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

8.9 DisableNormalCommunication ($28) Service. 
The purpose of this service is to prevent a device from 
transmitting or receiving all messages which are not the direct result of a diagnostic request. The primary use 
of the service is to set up a programming event. This is a required service that must be supported by all nodes. 
8.9.1 Service Description. 
When this service is activated, a node shall cease transmission of all normal mode 
(non-diagnostic) messages. Normal messages are halted by invoking a handler function(s) which performs the 
following tasks: 
1. Inhibits the transmission of all non-diagnostic signals/messages. Non-diagnostic frames already loaded 
into the protocol device do not have to be flushed, but no additional non-diagnostic frames shall be loaded. 
2. De-queue all queued messages and inhibit further queuing of non-diagnostic messages. 
3. Disable the processing of all received messages except for those corresponding to the specific diagnostic 
CAN Identifiers supported by the ECU. 
A node shall not send out any normal communication frames after sending the positive response to this 
service, until after this service is no longer active. Disabling the processing of normal mode messages is 
necessary to facilitate module programming for service and assembly. During the programming process, 
certain normal communication CAN Identifiers may be temporarily used as diagnostic CAN Identifiers. See the 
chapter in this specification on programming for further details on the use of these CAN Identifiers for 
programming. 
This service also disables the setting of all diagnostic trouble codes. A device receiving and complying with this 
request shall take failsoft action on all necessary parameters. 
Note:
A device which is connected to more than one GMLAN link shall be able to receive, process and 
respond to a service $28 request on each link to which it is connected. 
Note:
Use of this service during a programming event should always be targeted to all nodes using the 
AllNodes functional diagnostic request CANId ($101) and the AllNodes extended address ($FE). 
This service requires a periodic TesterPresent ($3E) service message to be transmitted by the tester to remain 
active. 
This service shall be terminated when: 
A node receives a $20 request. 
A P3
C
(TesterPresent) time-out occurs.