diff --git a/GCodes.h b/GCodes.h
index c44f98b..58f862d 100644
--- a/GCodes.h
+++ b/GCodes.h
@@ -41,9 +41,9 @@ class GCodeBuffer
     char* GetUnprecedentedString();
     char* GetString();
     char* Buffer();
-    bool Finished();
+    bool Finished() const;
     void SetFinished(bool f);
-    char* WritingFileDirectory();
+    char* WritingFileDirectory() const;
     void SetWritingFileDirectory(char* wfd);
     
   private:
@@ -75,8 +75,9 @@ class GCodes
     void QueueFileToPrint(char* fileName);
     bool GetProbeCoordinates(int count, float& x, float& y, float& z);
     char* GetCurrentCoordinates();
-    bool PrintingAFile();
+    bool PrintingAFile() const;
     void Diagnostics();
+    bool HaveIncomingData() const;
     
   private:
   
@@ -97,7 +98,7 @@ class GCodes
     bool SetOffsets(GCodeBuffer *gb);
     bool SetPositions(GCodeBuffer *gb);
     void LoadMoveBufferFromGCode(GCodeBuffer *gb, bool doingG92);
-    bool NoHome();
+    bool NoHome() const;
     bool Push();
     bool Pop();
     bool DisableDrives();
@@ -110,7 +111,7 @@ class GCodes
     void WriteHTMLToFile(char b, GCodeBuffer *gb);
     bool OffsetAxes(GCodeBuffer *gb);
 
-    int8_t Heater(int8_t head);
+    int8_t Heater(int8_t head) const;
     Platform* platform;
     bool active;
     Webserver* webserver;
@@ -172,7 +173,7 @@ inline char* GCodeBuffer::Buffer()
   return gcodeBuffer;
 }
 
-inline bool GCodeBuffer::Finished()
+inline bool GCodeBuffer::Finished() const
 {
   return finished;
 }
@@ -182,7 +183,7 @@ inline void GCodeBuffer::SetFinished(bool f)
   finished = f;
 }
 
-inline char* GCodeBuffer::WritingFileDirectory()
+inline char* GCodeBuffer::WritingFileDirectory() const
 {
 	return writingFileDirectory;
 }
@@ -192,12 +193,17 @@ inline void GCodeBuffer::SetWritingFileDirectory(char* wfd)
 	writingFileDirectory = wfd;
 }
 
-inline bool GCodes::PrintingAFile()
+inline bool GCodes::PrintingAFile() const
 {
   return fileBeingPrinted != NULL;
 }
 
-inline bool GCodes::NoHome()
+inline bool GCodes::HaveIncomingData() const
+{
+	return fileBeingPrinted != NULL || webserver->GCodeAvailable() || (platform->GetLine()->Status() & byteAvailable);
+}
+
+inline bool GCodes::NoHome() const
 {
    return !(homeX || homeY || homeZ || homeAxisMoveCount);
 }
@@ -205,7 +211,7 @@ inline bool GCodes::NoHome()
 // This function takes care of the fact that the heater and head indices 
 // don't match because the bed is heater 0.
 
-inline int8_t GCodes::Heater(int8_t head)
+inline int8_t GCodes::Heater(int8_t head) const
 {
    return head+1; 
 }
diff --git a/Move.cpp b/Move.cpp
index 453d5c7..6b99715 100644
--- a/Move.cpp
+++ b/Move.cpp
@@ -441,7 +441,7 @@ void Move::DoLookAhead()
   // or both to the maximum that can be achieved because of the requirements of
   // the adjacent moves. 
     
-  if(addNoMoreMoves || !gCodes->PrintingAFile() || lookAheadRingCount > LOOK_AHEAD)
+  if(addNoMoreMoves || !gCodes->HaveIncomingData() || lookAheadRingCount > LOOK_AHEAD)
   { 
     
     // Run up the moves
@@ -497,7 +497,7 @@ void Move::DoLookAhead()
   // If there are any new unprocessed moves in there, set their end speeds
   // according to the cosine of the angle between them.
   
-  if(addNoMoreMoves || !gCodes->PrintingAFile() || lookAheadRingCount > 1)
+  if(addNoMoreMoves || !gCodes->HaveIncomingData() || lookAheadRingCount > 1)
   {  
     n1 = lookAheadRingGetPointer;
     n0 = n1->Previous();
@@ -531,7 +531,7 @@ void Move::DoLookAhead()
     
     // If we are just doing one isolated move, set its end velocity to InstantDv(Z_AXIS).
     
-    if(addNoMoreMoves || !gCodes->PrintingAFile())
+    if(addNoMoreMoves || !gCodes->HaveIncomingData())
     {
       n1->SetV(platform->InstantDv(Z_AXIS));
       n1->SetProcessed(complete);
@@ -1163,7 +1163,7 @@ void LookAhead::Init(long ep[], float f, float vv, bool ce, int8_t mt)
   // are printing a file, so set processed
   // complete when we aren't.
   
