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

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GMW3110 
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February 2010 
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8.18.5.3 Example #3 - Send on Change Reporting of DTC Information. 
This messaging example 
(Table 186) shows how the tester can request DTC count information to be sent using a send-on-change 
message transmission model. The ECM will return the current number of DTCs that satisfy the masking criteria 
as soon as the tester
’s request is received. The ECM will then return fresh count information as soon as the 
node-resident periodic DTC scanning algorithm detects any change in the number of DTCs matching the 
masking criteria. 
Note:
The tester must periodically transmit service $3E TesterPresent messages to keep the send-on-change 
model active. 
As was mentioned above, the masking algorithm consists of a logical OR-ing operation between actual DTC 
statuses and the tester defined filtering mask. 

This example shows that there were four DTCs present in the ECM that matched the status mask $02 at 
the time of the tester
’s request (status mask = $02 corresponds to current DTCs). 

At some time later, the ECM returns an updated DTC count indicating that another DTC has satisfied the 
$02 filtering criteria, bringing the total number of matches to $00 05. (Aside: An off-board tester could use 
this message from the node to trigger a message $81 request for DTC information by status mask to 
determine which DTC(s) actually set). 

Note that the count is not monotonically increasing (i.e., If it is determined by a node that fewer DTCs 
currently match the masking criteria than that which was calculated previously, then the DTC count will 
decrement). This can be seen below as the count drops from $00 05 back down to $00 04. 

This example also shows how to terminate the send-on-change session by defining a status mask of $00. 
After positively acknowledging this request with a UUDT message containing a DTC count of $00 00, the 
ECM will halt all subsequent computation/transmission of send-on-change DTC count information.