> [!warning] > You are viewing the documentation for an unreleased product. # Transport Protocol The firmware update process uses J1939 Transport Protocol to send the firmware in packets of up to 7 bytes and multiple transfers of up to 1024 bytes. > [!info] > The PDM does have the ability to receive CAN FD frames with up to 64 bytes payloads, but this is not supported by J1939 and so not implemented in our firmware. ## Request to Send At the start of each 1kB transfer, the RTS (request to send) message is sent to the PDM informing it of the transfer parameters. If the parameters are invalid, the PDM will respond with a connection abort message instead of the usual CTS (clear to send). See J1939™-21 section 5.10.3.1 for more details. ## Clear to Send After each RTS or DT (data transfer) message, the PDM will respond with a CTS message unless the transfer is complete and as such with the end of message. See J1939™-21 section 5.10.3.2 for more details. ## End of Message Sent by the PDM to acknowledge that the transfer is complete. See J1939™-21 section 5.10.3.3 for more details. ## Connection Abort If any issue arrises with the transfer a connection abort message will be sent by the PDM. Upon receiving this message, the host will need to restart the transfer. See J1939™-21 section 5.10.3.4 for more details. ## Data Transfer Each data transfer packet contains a sequence number and up to 7 bytes of data. The host may See J1939™-21 section 5.10.4 for more details.