diff --git a/CommandStation-EX.ino b/CommandStation-EX.ino
index 0e282d1..2357638 100644
--- a/CommandStation-EX.ino
+++ b/CommandStation-EX.ino
@@ -127,7 +127,7 @@ void loop()
 #if ENABLE_FREE_MEM_WARNING
   static int ramLowWatermark = 32767; // replaced on first loop 
 
-  int freeNow = freeMemory();
+  int freeNow = updateMinimumFreeMemory();
   if (freeNow < ramLowWatermark)
   {
     ramLowWatermark = freeNow;
diff --git a/DCCWaveform.cpp b/DCCWaveform.cpp
index c1e0269..c7baa9f 100644
--- a/DCCWaveform.cpp
+++ b/DCCWaveform.cpp
@@ -23,6 +23,7 @@
 #include "DCCWaveform.h"
 #include "DCCTimer.h"
 #include "DIAG.h"
+#include "freeMemory.h"
  
 
 DCCWaveform  DCCWaveform::mainTrack(PREAMBLE_BITS_MAIN, true);
@@ -203,6 +204,9 @@ void DCCWaveform::interrupt2() {
   if (remainingPreambles > 0 ) {
     state=WAVE_MID_1;  // switch state to trigger LOW on next interrupt
     remainingPreambles--;
+    // Update free memory diagnostic as we don't have anything else to do this time.
+    // Allow for checkAck and its called functions using 22 bytes more.
+    updateMinimumFreeMemory(22); 
     return;
   }
 
diff --git a/freeMemory.cpp b/freeMemory.cpp
index 30fa16d..6735ac1 100644
--- a/freeMemory.cpp
+++ b/freeMemory.cpp
@@ -1,5 +1,6 @@
 /*
  *  © 2020, Harald Barth
+ *  © 2021, Neil McKechnie
  *  
  *  This file is part of Asbelos DCC-EX
  *
@@ -30,6 +31,9 @@ extern char *__malloc_heap_start;
 #endif
 
 
+static volatile int minimum_free_memory = 32767;
+
+
 int freeMemory() {
   char top;
 #if defined(__arm__)
@@ -40,3 +44,12 @@ int freeMemory() {
 #error bailed out alredy above
 #endif
 }
+
+// Update low ram level.  Allow for extra bytes to be specified
+// by estimation or inspection, that may be used by other 
+// called subroutines.
+int updateMinimumFreeMemory(unsigned char extraBytes) {
+  int spare = freeMemory()-extraBytes;
+  if (spare < minimum_free_memory) minimum_free_memory = spare;
+  return minimum_free_memory;
+}
diff --git a/freeMemory.h b/freeMemory.h
index 2bd35c1..bf93189 100644
--- a/freeMemory.h
+++ b/freeMemory.h
@@ -1,5 +1,6 @@
 /*
  *  © 2020, Harald Barth
+ *  © 2021, Neil McKechnie
  *  
  *  This file is part of DCC-EX
  *
@@ -20,4 +21,5 @@
 #ifndef freeMemory_h
 #define freeMemory_h
 int freeMemory();
+int updateMinimumFreeMemory(unsigned char extraBytes=0);
 #endif