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8.20.1 Service Description.
Device control is performed by manipulating predefined bits and/or bytes within a
message to indicate to the device which output(s) or control function(s) the tool wants to override. When a
CPID is defined with multiple output controls, the ECU shall support simultaneous control of multiple outputs
contained in that CPID as part of a single diagnostic request.
Note:
Certain combinations of outputs may not be allowed to be controlled at the same time due to safety or to
avoid product damage. Any such restrictions shall be negotiated between the DRE, service and manufacturing
engineering and documented in a CTS, SSTS, or supplemental diagnostic specification referenced by either of
the preceding documents.
Each device control message shall fit in a single frame and as such contains a maximum of six data bytes after
the service Identifier byte. The first data byte (after the service identifier) of a device control request message
is a Control Packet Identifier (CPID). The remaining five bytes are used to provide output control information.
However, it is not required that a CPID contain five bytes of control information. CPIDs are used to distinguish
between different output control packets for a given device. Valid CPID numbers can range from $01 thru $FE.
Corporate common CPIDs shall be defined starting at CPID $FE and numbered sequentially backwards (see
Appendix B for a list of corporate common CPIDs). A $00 in place of the CPID number in the request message
is used to completely cancel all active device controls. If a device supports multiple CPIDs, then receipt of a
second device control request with a different CPID number than that of a previous request, shall not result in
any change to the functions requested with the previous device control request.
The DeviceControl service shall be given the highest priority in the devices output control logic unless control
of the given output could result in vehicle damage, or there is potential risk to the vehicles occupants. If vehicle
damage or safety related concerns exist, DeviceControl limitations shall be built into the device’s software.
DeviceControl limitations are automatically active upon the receipt of an $AE message. If the device detects
that a DeviceControl limitation has been exceeded, all active DeviceControls shall be terminated and the
device shall send a $7F $AE $E3 $xx $yy (DeviceControlLimitsExceeded) reject message. The
DeviceControlLimitsExceeded reject message may be sent in response to a DeviceControl request message
or as an unsolicited response any time the application software detects that a limit has been exceeded. The
$xx $yy portion of the reject message shall contain an application specific state encoded value which will allow
the tool to determine which device control limit was exceeded. The master list of values for $xx $yy is
maintained on the GM Service and Parts Operations web page. The values for the device control limits
exceeded return codes implemented for a specific ECU must follow the master list and shall be documented in
the ECU CTS, SSTS, or supplemental diagnostic specification referenced by either of the preceeding
documents.
Note:
If a device control limit is exceeded which warrants an unsolicited negative response message and the
ECU is in process of receiving or transmitting a multi-frame USDT diagnostic message, then the unsolicited
negative response message shall be delayed until after the multi-frame message (reception or transmission)
has completed.
The ECU shall support reactivation of device controls (provided that the additional requests do not result in
restrictions being exceeded) after a device control restriction has been exceeded without requiring
extraordinary measures such as a key cycle or cycling of power. Any additional limitations imposed on a
device control after being initially exceeded (that are beyond those that caused the device control to be initially
exceeded) shall be agreed upon by the DRE, service and manufacturing engineering and shall be documented
in a CTS, SSTS, or supplemental diagnostic specification referenced by either of the preceeding documents.
Certain DeviceControl restrictions may be bypassed for the purposes of vehicle manufacturing testing. A tester
can bypass device control restrictions via the SecurityAccess ($27) service using the sub-parameters assigned
for device control limitations.
Note:
Security Access levels used for device control restrictions overrides are different than those for SPS
security. The SPS security levels are used for reprogramming and with secure CPIDs. The device control
security levels are used to accommodate manufacturing device control overrides. See pseudo code of this
service for more detail.
All device control limitations supported by an ECU along with the list of restrictions which can be bypassed in
manufacturing, must be documented in a CTS, SSTS, or a supplemental diagnostic specification referenced by
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WORLDWIDE ENGINEERING STANDARDS General Motors Local Area Network Enhanced Diagnostic Test Mode Specification
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