diff --git a/CHANGELOG.md b/CHANGELOG.md
index 44f1d2c3..efcd4a14 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -29,6 +29,7 @@ The git repositories are hosted at the following sites:
 
 ### Fixed
 
+* Fixed NDPU comparison functions that were missing segment-ack PDU. (#991)
 * Fixed WriteProperty NULL bypass which is only for present-value property
   of commandable objects. (#984)
 * Fixed the ghost Device ID 0 in the I-Am response when the actual
diff --git a/ports/at91sam7s/dlmstp.c b/ports/at91sam7s/dlmstp.c
index 7460d439..fbd3d161 100644
--- a/ports/at91sam7s/dlmstp.c
+++ b/ports/at91sam7s/dlmstp.c
@@ -264,6 +264,7 @@ static bool dlmstp_compare_data_expecting_reply(const uint8_t *request_pdu,
             break;
         case PDU_TYPE_REJECT:
         case PDU_TYPE_ABORT:
+        case PDU_TYPE_SEGMENT_ACK:
             reply.invoke_id = reply_pdu[offset + 1];
             break;
         default:
@@ -274,7 +275,8 @@ static bool dlmstp_compare_data_expecting_reply(const uint8_t *request_pdu,
     }
     /* these services don't have service choice included */
     if ((reply.pdu_type != PDU_TYPE_REJECT) &&
-        (reply.pdu_type != PDU_TYPE_ABORT)) {
+        (reply.pdu_type != PDU_TYPE_ABORT) &&
+        (reply.pdu_type != PDU_TYPE_SEGMENT_ACK)) {
         if (request.service_choice != reply.service_choice) {
             return false;
         }
diff --git a/ports/bdk-atxx4-mstp/dlmstp.c b/ports/bdk-atxx4-mstp/dlmstp.c
index 673b684f..023a67c1 100644
--- a/ports/bdk-atxx4-mstp/dlmstp.c
+++ b/ports/bdk-atxx4-mstp/dlmstp.c
@@ -265,6 +265,7 @@ static bool dlmstp_compare_data_expecting_reply(const uint8_t *request_pdu,
             break;
         case PDU_TYPE_REJECT:
         case PDU_TYPE_ABORT:
+        case PDU_TYPE_SEGMENT_ACK:
             reply.invoke_id = reply_pdu[offset + 1];
             break;
         default:
@@ -275,7 +276,8 @@ static bool dlmstp_compare_data_expecting_reply(const uint8_t *request_pdu,
     }
     /* these services don't have service choice included */
     if ((reply.pdu_type != PDU_TYPE_REJECT) &&
-        (reply.pdu_type != PDU_TYPE_ABORT)) {
+        (reply.pdu_type != PDU_TYPE_ABORT) &&
+        (reply.pdu_type != PDU_TYPE_SEGMENT_ACK)) {
         if (request.service_choice != reply.service_choice) {
             return false;
         }
diff --git a/ports/bsd/dlmstp.c b/ports/bsd/dlmstp.c
index 36bfafa5..3534df0e 100644
--- a/ports/bsd/dlmstp.c
+++ b/ports/bsd/dlmstp.c
@@ -488,6 +488,7 @@ static bool dlmstp_compare_data_expecting_reply(
             break;
         case PDU_TYPE_REJECT:
         case PDU_TYPE_ABORT:
+        case PDU_TYPE_SEGMENT_ACK:
             reply.invoke_id = reply_pdu[offset + 1];
             break;
         default:
@@ -495,7 +496,8 @@ static bool dlmstp_compare_data_expecting_reply(
     }
     /* these don't have service choice included */
     if ((reply.pdu_type == PDU_TYPE_REJECT) ||
-        (reply.pdu_type == PDU_TYPE_ABORT)) {
+        (reply.pdu_type == PDU_TYPE_ABORT) ||
+        (reply.pdu_type == PDU_TYPE_SEGMENT_ACK)) {
         if (request.invoke_id != reply.invoke_id) {
             debug_printf("DLMSTP: DER Compare failed: "
                          "Invoke ID mismatch.\n");
diff --git a/ports/bsd/dlmstp_port.c b/ports/bsd/dlmstp_port.c
index 6697169a..910e5b1b 100644
--- a/ports/bsd/dlmstp_port.c
+++ b/ports/bsd/dlmstp_port.c
@@ -526,6 +526,7 @@ static bool dlmstp_compare_data_expecting_reply(
