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Page 196
GM WORLDWIDE ENGINEERING STANDARDS
GMW3110
able to transmit the first ($81) UUDT response message (within the P2
C
* time window), each subsequent
UUDT response message(s) shall be transmitted within P2
C
.
If the ECU diagnostic application determines that the tester request is NOT acceptable (i.e., formatted
incorrectly), the ECU shall return an appropriate USDT-SF negative response within the P2
C 
time window
followed by NO ($81) UUDT message(s).
See Figure 33 for a graphical representation of the ECU UUDT timing requirements for this service.

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Figure 33: ECU UUDT Timing Parameter Definition for service $A9, $Level $81
8.18.1.3 Enabling the Node Resident DTC Count Algorithm. 
The tester can enable a node-resident DTC
count algorithm by requesting the $A9 service with the sub-parameter set to sendOnChangeDTCCount ($82).
If the requested mask value is not $00, the node shall enable a background algorithm that scans through the
list of supported DTCs to count the total number of DTCs that match the masking criteria. As soon as the
computation is complete, the node shall return a UUDT message back to the tester containing the 2-byte count
value. Thereafter, the node resident DTC count algorithm shall count the number of DTCs satisfying the tester
defined mask at a periodic rate of approximately 1 s (exact value to be documented in the component technical
specification). If the count is different from that which was calculated on the previous execution, the node shall
transmit a single UUDT message back to the tester with the updated 2-byte DTC count. The latest count shall
then be stored as a reference for the next calculation. Masking shall consist of a logical 
OR-ing 
operation and
shall only be performed on the status byte associated with each DTC/FailureType combination (unsupported
status bits are ignored).
Note: 
If the algorithm which calculates the number of DTCs cannot complete within the P2
C 
timing window, the
ECU shall respond with a $7F $A9 $78 RequestCorrectlyReceived-ResponsePending negative response to
institute P2
C
* timing for the pending UUDT positive response message. The ECU shall always send at least
one positive ($82) UUDT response message to the tester’s request. Subsequent ($82) UUDT responses will
only be returned if and when the background algorithm results in a different count.
A node shall support only one sendOnChangeDTCCount transmission arrangement at a time. Redefinition of
the sendOnChangeDTCCount status mask shall occur by sending another sendOnChangeDTCCount request
message with a different status mask. A UUDT message with a DTC count of $00 00 shall be returned in
response to a request in which the status mask = $00, and the receiving ECU shall halt all background
sendOnChangeDTCCount computations.
Once a sendOnChangeDTCCount request message has been sent, the tester must periodically transmit
TesterPresent ($3E) messages to keep this level of the service active within the node. If a P3
C 
(TesterPresent)
timeout occurs, the node shall deactivate sendOnChangeDTCCount reporting. The node shall also deactivate
this level if the tester transmits a ReturnToNormalMode ($20) service request, or the node is powered down. In
each of the three cases described, the node is NOT required to transmit any $A9 specific response message
to inform the tester that the send-on-change algorithm has been deactivated.
If a code clear request is received while this level is active, the send-on-change DTC count checking shall
remain active. To lessen the burden on the ECU, the send-on-change DTC count algorithm may be temporarily