diff --git a/MAC/APL/PIC/RSP_Protocol/src/RSP_Protocol.prot b/MAC/APL/PIC/RSP_Protocol/src/RSP_Protocol.prot
index 2c857166b2121c08abdc2d66418e26f083c11cf3..f13b30482c8640ef05010660fe21de18e01f0d6f 100644
--- a/MAC/APL/PIC/RSP_Protocol/src/RSP_Protocol.prot
+++ b/MAC/APL/PIC/RSP_Protocol/src/RSP_Protocol.prot
@@ -40,6 +40,7 @@ typedef	void*	memptr_t;
 static const int MAX_N_RSPBOARDS 	= 24;
 static const int RSP_SUCCESS       	= 1;
 static const int RSP_FAILURE       	= 0;
+static const int RSP_BUSY       	= -1;
 static const int RSP_RAW_BLOCK_SIZE = 1480;
 
 // The protocol implements the following messages.