Index: firmware/App/Services/SystemCommMessages.c
===================================================================
diff -u -r6c60d0d0300828750c76f2e397d4e7ccaa84bbb2 -r05b4d416b6bd5e7af4347f112a20f9f81a257064
--- firmware/App/Services/SystemCommMessages.c	(.../SystemCommMessages.c)	(revision 6c60d0d0300828750c76f2e397d4e7ccaa84bbb2)
+++ firmware/App/Services/SystemCommMessages.c	(.../SystemCommMessages.c)	(revision 05b4d416b6bd5e7af4347f112a20f9f81a257064)
@@ -109,16 +109,19 @@
     // prefix data with message sync byte
     data[ msgSize++ ] = MESSAGE_SYNC_BYTE;
 
-    // set sequence # and ACK bit
+    // set sequence # and ACK bit (unless this is an ACK to a received message)
 #ifndef ACK_NOT_IMPLEMENTED
-    // thread protect next sequence # access & increment
-    _disable_IRQ();
-        msg.hdr.seqNo = nextSeqNo;
-        nextSeqNo = INC_WRAP( nextSeqNo, MIN_MSG_SEQ_NO, MAX_MSG_SEQ_NO );
-    _enable_IRQ();
-    if ( TRUE == ackReq )
+    if ( msg.hdr.msgID != MSG_ID_ACK )
     {
-        msg.hdr.seqNo *= -1;
+        // thread protect next sequence # access & increment
+        _disable_IRQ();
+            msg.hdr.seqNo = nextSeqNo;
+            nextSeqNo = INC_WRAP( nextSeqNo, MIN_MSG_SEQ_NO, MAX_MSG_SEQ_NO );
+        _enable_IRQ();
+        if ( TRUE == ackReq )
+        {
+            msg.hdr.seqNo *= -1;
+        }
     }
 #endif