() >= deadlineMs_) {
     if (retries_++ >= LINK_PACKET_RETRIES) finish(false, FailReason::LINK_LOST);
-    else { Log::printf("ESP-NOW", "RESULT retry=%u", retries_); radio_.sendTo(peer_, pendingPacket_); deadlineMs_ = millis() + LINK_REPLY_TIMEOUT_MS; }
+    else { Log::printf("ESP-NOW", "RESULT retry=%u", retries_); radio_.sendBroadcast(pendingPacket_); deadlineMs_ = millis() + LINK_REPLY_TIMEOUT_MS; }
   }
 }
 
@@ -351,14 +416,33 @@ void App::abortTest() {
 
 void App::finish(bool pass, FailReason reason) {
   Log::printf("TEST", "finishing result=%s reason=%s", pass ? "PASS" : "FAIL", failName(reason));
+  const AppState failedState = state_;
+  const bool masterLinkLost = reason == FailReason::LINK_LOST &&
+    (failedState == AppState::MASTER_DISCOVER || failedState == AppState::MASTER_WAIT_READY ||
+     failedState == AppState::MASTER_WAIT_RESULT);
+  const bool slaveLinkLost = reason == FailReason::LINK_LOST &&
+    (failedState == AppState::SLAVE_READY || failedState == AppState::SLAVE_WAIT_START ||
+     failedState == AppState::SLAVE_MEASURE || failedState == AppState::SLAVE_WAIT_ACK);
   pwm_.stop(); receiver_.stop();
+  if (masterLinkLost) {
+    Log::event("ESP-NOW", "link lost; returning to continuous discovery");
+    startMasterDiscovery();
+    return;
+  }
   if (state_ != AppState::IDLE && state_ != AppState::MENU) radio_.end();
   state_ = AppState::FINISHED; pendingReason_ = reason;
+  if (slaveLinkLost) {
+    if (armSlave()) display_.show("MASTER LOST", "WAIT MASTER");
+    return;
+  }
+  if (static_cast<Role>(settings_.role) == Role::SLAVE) slaveRearmAtMs_ = millis() + 2000;
   char one[24];
   if (pass) display_.show("PASS", "REPEAT");
-  else {
+  else if (requestedHz_) {
     char frequency[12]; Display::formatFrequency(requestedHz_, frequency, sizeof(frequency));
     snprintf(one, sizeof(one), "FAIL %s", frequency); display_.show(one, failName(reason));
+  } else {
+    display_.show("TEST FAILED", failName(reason));
   }
 }
 
diff --git a/OpticalChannelTester/App.h b/OpticalChannelTester/App.h
index 52c3035..31bc87d 100644
--- a/OpticalChannelTester/App.h
+++ b/OpticalChannelTester/App.h
@@ -24,6 +24,7 @@ class App {
   void changeMenu(int direction);
   void sanitizeRange();
   void startTest();
+  bool armSlave();
   bool prepareStage();
   bool startLocalMeasurement(float hz, float duty);
   void startMasterDiscovery();
@@ -39,6 +40,7 @@ class App {
   uint64_t actualNominalTotalUs();
   ProtocolPacket makePacket(MessageType type) const;
   void sendCurrent(MessageType type);
+  void updateHeartbeat();
   bool packetForCurrent(const ProtocolPacket &p) const;
 
   Button startButton_, modeButton_;
@@ -61,8 +63,10 @@ class App {
   uint8_t peer_[6] = {};
   bool havePeer_ = false;
   uint32_t deadlineMs_ = 0, lastSendMs_ = 0;
+  uint32_t lastHeartbeatMs_ = 0, lastPeerSeenMs_ = 0;
   uint8_t retries_ = 0;
   ProtocolPacket pendingPacket_ = {};
   bool initialized_ = false, bootResetCandidate_ = false;
   uint32_t bootCheckStartedMs_ = 0;
+  uint32_t slaveRearmAtMs_ = 0;
 };
diff --git a/OpticalChannelTester/Config.h b/OpticalChannelTester/Config.h
index c402650..794bc18 100644
--- a/OpticalChannelTester/Config.h
+++ b/OpticalChannelTester/Config.h
@@ -7,22 +7,24 @@
 constexpr bool TARGET_IS_C3 = true;
 constexpr uint8_t GPIO_PWM = 3;
 constexpr uint8_t GPIO_RX = 4;
-constexpr uint8_t GPIO_BUTTON_START = 0;
-constexpr uint8_t GPIO_BUTTON_MODE = 1;
+constexpr uint8_t GPIO_BUTTON_MODE = 0;
+constexpr uint8_t GPIO_BUTTON_START = 1;
 constexpr uint8_t GPIO_SDA = 6;
 constexpr uint8_t GPIO_SCL = 7;
 #elif CONFIG_IDF_TARGET_ESP32S3
 constexpr bool TARGET_IS_C3 = false;
 constexpr uint8_t GPIO_PWM = 4;
 constexpr uint8_t GPIO_RX = 5;
-constexpr uint8_t GPIO_BUTTON_START = 6;
-constexpr uint8_t GPIO_BUTTON_MODE = 7;
+constexpr uint8_t GPIO_BUTTON_MODE = 6;
+constexpr uint8_t GPIO_BUTTON_START = 7;
 constexpr uint8_t GPIO_SDA = 8;
 constexpr uint8_t GPIO_SCL = 9;
 #else
 #error "Only ESP32-C3 and ESP32-S3 are supported"
 #endif
 
