Frame transport
ISO-TP segments diagnostic messages that exceed one CAN frame into first, consecutive and flow-control frames.
ISO-TP / Communication network
ISO 15765 CAN DIAGNOSTICS
OBD-II diagnostic transport over CAN
NETWORK GUIDE09/14
TOPOLOGYCAN physical layer carrying segmented diagnostic messages
PHYSICALCAN High and CAN Low
DATA RATETypically 250 or 500 kbit/s
Correct theory and operation
HOW ISO-TP
COMMUNICATES.
Understand the normal architecture, signaling and message rules before deciding that a voltage or waveform is incorrect.
Tester and ECU roles
The tester requests data and the ECU controls response pacing with flow-control parameters.
Common mistakes
AVOID THE
FALSE CLUE.
- Testing with the connector unplugged when the fault only appears under load.
- Using a generic voltage value instead of the vehicle-specific specification.
- Skipping power, ground and reference checks before condemning the component.
- Moving the harness or clearing codes before capturing the original evidence.
- Replacing a part without repeating the test.
Reading the signal
Good waveform / normal communication
On a healthy CAN physical layer, ISO-TP requests, flow-control frames and segmented responses arrive in the required sequence and timing. Confirm the correct addresses and diagnostic session.
Faulty waveform / fault indications
Missing flow control, sequence errors or timeouts can prevent a diagnostic exchange even when the bus voltage looks normal. Separate a physical CAN fault from transport addressing, timing or module-session issues.
COMPLETE TEST SEQUENCE
- Record addressing and session state.
- Confirm CAN physical-layer health.
- Capture a complete request and response sequence.
- Check sequence numbers and flow-control timing.
Confirm the network, topology, awake state and test conditions from vehicle-specific information.