General Specification Electrical Function gmw3110 General Motors Local Area Network Enhanced Diagnostic Test Mode Specification

4.4.2 CAN Identifier Memory Map Model. 
The following requirements apply to the GMLAN diagnostic CAN 
Identifiers used by all ECUs with the exception of any emission related ECU(s) that have opted to use their 
OBD/EOBD CAN Identifiers to support enhanced diagnostics (refer to paragraph 4.4.4.1 for rules regarding the 
optional assignment of OBD/EOBD CAN Identifiers by emissions related ECUs for enhanced diagnostics): 

The physical request CAN Identifiers shall be higher priority than the UUDT response CAN Identifiers to be 
able to shut down a node which is sending periodic messages. 

The UUDT response CAN Identifiers shall be higher priority than the USDT response CAN Identifiers to 
allow the periodic messages to be sent prior to a multiple frame response message which is transmitted to 
the tester. 

Since the priority of the CANId is inversely proportional to its numerical value (i.e., the lower the CANId 
number, the higher the priority), the following is true: 
USDT_RequestCANId < UUDT_ResponseCANId < USDT_ResponseCANId 
Figure 2 gives an overview of the reserved GMLAN diagnostic CAN Identifier ranges to provide a better 
understanding of where the GMLAN diagnostic CAN Identifiers are located. It also shows some of the GMLAN 
CAN Identifiers reserved for normal communication, such as Virtual Network Management Frames (VNMF). 
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GMW3110 
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February 2010 
Page 24 of 336 
Figure 2: CAN Identifier Memory Map Model 
4.4.3 CAN Identifier Table. 
Table 8
 
defines the ranges for the GMLAN diagnostic CAN Identifiers to be 
reserved for each GMLAN sub-network.