+constexpr uint8_t OLED_ROTATION = 0;
+
 constexpr uint8_t OLED_ADDRESS = 0x3C;
 constexpr uint8_t ESPNOW_WIFI_CHANNEL = 6;
 constexpr uint32_t SERIAL_BAUD = 115200;
@@ -41,10 +43,11 @@ constexpr uint32_t BUTTON_REPEAT_DELAY_MS = 600;
 constexpr uint32_t BUTTON_REPEAT_MS = 180;
 constexpr uint32_t FACTORY_RESET_HOLD_MS = 1500;
 
-constexpr uint32_t LINK_DISCOVERY_TIMEOUT_MS = 3000;
-constexpr uint32_t LINK_REPLY_TIMEOUT_MS = 800;
-constexpr uint8_t LINK_PACKET_RETRIES = 3;
-constexpr uint32_t LINK_RETRY_INTERVAL_MS = 100;
+constexpr uint32_t LINK_REPLY_TIMEOUT_MS = 1500;
+constexpr uint8_t LINK_PACKET_RETRIES = 10;
+constexpr uint32_t LINK_RETRY_INTERVAL_MS = 1000;
+constexpr uint32_t LINK_HEARTBEAT_INTERVAL_MS = 500;
+constexpr uint32_t LINK_HEARTBEAT_TIMEOUT_MS = 2500;
 constexpr uint8_t NO_SIGNAL_TIMEOUT_PERIODS = 8;
 constexpr uint16_t RMT_MIN_RECEIVE_SYMBOLS = 48;
 constexpr uint16_t RMT_MAX_RECEIVE_SYMBOLS = 512;
diff --git a/OpticalChannelTester/Display.cpp b/OpticalChannelTester/Display.cpp
index 6287371..3be14c8 100644
--- a/OpticalChannelTester/Display.cpp
+++ b/OpticalChannelTester/Display.cpp
@@ -2,14 +2,28 @@
 #include "Config.h"
 #include "Log.h"
 #include <Wire.h>
+#include <esp_log.h>
 #include <string.h>
 
 Display::Display() : oled_(128, 32, &Wire, -1) {}
 
 bool Display::begin() {
   Wire.begin(GPIO_SDA, GPIO_SCL);
+  // An absent optional OLED produces a large burst of ESP-IDF NACK messages.
+  // Probe it once and keep the I2C driver quiet when no display is connected.
+  esp_log_level_set("i2c.master", ESP_LOG_NONE);
+  Wire.beginTransmission(OLED_ADDRESS);
+  if (Wire.endTransmission() != 0) {
+    ok_ = false;
+    Log::printf("OLED", "not detected at I2C address=0x%02X", OLED_ADDRESS);
+    return false;
+  }
   ok_ = oled_.begin(SSD1306_SWITCHCAPVCC, OLED_ADDRESS);
-  if (ok_) { oled_.setTextColor(SSD1306_WHITE); oled_.setTextSize(1); }
+  if (ok_) {
+    oled_.setRotation(OLED_ROTATION);
+    oled_.setTextColor(SSD1306_WHITE);
+    oled_.setTextSize(1);
+  }
   Log::printf("OLED", "initialization %s, I2C address=0x%02X", ok_ ? "OK" : "FAILED", OLED_ADDRESS);
   return ok_;
 }
diff --git a/OpticalChannelTester/Log.cpp b/OpticalChannelTester/Log.cpp
index 4565f2f..cf0cfe6 100644
--- a/OpticalChannelTester/Log.cpp
+++ b/OpticalChannelTester/Log.cpp
@@ -7,7 +7,7 @@ namespace Log {
 void event(const char *component, const char *message) {
   if (!SERIAL_ACTION_LOG) return;
   if (SERIAL_MINIMAL_LOG && strcmp(component, "INPUT") && strcmp(component, "UI") &&
-      strcmp(component, "CONFIG") && strcmp(component, "RESULT")) return;
+      strcmp(component, "CONFIG") && strcmp(component, "RESULT") && strcmp(component, "ESP-NOW")) return;
   if (SERIAL_LOG_TIMESTAMPS) Serial.printf("[%10lu][%-8s] %s\n", millis(), component, message);
   else Serial.printf("[%-8s] %s\n", component, message);
 }
diff --git a/OpticalChannelTester/Protocol.cpp b/OpticalChannelTester/Protocol.cpp
index fe24d71..b4cd09f 100644
--- a/OpticalChannelTester/Protocol.cpp
+++ b/OpticalChannelTester/Protocol.cpp
@@ -3,7 +3,7 @@
 
