When MAC address length is zero, assume it is a broadcast. Updated the MS/TP to understand this.

bacnet-stack/ports/at91sam7s/dlmstp.c

@@ -1238,7 +1238,12 @@ int dlmstp_send_pdu(
             pkt->buffer[i] = pdu[i];
         }
         pkt->length = pdu_len;
-        pkt->destination_mac = dest->mac[0];
+        if (dest && dest->mac_len) {
+            pkt->destination_mac = dest->mac[0];
+        } else {
+            /* mac_len = 0 is a broadcast address */
+            pkt->destination_mac = MSTP_BROADCAST_ADDRESS;
+        }
         bytes_sent = pdu_len;
     }
 

bacnet-stack/ports/atmega168/dlmstp.c

@@ -987,7 +987,12 @@ int dlmstp_send_pdu(
         TransmitPacket = pdu;
         TransmitPacketLen = pdu_len;
         bytes_sent = pdu_len;
-        TransmitPacketDest = dest->mac[0];
+        if (dest && dest->mac_len) {
+            TransmitPacketDest = dest->mac[0];
+        } else {
+            /* mac_len = 0 is a broadcast address */
+            TransmitPacketDest = MSTP_BROADCAST_ADDRESS;
+        }
         MSTP_Flag.TransmitPacketPending = true;
     }
 

bacnet-stack/ports/bdk-atxx4-mstp/dlmstp.c

@@ -1201,7 +1201,12 @@ int dlmstp_send_pdu(
             pkt->buffer[i] = pdu[i];
         }
         pkt->length = pdu_len;
-        pkt->destination_mac = dest->mac[0];
+        if (dest && dest->mac_len) {
+            pkt->destination_mac = dest->mac[0];
+        } else {
+            /* mac_len = 0 is a broadcast address */
+            pkt->destination_mac = MSTP_BROADCAST_ADDRESS;
+        }
         bytes_sent = pdu_len;
     }
 

bacnet-stack/ports/linux/dlmstp.c

@@ -161,7 +161,12 @@ int dlmstp_send_pdu(
             pkt->buffer[i] = pdu[i];
         }
         pkt->length = pdu_len;
-        pkt->destination_mac = dest->mac[0];
+        if (dest && dest->mac_len) {
+            pkt->destination_mac = dest->mac[0];
+        } else {
+            /* mac_len = 0 is a broadcast address */
+            pkt->destination_mac = MSTP_BROADCAST_ADDRESS;
+        }
         bytes_sent = pdu_len;
     }
 

bacnet-stack/ports/pic18f6720/dlmstp.c

@@ -105,10 +105,11 @@ int dlmstp_send_pdu(
             MSTP_Port.TxFrameType = FRAME_TYPE_BACNET_DATA_NOT_EXPECTING_REPLY;
 
         /* load destination MAC address */
-        if (dest && dest->mac_len == 1) {
+        if (dest && dest->mac_len) {
             MSTP_Port.TxDestination = dest->mac[0];
         } else {
-            return -2;
+            /* mac_len = 0 is a broadcast address */
+            MSTP_Port.TxDestination = MSTP_BROADCAST_ADDRESS;
         }
         dlmstp_get_my_address(&src);
         if ((MAX_HEADER + pdu_len) > MAX_MPDU) {

bacnet-stack/ports/rtos32/dlmstp.c

@@ -74,19 +74,17 @@ int dlmstp_send_pdu(
     unsigned mtu_len = 0;
 
     if (MSTP_Port.TxReady == false) {
-        if (npdu_data->confirmed_message)
+        if (npdu_data->confirmed_message) {
             MSTP_Port.TxFrameType = FRAME_TYPE_BACNET_DATA_EXPECTING_REPLY;
-        else
+        } else {
             MSTP_Port.TxFrameType = FRAME_TYPE_BACNET_DATA_NOT_EXPECTING_REPLY;
-
+        }
         /* load destination MAC address */
-        if (dest && dest->mac_len == 1) {
+        if (dest->mac_len) {
             destination = dest->mac[0];
         } else {
-#if PRINT_ENABLED
-            fprintf(stderr, "mstp: invalid destination MAC address!\n");
-#endif
-            return -2;
+            /* mac_len = 0 is a broadcast address */
+            destination = MSTP_BROADCAST_ADDRESS;
         }
         /* header len */
         mtu_len = MAX_HEADER - 2 /* data crc */ ;

bacnet-stack/ports/stm32f10x/dlmstp.c

@@ -1281,10 +1281,10 @@ int dlmstp_send_pdu(
             pkt->buffer[i] = pdu[i];
         }
         pkt->length = pdu_len;
-        if (dest->mac_len == 0) {
-            pkt->destination_mac = MSTP_BROADCAST_ADDRESS;
-        } else {
+        if (dest && dest->mac_len) {
             pkt->destination_mac = dest->mac[0];
+        } else {
+            pkt->destination_mac = MSTP_BROADCAST_ADDRESS;
         }
         bytes_sent = pdu_len;
     }

bacnet-stack/ports/win32/dlmstp-mm.c

@@ -291,10 +291,11 @@ uint16_t MSTP_Get_Send(
         return 0;
     }
     /* load destination MAC address */
-    if (Transmit_Packet.address.mac_len == 1) {
+    /* load destination MAC address */
+    if (Transmit_Packet.address.mac_len) {
         destination = Transmit_Packet.address.mac[0];
     } else {
-        return 0;
+        destination = MSTP_BROADCAST_ADDRESS;
     }
     if ((MAX_HEADER + Transmit_Packet.pdu_len) > MAX_MPDU) {
         return 0;

bacnet-stack/ports/win32/dlmstp.c

@@ -279,10 +279,10 @@ uint16_t MSTP_Get_Send(
         return 0;
     }
     /* load destination MAC address */
-    if (Transmit_Packet.address.mac_len == 1) {
+    if (Transmit_Packet.address.mac_len) {
         destination = Transmit_Packet.address.mac[0];
     } else {
-        return 0;
+        destination = MSTP_BROADCAST_ADDRESS;
     }
     if ((MAX_HEADER + Transmit_Packet.pdu_len) > MAX_MPDU) {
         return 0;