USDT and UUDT diagnostic responses in CAN define a method of transmitting diagnostic data where large amounts of diagnostic information is divided into smaller segments or frames for transmission.
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What are USDT and UUDT diagnostic responses?
USDT and UUDT diagnostic responses are the approaches taken while responding to a CAN diagnostic request. These are explained in detail here:
USDT Response
If a Diagnostic CAN message response does not fit in a single CAN frame (more than 8 bytes of data), it is split into two or more parts. Such Diagnostic responses are called Unacknowledged Segmented Data Transfer (USDT) responses. This process of Diagnostic communication on CAN with Data Segmentation is called USDT. It is provided by the OSI model Transport and Network layer. It is the default process.
Note: There is no data segmentation for Onboard communication.
USDT does not automatically mean a message does not fit in a single CAN frame. PCI (Protocol Control Information) byte is required with USDT. The PCI byte is the first byte of the CAN data field and controls the data segmentation.
UUDT Response
To optimize data transfer with CAN frames, the PCI byte can be removed from the Diagnostic response, which saves an additional byte of data. This type of data transfer is called Unacknowledged Unsegmented Data Transfer (UUDT).
If UDS Diagnostic protocol is employed, the vehicle manufacturers (OEMs) and suppliers must agree on additional Physical response CAN-IDs for UUDT, also known as UUDT response CAN-identifiers.
Examples of USDT and UUDT diagnostic responses
In the example below, the tester sends a physical address CAN-ID request to the TCM (Transmission Control Module). TCM then responds with a UUDT response.
USDT CAN-IDs for ECM and TCM would be $7E8 and $7E9 respectively. Similarly, UUDT CAN-IDs for ECM and TCM would be $6E8 and $6E9 respectively.
Note: The priority of CAN-ID $6E9 (UUDT) is higher than that of $7E9 (USDT).
Functional request with USDT Response
A maximum of 7 bytes of data is transmitted as 1 byte is occupied by a PCI byte.
Physical request with USDT Response
Increased bus load as four diagnostic messages have to be sent, i.e., $7E0 -> $7E8 and $7E1 -> $7E9.
Physical request with UUDT Response
A maximum of 8 bytes as the PCI byte can be omitted in case of UUDT response.
Conclusion
The choice between USDT and UUDT approaches would depend on factors such as the size of the diagnostic data payload, the available bandwidth on the CAN bus, and the specific requirements of the diagnostic application. Both methods aim to efficiently transmit diagnostic information over the CAN network while considering speed, reliability, and resource utilization.
