Unified Diagnostic Services (UDS)

Unified Diagnostic Services (UDS) specifies data link independent requirements of automotive diagnostic services in road vehicles. UDS is codified in ISO 142291:2013 and allows diagnostics to control functions on an invehicle Electronic Control Unit (ECU). Typical functions ECUs control are electronic fuel injection(EFI), automatic gear box, antilock braking system, etc. all connected to a serial data link embedded in a road vehicle. The diagnostic communication over controller area network (DoCAN) is specified in ISO 157653 and ISO 142293.

What is Vehicle Diagnostics ?

• Modern vehicles are packed full of modules, all continuously  monitoring themselves and reporting their status.
• Detecting a failure in this complex machine would be a tedious task. However, most of the vehicles today include computers (Electronic Control Unit (ECU)), which monitors several sensors, located throughout the vehicle.
• When the computer system of the car detects a fault, two things are supposed to happen/monitored.
• First, a warning light on the dashboard (MIL – Malfunction Indication Light) is set, to inform the driver that a problem exists.
• Second the code is recorded in the computer’s memory (EEPROM) so that it can later be retrieved by a technician for diagnosis and repair.
• Diagnostics, as the word suggests, is to identify the cause of a problem or a situation. Whenever the ECU finds a problem, it stores that problem as a Diagnostics Trouble Code (DTC) in the ElectricallyErasableProgrammableRead-OnlyMemory (EEPROM) for later retrieval.
• A diagnostic equipment allows you to diagnose and fix the problem with the vehicle. Diagnostic Tools are used to read data (DTC’s) from the EEPROM to analyze the cause of failure.
• Such an equipment will communicate with the vehicle and for this, it requires basically a communication medium and a communication protocol.

ECU – Electronic Control Unit

Diagnostic Protocols

• KWP2000 – Keyword Protocol 2000 – used for on-board vehicle diagnostics systems (OBD)
• J1939 –Data protocol for heavy duty commercial vehicles
• GMLAN – GM Specific Diagnostic Protocol
• UDS – Unified Diagnostic Services

UDS ISO 14229

1. UDS (Unified Diagnostic Service) is based on the standards KWP2000 for K-line and CAN. It was developed with the goal of standardizing different implementations of the predecessor standards and new requirements stemming from further developments in technology and new standards to form one generally valid diagnostic protocol.
2. UDS describes the layer 7 protocol (diagnostic services) but is based on an extended version of the diagnostic protocol of KWP2000 on CAN (ISO 15765-2).
3. UDS is currently being used widely areas of the automobile industry.

Implementation of UDS on CAN in OSI model

ISO 14229-1 & 15765-2

• ISO 14229-1 has been established in order to define common requirements for diagnostic systems, whatever the serial data link is.
• ISO 15765-2, or ISO-TP is an international standard for sending data packets over a CANBus.
  – The protocol allows for the transport of messages that exceed the eight byte maximum payload of CAN frames.
  – ISO-TP segments longer messages into multiple frames, adding metadata that allows the interpretation of individual frames and reassembly into a complete message packet by the recipient.
  – It can carry up to 4095 bytes of payload per message packet.

Basic Principle

The request / response communication principle is used.

1. The diagnostic tester sends a request, the request to one or more control devices.
2. Then the control unit sends a positive or negative response.
3. Within the request message is the first byte, called a service identifier to distinguish the various diagnostic services and the tasks to be done.
4. The remaining bytes contain sub-level identifiers, parameters and data.

Basic Terms

1. Diagnostic Service: information exchange initiated by a client in order to require diagnostic information from a server and/or to modify its behavior for diagnostic purposes.
2. Service identifier (SI): Each request service shall be assigned a unique SI value. Each positive response service shall be assigned a corresponding unique SI value. All request messages have SI bit 6 = 0. All positive response messages have SI bit 6 = 1
3. Addressing mode: For communicating with the ECU, the diagnostic tool uses either Physical addressing or Functional addressing method.
   – Physical addressing is the kind of addressing where the Diagnostics tool communicates with a single ECU.
   – Functional addressing is where the Diagnostics tool communicates with multiple ECUs.

Service identifier (SI) values

• There is a one-to-one correspondence between service identifiers for request messages and service identifiers for positive response messages, with bit 6 of the SI hex value indicating the service type.
• All request messages have SI bit 6 = 0.
• All positive response messages have SI bit 6 = 1

Sub-Function of a Service

List of Complete Services

Diagnostic & Communication Management Functional Unit