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

8.18.8 Tester Implications. 
The send-on-change status mask and DTC count information are lost upon a $3E 
time-out, after a $20 service request, or after power is cycled. 
The tester shall take into account the execution rate of the background send on change DTC algorithm when 
determining how long to wait for the UUDT response to a $Level $82 request. 
The test device shall use the endOfDTCReport message to determine when all UUDT responses to a $Level 
$81 request have been sent. For a given node, transmitting another $Level $81 request prior to receiving the 
endOfDTCReport message may result in undefined operation. Refer to the $A9 $Level $81 Service 
Description section for more timing details. 
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GM WORLDWIDE ENGINEERING STANDARDS 
GMW3110 
© Copyright 2010 General Motors All Rights Reserved 
February 2010 
Page 211 of 336 
8.19 ReadDataByPacketIdentifier ($AA) Service. 
The purpose of the ReadDataByPacketIdentifier($AA) 
service is to allow a tester to request data packets that contain diagnostic information (e.g., sensor input or 
output values) which are packaged in a UUDT diagnostic message format. Refer to paragraph 4.5.1.2 for more 
information on UUDT diagnostic message format. Each diagnostic data packet includes one byte that contains 
a Data Packet IDentifier (DPID) number, and one to seven bytes of additional data. The DPID number 
occupies the message number byte position of the UUDT diagnostic response message and is used by the 
tester to determine the data contents of the remaining bytes of the message. 
This service is intended to be used to retrieve ECU data which is most likely changing during normal operation 
(e.g., ECU sensor inputs, ECU commanded output states, etc). Static information such as VIN or Part 
Numbers should be retrieved via the ReadDataByIdentifier ($1A) service. 
The DPIDs requested via this service can be sent as a one-time response or scheduled periodically. Each 
DPID scheduled can be transmitted at one of three predefined periodic rates (slow, medium, or fast). Periodic 
rates require a TesterPresent ($3E) message to be sent on the bus to keep the Periodic DPID Scheduler 
(PDS) active (reference $3E service description).