diff --git a/bacnet-stack/ports/atmega168/hardware.ods b/bacnet-stack/ports/atmega168/hardware.ods
index a71bdf8f..ce38e622 100644
Binary files a/bacnet-stack/ports/atmega168/hardware.ods and b/bacnet-stack/ports/atmega168/hardware.ods differ
diff --git a/bacnet-stack/ports/pic18f6720/dlmstp.c b/bacnet-stack/ports/pic18f6720/dlmstp.c
index 433f15ed..82fbb7bc 100644
--- a/bacnet-stack/ports/pic18f6720/dlmstp.c
+++ b/bacnet-stack/ports/pic18f6720/dlmstp.c
@@ -112,7 +112,7 @@ int dlmstp_send_pdu(
             return -2;
         }
         dlmstp_get_my_address(&src);
-        if ((8 /* header len */  + pdu_len) > MAX_MPDU) {
+        if ((MAX_HEADER + pdu_len) > MAX_MPDU) {
             return -4;
         }
         bytes_sent =
diff --git a/bacnet-stack/ports/rtos32/dlmstp.c b/bacnet-stack/ports/rtos32/dlmstp.c
index 2c15977a..01b2cf5e 100644
--- a/bacnet-stack/ports/rtos32/dlmstp.c
+++ b/bacnet-stack/ports/rtos32/dlmstp.c
@@ -89,8 +89,8 @@ int dlmstp_send_pdu(
             return -2;
         }
         /* header len */
-        mtu_len = 8;
-        if ((mtu_len + pdu_len) > MAX_MPDU) {
+        mtu_len = MAX_HEADER - 2 /* data crc */;
+        if ((MAX_HEADER + pdu_len) > MAX_MPDU) {
 #if PRINT_ENABLED
             fprintf(stderr, "mstp: PDU is too big to send!\n");
 #endif