diff --git a/DCC.cpp b/DCC.cpp
index e60cf25b..b2237617 100644
--- a/DCC.cpp
+++ b/DCC.cpp
@@ -309,6 +309,7 @@ void DCC::setAccessory(int address, byte port, bool gate, byte onoff /*= 2*/) {
   b[0] = address % 64 + 128;
   b[1] = ((((address / 64) % 8) << 4) + (port % 4 << 1) + gate % 2) ^ 0xF8;
   if (onoff != 0) {
+    DIAG(F("DCC::setAccessory(%d,%d,%d) ON"), address, port, gate);
     DCCWaveform::mainTrack.schedulePacket(b, 2, 3);      // Repeat on packet three times
 #if defined(EXRAIL_ACTIVE)
     RMFT2::activateEvent(address<<2|port,gate);
@@ -316,6 +317,7 @@ void DCC::setAccessory(int address, byte port, bool gate, byte onoff /*= 2*/) {
   }
   if (onoff != 1) {
     b[1] &= ~0x08; // set C to 0
+    DIAG(F("DCC::setAccessory(%d,%d,%d) OFF"), address, port, gate);
     DCCWaveform::mainTrack.schedulePacket(b, 2, 3);      // Repeat off packet three times
   }
 }
diff --git a/DCCRMT.cpp b/DCCRMT.cpp
index 24a6e000..f03f4908 100644
--- a/DCCRMT.cpp
+++ b/DCCRMT.cpp
@@ -86,6 +86,14 @@ void setDCCBit0Long(rmt_item32_t* item) {
   item->duration1 = DCC_0_HALFPERIOD + DCC_0_HALFPERIOD/10;
 }
 
+// special short one to trigger scope
+void setDCCBit1Short(rmt_item32_t* item) {
+  item->level0    = 1;
+  item->duration0 = DCC_1_HALFPERIOD/2;
+  item->level1    = 0;
+  item->duration1 = DCC_1_HALFPERIOD/2;
+}
+
 void setEOT(rmt_item32_t* item) {
   item->val = 0;
 }
@@ -135,11 +143,17 @@ RMTChannel::RMTChannel(pinpair pins, bool isMain) {
   // preamble
   preambleLen = plen+2; // plen 1 bits, one 0 bit and one EOF marker
   preamble = (rmt_item32_t*)malloc(preambleLen*sizeof(rmt_item32_t));
-  for (byte n=0; n<plen; n++)
-    setDCCBit1(preamble + n);      // preamble bits
 #ifdef SCOPE
+  for (byte n=0; n<plen; n++) {
+    if (n == 0)
+      setDCCBit1Short(preamble + n); // start of preamble 1 bit short version
+    else
+      setDCCBit1(preamble + n);
+  }
   setDCCBit0Long(preamble + plen); // start of packet 0 bit long version
 #else
+  for (byte n=0; n<plen; n++) {
+    setDCCBit1(preamble + n);      // preamble bits
   setDCCBit0(preamble + plen);     // start of packet 0 bit normal version
 #endif
   setEOT(preamble + plen + 1);     // EOT marker
@@ -204,6 +218,7 @@ void RMTChannel::RMTprefill() {
 const byte transmitMask[] = {0x80, 0x40, 0x20, 0x10, 0x08, 0x04, 0x02, 0x01};
 
 int RMTChannel::RMTfillData(const byte buffer[], byte byteCount, byte repeatCount=0) {
+  int ret = 0;
   //int RMTChannel::RMTfillData(dccPacket packet) {
   // dataReady: Signals to then interrupt routine. It is set when
   // we have data in the channel buffer which can be copied out
@@ -211,9 +226,14 @@ int RMTChannel::RMTfillData(const byte buffer[], byte byteCount, byte repeatCoun
   // the caller of this function if the data has been sent enough
   // times (0 to 3 means 1 to 4 times in total).
   if (dataRepeat > 0) // we have still old work to do
-    return dataRepeat;
+    ret = dataRepeat;
   if (dataReady == true) // the packet is not copied out yet
-    return 1000;
+    ret = 1000;
+  if (buffer[0] == 129) {
+    DIAG(F("RMTfillData 129 %d %d"), buffer[1], ret);
+  }
+  if (ret > 0)
+    return ret;
   if (DATA_LEN(byteCount) > maxDataLen) {  // this would overun our allocated memory for data
     DIAG(F("Can not convert DCC bytes # %d to DCC bits %d, buffer too small"), byteCount, maxDataLen);
     return -1;                          // something very broken, can not convert packet
diff --git a/DCCRMT.h b/DCCRMT.h
index 33257a04..691ca15c 100644
--- a/DCCRMT.h
+++ b/DCCRMT.h
@@ -39,6 +39,13 @@ class RMTChannel {
   void RMTprefill();
   //int RMTfillData(dccPacket packet);
   int RMTfillData(const byte buffer[], byte byteCount, byte repeatCount);
+  inline void waitForDataCopy() {
+    while(1) {
+      if (dataReady == false)
+	break;
+      //DIAG(F("%d %d"), dataReady, dataRepeat);
+    } //do nothing and wait for interrupt to happen
+  };
   inline bool busy() {
     if (dataRepeat > 0) // we have still old work to do
       return true;
diff --git a/DCCWaveform.cpp b/DCCWaveform.cpp
index 3d77e9e8..54615a29 100644
--- a/DCCWaveform.cpp
+++ b/DCCWaveform.cpp
@@ -280,6 +280,13 @@ void DCCWaveform::schedulePacket(const byte buffer[], byte byteCount, byte repea
   if (byteCount > MAX_PACKET_SIZE) return; // allow for chksum
   
   byte checksum = 0;
+  if(isMainTrack) {
+    if (rmtMainChannel != NULL)
+      rmtMainChannel->waitForDataCopy();
+  } else {
+    if (rmtProgChannel != NULL)
+      rmtProgChannel->waitForDataCopy();
+  }
   for (byte b = 0; b < byteCount; b++) {
     checksum ^= buffer[b];
     pendingPacket[b] = buffer[b];
diff --git a/GITHUB_SHA.h b/GITHUB_SHA.h
index f47fd82c..52e43b04 100644
--- a/GITHUB_SHA.h
+++ b/GITHUB_SHA.h
@@ -1 +1 @@
-#define GITHUB_SHA "c389fe9"
+#define GITHUB_SHA "master-packetloss-202501161204Z"
diff --git a/version.h b/version.h
index c4c7ba4a..862f07bb 100644
--- a/version.h
+++ b/version.h
@@ -3,7 +3,8 @@
 
 #include "StringFormatter.h"
 
-#define VERSION "5.4.0"
+#define VERSION "5.4.X"
+// 5.4.X  - bugfix branch
 // 5.4.0  - New version on master
 // 5.2.96 - EXRAIL additions XFWD() and XREV()
 // 5.2.95 - Release candidate for 5.4