Changed the MSTP_MASTER_STATE_ANSWER_DATA_REQUEST state to always send a REPLY_POSTPONED by

commenting out the section that checks for new packets to send.  It was not the intent of the BACnet standard, but it makes the Master Node FSM compliant.  It's either that or re-parse the message for a matching apdu and invoke id, and then defer if the next message does not match.  The problem happens when an unconfirmed request (i.e. WhoIs) is followed immediated by a confirmed request (i.e. ReadProperty) before the device can respond to the unconfirmed request (i.e. the I-Am is queued in the Transmit buffer).

bacnet-stack/dlmstp.h

@@ -45,6 +45,55 @@
 #define MAX_HEADER (2+1+1+1+2+1+2+1)
 #define MAX_MPDU (MAX_HEADER+MAX_PDU)
 
+/*  The value 255 is used to denote broadcast when used as a */
+/* destination address but is not allowed as a value for a station. */
+/* Station addresses for master nodes can be 0-127.  */
+/* Station addresses for slave nodes can be 127-254.  */
+#define MSTP_BROADCAST_ADDRESS 255
+        
+/* MS/TP Frame Type */
+/* Frame Types 8 through 127 are reserved by ASHRAE. */
+#define FRAME_TYPE_TOKEN 0
+#define FRAME_TYPE_POLL_FOR_MASTER 1
+#define FRAME_TYPE_REPLY_TO_POLL_FOR_MASTER 2
+#define FRAME_TYPE_TEST_REQUEST 3
+#define FRAME_TYPE_TEST_RESPONSE 4
+#define FRAME_TYPE_BACNET_DATA_EXPECTING_REPLY 5
+#define FRAME_TYPE_BACNET_DATA_NOT_EXPECTING_REPLY 6
+#define FRAME_TYPE_REPLY_POSTPONED 7
+/* Frame Types 128 through 255: Proprietary Frames */
+/* These frames are available to vendors as proprietary (non-BACnet) frames. */
+/* The first two octets of the Data field shall specify the unique vendor */
+/* identification code, most significant octet first, for the type of */
+/* vendor-proprietary frame to be conveyed. The length of the data portion */
+/* of a Proprietary frame shall be in the range of 2 to 501 octets. */
+#define FRAME_TYPE_PROPRIETARY_MIN 128
+#define FRAME_TYPE_PROPRIETARY_MAX 255
+/* The initial CRC16 checksum value */
+#define CRC16_INITIAL_VALUE (0xFFFF)
+
+/* receive FSM states */
+typedef enum {
+    MSTP_RECEIVE_STATE_IDLE = 0,
+    MSTP_RECEIVE_STATE_PREAMBLE = 1,
+    MSTP_RECEIVE_STATE_HEADER = 2,
+    MSTP_RECEIVE_STATE_HEADER_CRC = 3,
+    MSTP_RECEIVE_STATE_DATA = 4
+} MSTP_RECEIVE_STATE;
+
+/* master node FSM states */
+typedef enum {
+    MSTP_MASTER_STATE_INITIALIZE = 0,
+    MSTP_MASTER_STATE_IDLE = 1,
+    MSTP_MASTER_STATE_USE_TOKEN = 2,
+    MSTP_MASTER_STATE_WAIT_FOR_REPLY = 3,
+    MSTP_MASTER_STATE_DONE_WITH_TOKEN = 4,
+    MSTP_MASTER_STATE_PASS_TOKEN = 5,
+    MSTP_MASTER_STATE_NO_TOKEN = 6,
+    MSTP_MASTER_STATE_POLL_FOR_MASTER = 7,
+    MSTP_MASTER_STATE_ANSWER_DATA_REQUEST = 8
+} MSTP_MASTER_STATE;
+
 typedef struct dlmstp_packet {
     bool ready;                 /* true if ready to be sent or received */
     BACNET_ADDRESS address;     /* source address */