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

($Level) Definition. 
A sub-function parameter is 
used when a diagnostic service supports multiple 
levels of functionality. The sub-function parameter 
tells the diagnostic application which functionality is 
being requested. A sub-function parameter is used 
in a tester to ECU request message and when 
used, shall always occupy the first byte after the 
Service Identifier. 
The $Level designation is included in the heading 
to indicate that the pseudo code shall reference the 
sub-function parameter using $Level as a variable. 
3.2 
Request 
Message 
Data 
Parameter 
Definition. 
A request message data parameter is 
used to indicate to the ECU diagnostic application 
which specific information is being requested (e.g., 
specific ECU input or output status variables, 
calculated variables, etc.) A request message data 
parameter can affect the data contained in a 
response message but it does not affect the 
functionality of the diagnostic service used to 
retrieve it. 
Example:
If a diagnostic service is used to provide 
ECU input statuses, output statuses, etc., then a 
data parameter would be passed in with the 
diagnostic request message to identify which 
inputs, outputs, etc., that the tester is requesting. A 
sub-function parameter would be contained in the 
request message for this service to tell the 
diagnostic application if the data is to be sent as a 
one time response, sent periodically, or to stop 
sending previously requested data. 
3.3 Positive Response Message Data Parameter 
Definition. 
A data parameter in a positive 
message is used to provide the information to the 
tester that was requested via a diagnostic request 
message.