             break;
         case PDU_TYPE_REJECT:
         case PDU_TYPE_ABORT:
+        case PDU_TYPE_SEGMENT_ACK:
             reply.invoke_id = reply_pdu[offset + 1];
             break;
         default:
@@ -533,7 +534,8 @@ static bool dlmstp_compare_data_expecting_reply(
     }
     /* these don't have service choice included */
     if ((reply.pdu_type == PDU_TYPE_REJECT) ||
-        (reply.pdu_type == PDU_TYPE_ABORT)) {
+        (reply.pdu_type == PDU_TYPE_ABORT) ||
+        (reply.pdu_type == PDU_TYPE_SEGMENT_ACK)) {
         if (request.invoke_id != reply.invoke_id) {
             debug_printf("DLMSTP: DER Compare failed: "
                          "Invoke ID mismatch.\n");
diff --git a/ports/linux/dlmstp.c b/ports/linux/dlmstp.c
index a1783a17..a16bc547 100644
--- a/ports/linux/dlmstp.c
+++ b/ports/linux/dlmstp.c
@@ -489,6 +489,7 @@ static bool dlmstp_compare_data_expecting_reply(
             break;
         case PDU_TYPE_REJECT:
         case PDU_TYPE_ABORT:
+        case PDU_TYPE_SEGMENT_ACK:
             reply.invoke_id = reply_pdu[offset + 1];
             break;
         default:
@@ -496,7 +497,8 @@ static bool dlmstp_compare_data_expecting_reply(
     }
     /* these don't have service choice included */
     if ((reply.pdu_type == PDU_TYPE_REJECT) ||
-        (reply.pdu_type == PDU_TYPE_ABORT)) {
+        (reply.pdu_type == PDU_TYPE_ABORT) ||
+        (reply.pdu_type == PDU_TYPE_SEGMENT_ACK)) {
         if (request.invoke_id != reply.invoke_id) {
             debug_printf("DLMSTP: DER Compare failed: "
                          "Invoke ID mismatch.\n");
diff --git a/ports/linux/dlmstp_port.c b/ports/linux/dlmstp_port.c
index f1019988..c25a98fb 100644
--- a/ports/linux/dlmstp_port.c
+++ b/ports/linux/dlmstp_port.c
@@ -471,6 +471,7 @@ static bool dlmstp_compare_data_expecting_reply(
             break;
         case PDU_TYPE_REJECT:
         case PDU_TYPE_ABORT:
+        case PDU_TYPE_SEGMENT_ACK:
             reply.invoke_id = reply_pdu[offset + 1];
             break;
         default:
@@ -478,7 +479,8 @@ static bool dlmstp_compare_data_expecting_reply(
     }
     /* these don't have service choice included */
     if ((reply.pdu_type == PDU_TYPE_REJECT) ||
-        (reply.pdu_type == PDU_TYPE_ABORT)) {
+        (reply.pdu_type == PDU_TYPE_ABORT) ||
+        (reply.pdu_type == PDU_TYPE_SEGMENT_ACK)) {
         if (request.invoke_id != reply.invoke_id) {
             debug_printf("DLMSTP: DER Compare failed: "
                          "Invoke ID mismatch.\n");
diff --git a/ports/pic18f6720/mstp.c b/ports/pic18f6720/mstp.c
index a9dc75a1..a57f1aac 100644
--- a/ports/pic18f6720/mstp.c
+++ b/ports/pic18f6720/mstp.c
@@ -586,6 +586,7 @@ static bool mstp_compare_data_expecting_reply(uint8_t *request_pdu,
             break;
         case PDU_TYPE_REJECT:
         case PDU_TYPE_ABORT:
+        case PDU_TYPE_SEGMENT_ACK:
             reply.invoke_id = reply_pdu[offset + 1];
             break;
         default:
@@ -596,7 +597,8 @@ static bool mstp_compare_data_expecting_reply(uint8_t *request_pdu,
     }
     /* these services don't have service choice included */
     if ((reply.pdu_type != PDU_TYPE_REJECT) &&
-        (reply.pdu_type != PDU_TYPE_ABORT)) {
+        (reply.pdu_type != PDU_TYPE_ABORT) &&
+        (reply.pdu_type != PDU_TYPE_SEGMENT_ACK)) {
         if (request.service_choice != reply.service_choice) {
             return false;
         }
