send bytes to the message

Author: Christian-B

spinn_front_end_common/utility_models/data_speed_up_packet_gatherer_machine_vertex.py

@@ -1243,7 +1243,7 @@ def _determine_and_retransmit_missing_seq_nums(
 
             # build SDP message and send it to the core
             connection.send_sdp_message(self.__make_data_out_message(
-                placement, data))
+                placement, bytes(data)))
 
             # sleep for ensuring core doesn't lose packets
             time.sleep(self._TIMEOUT_FOR_SENDING_IN_SECONDS)