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

GM WORLDWIDE ENGINEERING STANDARDS 
GMW3110 
© Copyright 2010 General Motors All Rights Reserved 
February 2010 
Page 35 of 336 
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If the ReadMemoryByAddress ($23) service is supported in an ECU, then the DRE, the appropriate 
Service Operations engineer(s), and the appropriate Manufacturing engineer(s) must determine the maximum 
amount of memory required to be retrieved via a single request of this service and take this into consideration 
when determining the network layer buffer requirements. 
The actual buffer size shall be documented in a CTS, SSTS, or supplemental diagnostic specification 
referenced by either of the preceding documents. 
4.8.2 Buffer Requirements During SPS Programming. 
During SPS programming, an ECU shall maximize 
the size network layer buffer to the largest possible value while taking into account ECU RAM resources, the 
size of the programming algorithm to be downloaded, and the 4095 byte restriction provided by the USDT 
protocol. 
An ECU may have a different buffer size during SPS programming than it would during normal operations. The 
RequestDownload ($34) service is used during a programming event to tell an ECU that it is about to be 
programmed. An ECU can use the receipt of this diagnostic service to free up RAM resources for programming 
which would otherwise be used to support normal vehicle functionality. All ECUs shall perform a software reset 
at the conclusion of a programming event at which time the ECU would reinitialize RAM and once again begin 
using the allocated memory for normal operations. 
4.9 Diagnostic Message Sequence Examples. 
4.9.1 General USDT and UUDT Request/Response Sequence Examples. 
Figure 5 shows the general 
structure of a request/response scheme for GMLAN. In this example, the request message is a multi-frame 
USDT message and the response message is a single frame USDT message. Figure 5 and Table 24 illustrate 
how the sequence number embedded in each consecutive frame of a multi-frame message shall be treated.