diff --git a/ports/pic18f97j60/mstp.c b/ports/pic18f97j60/mstp.c
index 373acfc7..2c71e2ed 100644
--- a/ports/pic18f97j60/mstp.c
+++ b/ports/pic18f97j60/mstp.c
@@ -594,6 +594,7 @@ static bool mstp_compare_data_expecting_reply(uint8_t *request_pdu,
             break;
         case PDU_TYPE_REJECT:
         case PDU_TYPE_ABORT:
+        case PDU_TYPE_SEGMENT_ACK:
             reply.invoke_id = reply_pdu[offset + 1];
             break;
         default:
@@ -604,7 +605,8 @@ static bool mstp_compare_data_expecting_reply(uint8_t *request_pdu,
     }
     /* these services don't have service choice included */
     if ((reply.pdu_type != PDU_TYPE_REJECT) &&
-        (reply.pdu_type != PDU_TYPE_ABORT)) {
+        (reply.pdu_type != PDU_TYPE_ABORT) &&
+        (reply.pdu_type != PDU_TYPE_SEGMENT_ACK)) {
         if (request.service_choice != reply.service_choice) {
             return false;
         }
diff --git a/ports/win32/dlmstp-mm.c b/ports/win32/dlmstp-mm.c
index 5cdb7ceb..3c816800 100644
--- a/ports/win32/dlmstp-mm.c
+++ b/ports/win32/dlmstp-mm.c
@@ -377,6 +377,7 @@ bool dlmstp_compare_data_expecting_reply(
             break;
         case PDU_TYPE_REJECT:
         case PDU_TYPE_ABORT:
+        case PDU_TYPE_SEGMENT_ACK:
             reply.invoke_id = reply_pdu[offset + 1];
             break;
         default:
@@ -384,7 +385,8 @@ bool dlmstp_compare_data_expecting_reply(
     }
     /* these don't have service choice included */
     if ((reply.pdu_type == PDU_TYPE_REJECT) ||
-        (reply.pdu_type == PDU_TYPE_ABORT)) {
+        (reply.pdu_type == PDU_TYPE_ABORT) ||
+        (reply.pdu_type == PDU_TYPE_SEGMENT_ACK)) {
         if (request.invoke_id != reply.invoke_id) {
             return false;
         }
diff --git a/ports/win32/dlmstp.c b/ports/win32/dlmstp.c
index 144396af..d8c0ae23 100644
--- a/ports/win32/dlmstp.c
+++ b/ports/win32/dlmstp.c
@@ -364,6 +364,7 @@ static bool dlmstp_compare_data_expecting_reply(
             break;
         case PDU_TYPE_REJECT:
         case PDU_TYPE_ABORT:
+        case PDU_TYPE_SEGMENT_ACK:
             reply.invoke_id = reply_pdu[offset + 1];
             break;
         default:
@@ -371,7 +372,8 @@ static bool dlmstp_compare_data_expecting_reply(
     }
     /* these don't have service choice included */
     if ((reply.pdu_type == PDU_TYPE_REJECT) ||
-        (reply.pdu_type == PDU_TYPE_ABORT)) {
+        (reply.pdu_type == PDU_TYPE_ABORT) ||
+        (reply.pdu_type == PDU_TYPE_SEGMENT_ACK)) {
         if (request.invoke_id != reply.invoke_id) {
             return false;
         }
diff --git a/src/bacnet/datalink/dlmstp.c b/src/bacnet/datalink/dlmstp.c
index 5af8458b..8852d40b 100644
--- a/src/bacnet/datalink/dlmstp.c
+++ b/src/bacnet/datalink/dlmstp.c
@@ -200,6 +200,7 @@ static bool MSTP_Compare_Data_Expecting_Reply(
             break;
         case PDU_TYPE_REJECT:
         case PDU_TYPE_ABORT:
+        case PDU_TYPE_SEGMENT_ACK:
             reply.invoke_id = reply_pdu[offset + 1];
             break;
         default:
@@ -207,7 +208,8 @@ static bool MSTP_Compare_Data_Expecting_Reply(
     }
     /* these don't have service choice included */
     if ((reply.pdu_type == PDU_TYPE_REJECT) ||
-        (reply.pdu_type == PDU_TYPE_ABORT)) {
+        (reply.pdu_type == PDU_TYPE_ABORT) ||
+        (reply.pdu_type == PDU_TYPE_SEGMENT_ACK)) {
         if (request.invoke_id != reply.invoke_id) {
             return false;
         }