 const char *messageName(MessageType type) {
   static const char *names[] = {"DISCOVER", "DISCOVER_ACK", "PREPARE", "READY",
-    "START_STAGE", "RESULT", "ACK", "ABORT"};
+    "START_STAGE", "RESULT", "ACK", "ABORT", "HEARTBEAT", "HEARTBEAT_ACK"};
   const uint8_t index = static_cast<uint8_t>(type);
   return index < sizeof(names) / sizeof(names[0]) ? names[index] : "UNKNOWN";
 }
@@ -24,5 +24,5 @@ void finalizePacket(ProtocolPacket &p) {
 
 bool validPacket(const ProtocolPacket &p) {
   return p.magic == PROTOCOL_MAGIC && p.version == PROTOCOL_VERSION &&
-    p.type <= static_cast<uint8_t>(MessageType::ABORT) && p.crc == packetCrc(p);
+    p.type <= static_cast<uint8_t>(MessageType::HEARTBEAT_ACK) && p.crc == packetCrc(p);
 }
diff --git a/OpticalChannelTester/Protocol.h b/OpticalChannelTester/Protocol.h
index 61f1f80..529ca10 100644
--- a/OpticalChannelTester/Protocol.h
+++ b/OpticalChannelTester/Protocol.h
@@ -2,10 +2,11 @@
 #include "Core.h"
 
 constexpr uint16_t PROTOCOL_MAGIC = 0x4F43;
-constexpr uint8_t PROTOCOL_VERSION = 1;
+constexpr uint8_t PROTOCOL_VERSION = 2;
 
 enum class MessageType : uint8_t {
-  DISCOVER, DISCOVER_ACK, PREPARE, READY, START_STAGE, RESULT, ACK, ABORT
+  DISCOVER, DISCOVER_ACK, PREPARE, READY, START_STAGE, RESULT, ACK, ABORT,
+  HEARTBEAT, HEARTBEAT_ACK
 };
 
 const char *messageName(MessageType type);
diff --git a/OpticalChannelTester/Radio.cpp b/OpticalChannelTester/Radio.cpp
index a8011b4..3690a97 100644
--- a/OpticalChannelTester/Radio.cpp
+++ b/OpticalChannelTester/Radio.cpp
@@ -10,17 +10,36 @@ static const uint8_t BROADCAST_MAC[6] = {0xff, 0xff, 0xff, 0xff, 0xff, 0xff};
 
 bool Radio::begin() {
   if (active_) { Log::event("ESP-NOW", "already active"); return true; }
-  WiFi.mode(WIFI_STA); WiFi.disconnect();
+  // Keep both devices on a dedicated, fixed STA channel. Stored access-point
+  // credentials must not reconnect in the background and move ESP-NOW to the
+  // AP's channel.
+  WiFi.persistent(false);
+  WiFi.mode(WIFI_STA);
+  WiFi.setAutoReconnect(false);
+  WiFi.disconnect(false, false);
+  WiFi.setSleep(false);
+  const bool txPowerOk = WiFi.setTxPower(WIFI_POWER_8_5dBm);
+  delay(10);
   if (esp_wifi_set_channel(ESPNOW_WIFI_CHANNEL, WIFI_SECOND_CHAN_NONE) != ESP_OK) {
     Log::event("ESP-NOW", "Wi-Fi channel setup FAILED"); return false;
   }
   queue_ = xQueueCreate(8, sizeof(ReceivedPacket));
-  if (!queue_ || esp_now_init() != ESP_OK) { Log::event("ESP-NOW", "initialization FAILED"); return false; }
+  if (!queue_) { Log::event("ESP-NOW", "receive queue creation FAILED"); return false; }
+  if (esp_now_init() != ESP_OK) {
+    vQueueDelete(queue_); queue_ = nullptr;
+    Log::event("ESP-NOW", "initialization FAILED"); return false;
+  }
+  const esp_err_t rateResult = esp_wifi_config_espnow_rate(WIFI_IF_STA, WIFI_PHY_RATE_1M_L);
   instance_ = this;
   if (esp_now_register_recv_cb(onReceive) != ESP_OK) { end(); return false; }
   active_ = true;
   const bool ok = ensurePeer(BROADCAST_MAC);
-  Log::printf("ESP-NOW", "started channel=%u broadcast-peer=%s", ESPNOW_WIFI_CHANNEL, ok ? "OK" : "FAILED");
+  uint8_t primaryChannel = 0;
+  wifi_second_chan_t secondaryChannel = WIFI_SECOND_CHAN_NONE;
+  esp_wifi_get_channel(&primaryChannel, &secondaryChannel);
+  Log::printf("ESP-NOW", "started channel=%u expected=%u rate=1M%s tx-power=max%s broadcast-peer=%s",
+    primaryChannel, ESPNOW_WIFI_CHANNEL, rateResult == ESP_OK ? "" : "-FAILED",
+    txPowerOk ? "" : "-FAILED", ok ? "OK" : "FAILED");
   return ok;
 }
 
