CAN Bus vs. UDS Protocol
Comparing 2 standards across 13 specifications. 13 key differences identified.
Controller Area Network (CAN Bus, invented by Robert Bosch GmbH in 1986 and standardized under ISO 11898) is the multi-master, message-broadcast serial bus governing electronic control units (ECUs). Operating at 500 kbps (High-Speed CAN) over a twisted pair with 120-ohm termination resistors, utilizing differential voltage signaling (CAN_H / CAN_L) and non-destructive bitwise arbitration.
Unified Diagnostic Services (UDS, standardized under ISO 14229) is the international application-layer diagnostic communication protocol used across nearly all modern automotive electronic control units (ECUs). Defining standardized diagnostic request/response service IDs (SIDs) for reading Diagnostic Trouble Codes (DTCs), reading real-time live sensor parameters, running actuator self-tests, and cryptographic ECU security access.
Specification Comparison Table
13 Parameters| Parameter / Spec | ||
|---|---|---|
|
Standardization
|
ISO 11898-2 (High-Speed physical medium) / SAE J2284 | ISO 14229-1 (UDS Application Layer) / ISO 14229-2 (Session Management) |
|
Transmission Speed
|
500 kbps standard for powertrain (Engine, Transmission, ABS, Airbag); 125-250 kbps for body electronics | — |
|
Differential Voltage Levels
|
Recessive state (Logic 1): CAN_H = 2.5V, CAN_L = 2.5V (Diff = 0V) | Dominant state (Logic 0): CAN_H = 3.5V, CAN_L = 1.5V (Diff = 2.0V) | — |
|
Bus Termination
|
Exactly two 120 Ω terminating resistors at opposite physical ends of the bus (total measured resistance between CAN_H & CAN_L = 60.0 Ω) | — |
|
Frame Format (CAN 2.0B)
|
11-bit standard identifier or 29-bit extended identifier with up to 8 data payload bytes | — |
|
Bitwise Arbitration
|
Dominant zero bits overwrite recessive one bits; the lowest numerical CAN ID has the highest priority and never loses arbitration | — |
|
Fault Confinement
|
Hardware error counters automatically transition faulty nodes from Error-Active -> Error-Passive -> Bus-Off isolation | — |
|
Diagnostic Session Control (SID 0x10)
|
— | 0x01: Default Session | 0x02: Programming Session (Flashing) | 0x03: Extended Diagnostic Session |
|
Security Access (SID 0x27)
|
— | Challenge-response seed & key cryptographic authentication unlocking protected ECU coding and calibration memory |
|
Read Data By Identifier (SID 0x22)
|
— | Queries 2-byte Data Identifiers (DIDs, e.g. coolant temp, fuel pressure, engine RPM, battery state-of-charge) |
|
Read DTC Information (SID 0x19)
|
— | Reads stored, pending, and permanent 3-byte Diagnostic Trouble Codes with freeze-frame environmental data |
|
Clear Diagnostic Info (SID 0x14)
|
— | Erases stored DTCs, freeze frames, and resets emissions monitor readiness flags |
|
Negative Response Code (NRC 0x7F)
|
— | Standardized error code returned when service is rejected (e.g. 0x33 Security Denied, 0x31 Request Out Of Range) |