National Instruments CAN User Manual page 197

Description
For the VW TP 2.0, you must establish a connection to the ECU before any diagnostic
communication can occur. This function sets up a unique communication channel to an ECU
that you can use in subsequent diagnostic service requests.
You must maintain the communication link thus created by periodically (at least once a
second) calling
No equivalent exists for the ISO TP (ISO 15765-2), as the ISO TP does not use a special
communication link.
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ndVWTPConnectionTest
Chapter 6 Automotive Diagnostic Command Set API for C
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