Why positive response is always plus 40 and negative response is 7F in UDS,KWP diagnostic protocol in automotive?

Question

i want logical reason why positive response is +40 and negative response is 7F in UDS i tried some diagnostic services but getting always +40 and 7F but not getting why always it is increment by +40

Answer 1

In UDS the first byte of a positive response is always SID + 40 hex (SID = Service Identifier, the first byte of the request) and the first byte of a negative response is always 7F hex: simply because it is defined like this in the international standards ISO14229-1 (former ISO 15765-3). There is no further logical reason - theoretically other proprietary diagnostic protocols could use different offset values to indicate positive/negative responses: It's an arbitrary choice when designing a diagnostic protocol.

But historically using 40 hex as offset for positive responses has a long tradition in diagnostics; used i.e. in KWP2000 (ISO 14230) and OBD (SAE J1979 / ISO 15031) protocols respectively.

Using a single bit (Bit 6) to differentiate between requests and responses has certain advantages; it is trivial to distinguish between them for any application which needs to interpret the data; If Bit 6 is set the byte needs to be checked only against the constant value 7F hex to know if the response is positive or negative.