@@ -34,13 +53,17 @@ void Radio::end() {
 bool Radio::ensurePeer(const uint8_t mac[6]) {
   if (esp_now_is_peer_exist(mac)) return true;
   esp_now_peer_info_t peer = {};
-  memcpy(peer.peer_addr, mac, 6); peer.channel = ESPNOW_WIFI_CHANNEL; peer.encrypt = false;
+  memcpy(peer.peer_addr, mac, 6);
+  peer.channel = ESPNOW_WIFI_CHANNEL;
+  peer.ifidx = WIFI_IF_STA;
+  peer.encrypt = false;
   return esp_now_add_peer(&peer) == ESP_OK;
 }
 
 bool Radio::sendBroadcast(ProtocolPacket p) { return sendTo(BROADCAST_MAC, p); }
 
 bool Radio::sendTo(const uint8_t mac[6], ProtocolPacket p) {
+  maintainChannel();
   char peer[20]; macText(mac, peer, sizeof(peer));
   if (!active_ || !ensurePeer(mac)) {
     Log::printf("ESP-NOW", "TX %s to %s FAILED: inactive/peer", messageName(static_cast<MessageType>(p.type)), peer);
@@ -48,15 +71,32 @@ bool Radio::sendTo(const uint8_t mac[6], ProtocolPacket p) {
   }
   finalizePacket(p);
   const bool ok = esp_now_send(mac, reinterpret_cast<const uint8_t *>(&p), sizeof(p)) == ESP_OK;
-  Log::printf("ESP-NOW", "TX %s to %s session=%08lX stage=%u seq=%u %s",
-    messageName(static_cast<MessageType>(p.type)), peer, p.session, p.stage, p.sequence, ok ? "QUEUED" : "FAILED");
+  const MessageType type = static_cast<MessageType>(p.type);
+  if (type != MessageType::HEARTBEAT && type != MessageType::HEARTBEAT_ACK)
+    Log::printf("ESP-NOW", "TX %s to %s session=%08lX stage=%u seq=%u %s",
+      messageName(type), peer, p.session, p.stage, p.sequence, ok ? "QUEUED" : "FAILED");
   return ok;
 }
 
 bool Radio::receive(ReceivedPacket &r) {
+  maintainChannel();
   return queue_ && xQueueReceive(queue_, &r, 0) == pdTRUE;
 }
 
+void Radio::maintainChannel() {
+  if (!active_) return;
+  const uint32_t now = millis();
+  if (now - lastChannelCheckMs_ < 250) return;
+  lastChannelCheckMs_ = now;
+  uint8_t primaryChannel = 0;
+  wifi_second_chan_t secondaryChannel = WIFI_SECOND_CHAN_NONE;
+  if (esp_wifi_get_channel(&primaryChannel, &secondaryChannel) != ESP_OK ||
+      primaryChannel == ESPNOW_WIFI_CHANNEL) return;
+  const bool restored = esp_wifi_set_channel(ESPNOW_WIFI_CHANNEL, WIFI_SECOND_CHAN_NONE) == ESP_OK;
+  Log::printf("ESP-NOW", "channel drift %u->%u %s", primaryChannel, ESPNOW_WIFI_CHANNEL,
+    restored ? "RESTORED" : "FAILED");
+}
+
 void Radio::flush() { if (queue_) xQueueReset(queue_); }
 
 void Radio::onReceive(const esp_now_recv_info_t *info, const uint8_t *data, int length) {
diff --git a/OpticalChannelTester/Radio.h b/OpticalChannelTester/Radio.h
index 2488254..8038e51 100644
--- a/OpticalChannelTester/Radio.h
+++ b/OpticalChannelTester/Radio.h
@@ -17,8 +17,10 @@ class Radio {
  private:
   static void onReceive(const esp_now_recv_info_t *info, const uint8_t *data, int length);
   bool ensurePeer(const uint8_t mac[6]);
+  void maintainChannel();
   static Radio *instance_;
   QueueHandle_t queue_ = nullptr;
   bool active_ = false;
+  uint32_t lastChannelCheckMs_ = 0;
 };