-  if(reprap.GetGCodes()->PrintingAFile())
+  if(reprap.GetGCodes()->HaveIncomingData())
     processed = unprocessed;
   else
     processed = complete|vCosineSet|upPass;
diff --git a/Platform.cpp b/Platform.cpp
index 2177d78..553b0c6 100644
--- a/Platform.cpp
+++ b/Platform.cpp
@@ -835,6 +835,8 @@ Line::Line()
 
 void Line::Init()
 {
+	getIndex = 0;
+	numChars = 0;
 //	alternateInput = NULL;
 //	alternateOutput = NULL;
 	SerialUSB.begin(BAUD_RATE);
@@ -1087,7 +1089,7 @@ void Network::ReceiveInput(char* data, int length, void* pbuf, void* pcb, void*
 
 
 
-bool Network::CanWrite()
+bool Network::CanWrite() const
 {
 	return writeEnabled;
 }
@@ -1138,7 +1140,7 @@ void Network::Close()
 	//Reset();
 }
 
-int8_t Network::Status()
+int8_t Network::Status() const
 {
 	if(inputPointer >= inputLength)
 		return status;
diff --git a/Platform.h b/Platform.h
index f701f88..743ce8d 100644
--- a/Platform.h
+++ b/Platform.h
@@ -183,13 +183,15 @@ Licence: GPL
 
 #define BAUD_RATE 115200 // Communication speed of the USB if needed.
 
+const uint16_t lineBufsize = 256;	// use a power of 2 for good performance
+
 /****************************************************************************************************/
 
 enum EndStopHit
 {
-  noStop = 0,
-  lowHit = 1,
-  highHit = 2
+  noStop = 0,		// no enstop hit
+  lowHit = 1,		// low switch hit, or Z-probe in use and above threshold
+  highHit = 2		// high stop hit
 };
 
 /***************************************************************************************************/
@@ -264,9 +266,9 @@ class Network //: public InputOutput
 {
 public:
 
-	int8_t Status(); // Returns OR of IOStatus
+	int8_t Status() const; // Returns OR of IOStatus
 	bool Read(char& b);
-	bool CanWrite();
+	bool CanWrite() const;
 	void SetWriteEnable(bool enable);
 	void Write(char b);
 	void Write(char* s);
@@ -274,7 +276,7 @@ public:
 	void ReceiveInput(char* data, int length, void* pb, void* pc, void* h);
 	void InputBufferReleased(void* pb);
 	void ConnectionError(void* h);
-	bool Active();
+	bool Active() const;
 	bool LinkIsUp();
 
 friend class Platform;
@@ -308,7 +310,7 @@ class Line //: public InputOutput
 {
 public:
 
-	int8_t Status(); // Returns OR of IOStatus
+	int8_t Status() const; // Returns OR of IOStatus
 	int Read(char& b);
 	void Write(char b);
 	void Write(char* s);
@@ -324,6 +326,9 @@ protected:
 	void Spin();
 
 private:
+	char buffer[lineBufsize];
+	uint16_t getIndex;
+	uint16_t numChars;
 };
 
 class MassStorage
@@ -981,16 +986,24 @@ inline Line* Platform::GetLine()
 
 inline void Line::Spin()
 {
+	// Read the serial data in blocks to avoid excessive flow control
+	if (numChars <= lineBufsize/2)
+	{
+		while (SerialUSB.available() > 0 && numChars < lineBufsize)
+		{
+			int incomingByte = SerialUSB.read();
+			if (incomingByte < 0) break;
+			buffer[(getIndex + numChars) % lineBufsize] = (char)incomingByte;
+			++numChars;
+		}
+	}
 }
 
-inline int8_t Line::Status()
+inline int8_t Line::Status() const
 {
 //	if(alternateInput != NULL)
 //		return alternateInput->Status();
-
-	if(SerialUSB.available() > 0)
-		return byteAvailable;
-	return nothing;
+	return numChars == 0 ? nothing : byteAvailable;
 }
 
 inline int Line::Read(char& b)
@@ -998,11 +1011,11 @@ inline int Line::Read(char& b)
 //  if(alternateInput != NULL)
 //	return alternateInput->Read(b);
 
-  int incomingByte = SerialUSB.read();
-  if(incomingByte < 0)
-    return 0;
-  b = (char)incomingByte;
-  return true;
+  if (numChars == 0) return 0;
+  b = buffer[getIndex];
+  getIndex = (getIndex + 1) % lineBufsize;
+  --numChars;
+  return 1;
 }
 
 inline void Line::Write(char b)
@@ -1046,7 +1059,7 @@ inline bool Network::LinkIsUp()
 	return status_link_up();
 }
 
-inline bool Network::Active()
+inline bool Network::Active() const
 {
 	return active;
 }