Index: firmware/App/Services/SystemComm.c
===================================================================
diff -u -r9a9d04b84f4345fca87fb14d26f09d497b08aae8 -r6c60d0d0300828750c76f2e397d4e7ccaa84bbb2
--- firmware/App/Services/SystemComm.c	(.../SystemComm.c)	(revision 9a9d04b84f4345fca87fb14d26f09d497b08aae8)
+++ firmware/App/Services/SystemComm.c	(.../SystemComm.c)	(revision 6c60d0d0300828750c76f2e397d4e7ccaa84bbb2)
@@ -93,7 +93,7 @@
 static U08 pcXmitPacket[ PC_MESSAGE_PACKET_SIZE ] = { 0, 0, 0, 0, 0, 0, 0, 0 };
 static U08 pcRecvPacket[ PC_MESSAGE_PACKET_SIZE ] = { 0, 0, 0, 0, 0, 0, 0, 0 };
 
-#ifdef ACK_IMPLEMENTED
+#ifndef ACK_NOT_IMPLEMENTED
 static PENDING_ACK_RECORD_T pendingAckList[ PENDING_ACK_LIST_SIZE ];    // list of outgoing messages that are awaiting an ACK
 #endif
 
@@ -130,8 +130,8 @@
 static void checkForCommTimeouts( void );
 static void checkTooManyBadMsgCRCs( U16 msgID );
 
-#ifdef ACK_IMPLEMENTED
-static BOOL matchACKtoPendingACKList( U16 seqNo );
+#ifndef ACK_NOT_IMPLEMENTED
+static BOOL matchACKtoPendingACKList( S16 seqNo );
 static void checkPendingACKList( void );
 #endif
 
@@ -157,7 +157,7 @@
         badCRCTimeStamps[ i ] = 0;
     }
 
-#ifdef ACK_IMPLEMENTED
+#ifndef ACK_NOT_IMPLEMENTED
     // initialize pending ACK list
     for ( i = 0; i < PENDING_ACK_LIST_SIZE; i++ )
     {
@@ -271,7 +271,7 @@
     // check for sub-system comm timeouts
     checkForCommTimeouts();
 
-#ifdef ACK_IMPLEMENTED
+#ifndef ACK_NOT_IMPLEMENTED
     // check ACK list for messages that need to be re-sent because they haven't been ACK'd
     checkPendingACKList();
 #endif
@@ -787,7 +787,7 @@
             // TODO - check CRC before processing a message
             if ( message.crc == crc8( (U08*)(&message), sizeof(MESSAGE_HEADER_T) + message.msg.hdr.payloadLen ) )
             {
-#ifdef ACK_IMPLEMENTED
+#ifndef ACK_NOT_IMPLEMENTED
                 // if ACK,
                 if ( MSG_ID_ACK == message.msg.hdr.msgID )
                 {
@@ -859,7 +859,7 @@
     }
 }
 
-#ifdef ACK_IMPLEMENTED
+#ifndef ACK_NOT_IMPLEMENTED
 /*************************************************************************
  * @brief addMsgToPendingACKList
  * The addMsgToPendingACKList function adds a given message to the pending \n
@@ -884,13 +884,15 @@
         _disable_IRQ();  // slot selection needs interrupt protection
         if ( FALSE == pendingAckList[ i ].used )
         {
+            S16 seqNo = msg->hdr.seqNo;
+
             pendingAckList[ i ].used = TRUE;
             _enable_IRQ();
-            if ( msg->hdr.seqNo < 0 )
+            if ( seqNo < 0 )
             {
-                msg->hdr.seqNo *= -1;
+                seqNo *= -1;
             }
-            pendingAckList[ i ].seqNo = msg->hdr.seqNo;
+            pendingAckList[ i ].seqNo = seqNo;
             pendingAckList[ i ].channel = channel;
             pendingAckList[ i ].timeStamp = getMSTimerCount();
             pendingAckList[ i ].retries = MSG_NOT_ACKED_MAX_RETRIES;
@@ -919,7 +921,7 @@
  * @param seqNo : sequence # to match to an entry in the list
  * @return TRUE if a match was found, FALSE if not
  *************************************************************************/
-static BOOL matchACKtoPendingACKList( U16 seqNo )
+static BOOL matchACKtoPendingACKList( S16 seqNo )
 {
     BOOL result = FALSE;
     U32  i;
@@ -928,7 +930,7 @@
     for ( i = 0; i < PENDING_ACK_LIST_SIZE; i++ )
     {
         if ( ( TRUE == pendingAckList[ i ].used ) && ( pendingAckList[ i ].seqNo == seqNo ) )
-        {
+        {   // remove message pending ACK from list
             pendingAckList[ i ].used = FALSE;
             result = TRUE;
             break;
@@ -989,9 +991,9 @@
 {
     U16 msgID = message->hdr.msgID;
 
-#ifdef ACK_IMPLEMENTED
+#ifndef ACK_NOT_IMPLEMENTED
     // if received message requires ACK, queue one up
-    if ( ( message->hdr.seqNo & MSG_ACK_BIT ) != 0 )
+    if ( message->hdr.seqNo < 0 )
     {
         sendACKMsg( message );
     }