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Diagnostic services have a common message format. Each service defines a Request Message, a Positive Response Message, and a Negative Response Message. The general format of the diagnostic services complies with the KWP2000 definition; most of the Service Ids also comply with KWP2000. The Request Message has the ServiceId as first byte, plus additional service-defined parameters. The Positive Response Message has an echo of the ServiceId with bit 6 set as first byte, plus the service-defined response parameters.
|Note Some parameters to both the Request and Positive Response Messages are optional. Each service defines these parameters. Also, the standard does not define all parameters.|
The Negative Response Message is usually a three-byte message: it has the Negative Response ServiceId (0x7F) as first byte, an echo of the original ServiceId as second byte, and a ResponseCode as third byte. The UDS standard partly defines the ResponseCodes, but there is room left for manufacturer-specific extensions. For some of the ResponseCodes, UDS defines an error handling procedure.
Because both positive and negative responses have an echo of the requested service, you always can assign the responses to their corresponding request.