diff --git a/src/main/python/main/ayab/engine/communication_mock.py b/src/main/python/main/ayab/engine/communication_mock.py
index 837f132e..f90415a5 100644
--- a/src/main/python/main/ayab/engine/communication_mock.py
+++ b/src/main/python/main/ayab/engine/communication_mock.py
@@ -22,7 +22,6 @@
 
 import logging
 from time import sleep
-from collections import deque
 
 from PySide6.QtWidgets import QMessageBox
 
@@ -47,8 +46,9 @@ def __del__(self) -> None:
     def reset(self):
         self.__is_open = False
         self.__is_started = False
-        self.rx_msg_list = deque([], maxlen=100)
+        self.rx_msg_list = list()
         self.__line_count = 0
+        self.__started_row = False
 
     def is_open(self) -> bool:
         """Return status of the interface."""
@@ -109,29 +109,36 @@ def cnf_line_API6(self, line_number, color, flags, line_data) -> bool:
         """Send a row of stitch data."""
         return True
 
-    def update_API6(self) -> tuple[bytes, Token, int]:
+    def update_API6(self) -> tuple[bytes | None, Token, int]:
         """Read and parse data packet."""
         if self.__is_open and self.__is_started:
-            reqLine = bytes([Token.reqLine.value, self.__line_count])
-            self.__line_count += 1
-            self.__line_count %= 256
-            self.rx_msg_list.append(reqLine)
-            if self.__delay:
-                sleep(1)  # wait for knitting progress dialog to update
-            # step through output line by line
-            if self.__step:
-                # pop up box waits for user input before moving on to next line
-                msg = QMessageBox()
-                msg.setIcon(QMessageBox.Icon.Information)
-                msg.setText("Line number = " + str(self.__line_count))
-                msg.setStandardButtons(
-                    QMessageBox.StandardButton.Ok | QMessageBox.StandardButton.Cancel
-                )
-                ret = None
-                ret = msg.exec_()
-                while ret is None:
-                    pass
+            # Alternate between reqLine and no message
+            # (so that the UI makes the end-of-line sound for each row)
+            if self.__started_row:
+                self.__started_row = False
+                reqLine = bytes([Token.reqLine.value, self.__line_count])
+                self.__line_count += 1
+                self.__line_count %= 256
+                self.rx_msg_list.append(reqLine)
+                if self.__delay:
+                    sleep(1)  # wait for knitting progress dialog to update
+                # step through output line by line
+                if self.__step:
+                    # pop up box waits for user input before moving on to next line
+                    msg = QMessageBox()
+                    msg.setIcon(QMessageBox.Icon.Information)
+                    msg.setText("Line number = " + str(self.__line_count))
+                    msg.setStandardButtons(
+                        QMessageBox.StandardButton.Ok
+                        | QMessageBox.StandardButton.Cancel
+                    )
+                    ret = None
+                    ret = msg.exec_()
+                    while ret is None:
+                        pass
+            else:
+                self.__started_row = True
         if len(self.rx_msg_list) > 0:
-            return self.parse_API6(self.rx_msg_list.popleft())  # FIFO
+            return self.parse_API6(self.rx_msg_list.pop(0))  # FIFO
         # else
         return self.parse_API6(None)
diff --git a/src/main/python/main/ayab/tests/test_communication_mock.py b/src/main/python/main/ayab/tests/test_communication_mock.py
index 8bee5a86..452efc6b 100644
--- a/src/main/python/main/ayab/tests/test_communication_mock.py
+++ b/src/main/python/main/ayab/tests/test_communication_mock.py
@@ -107,6 +107,9 @@ def test_req_line_API6(self):
         )
         self.comm_dummy.update_API6()  # cnfStart
 
+        # Alternates between requesting a line and no output
         for i in range(0, 256):
             bytes_read = self.comm_dummy.update_API6()
             assert bytes_read == (bytearray([Token.reqLine.value, i]), Token.reqLine, i)
+            bytes_read = self.comm_dummy.update_API6()
+            assert bytes_read == (None, Token.none, 0)