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

9.3.2.4.4 Service $27 - Security Access. 
This service shall be supported in the boot software if the ECU is 
theft, emission, safety related, or if the ECU contains functionality which requires the use of this service to 
meet legal requirements. If implemented, the minimum sub-function levels required are: 
1. Level $01 
– SPSrequestSeed. 
2. Level $02 - SPSsendKey - This sub-function is only required for ECUs that require this service and are 
sent to the field as an SPS_TYPE_B or SPS_TYPE_C ECU. 
Note:
An ECU that is sent to the field with operational software and a dummy calibration (fully programmed but 
not completely functional) is considered to be an SPS_TYPE_A ECU. In this case it is assumed that the 
dummy calibration would activate the security feature and it would not be necessary to support the Level $02 
in the boot software. 
9.3.2.4.4.1 Pseudo Code. 
(Refer to the data dictionary of the application implementation of this service for 
variable definitions.)
 
BEGINFUNCTION Boot_27_Msg_Recvd() 
IF ($Level = $01) THEN 
Send ($67 $01 $00 $00) message 
ENDIF 
ENDFUNCTION 
Note: 
The above pseudo code assumes that the ECU is unlocked when operating out of the boot software. 
9.3.2.4.5 Service $28 - Disable Normal Message Transmission. 
This mode is required to be supported to 
allow a common utility file for Service and Manufacturing, and to assure no conflicts between programming 
messages and normal communication messages.

An SPS_TYPE_C ECU which only has knowledge of its SPS_Prime CAN Identifiers does not respond 
unless the SPS_Prime CAN Identifiers have already been enabled. Refer to paragraph 9.1.1 - Enabling 
Diagnostic Responses on SPS_TYPE_C ECUs for more detail. 
9.3.2.4.5.1 Pseudo Code. 
(Refer to the data dictionary of the application implementation of this service for 
variable definitions.)
 
BEGINFUNCTION Boot _28_Msg_Recvd() 
Service_28_MsgReceived 

YES 
TesterPresent_Timer_State 

ACTIVE 
IF (diagnostic_responses_enabled = YES) THEN
Send ($68) response message 
ENDIF 
ENDFUNCTION