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

4.3.4 Special Considerations for Nodes Operating On Multiple GMLAN Subnets. 

Gateways shall not transmit diagnostic messages across subnets. Any diagnostic message which is 
targeted for all GMLAN subnets must be transmitted by the tester on each specific subnet. Any diagnostic 
message which is targeted to a functional system that spans one or multiple GMLAN subnets must be 
transmitted by the tester on each subnet where nodes of the addressed functional system reside on. 
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GM WORLDWIDE ENGINEERING STANDARDS 
GMW3110 
© Copyright 2010 General Motors All Rights Reserved 
February 2010 
Page 22 of 336 

A Multiple Subnet Signal Consumer (MSSC) shall process diagnostic services supported on multiple 
subnets individually. For example, a service $28 received on one subnet would only stop normal 
communications on the subnet which it is received. The ECU would continue normal communications 
messages (if otherwise active) on other subnet(s) to which it is attached. Another example of this would be 
that a tester present request received on a subnet would not reset the P3
C
timer on another subnet. 

A MSSC device shall support simultaneous programming events on each of the mulitple subnets to which 
it is connected. A programming event on one subnet shall not impact or preclude initiating a programming 
event on another subnet. This requires processing the following services separately on each subnet: 
service $20, service $28, service $A5, service $1A, and service $3E. 
Note:
See service $A5 for more information on enabling programming events. 
Note:
A node shall support the DIDs required for programming on both subnets. Reference the programming 
chapter within this specification for specific requirements. The other DIDs needed for diagnostics may only be 
supported on one subnet. 

A MSSC device shall be capable of being diagnosed fully from at least one subnet to which it is connected. 

A MSSC shall document in a Component Technical Specification (CTS), Subsystem Technical 
Specification (SSTS), or other specification referenced in a CTS or SSTS the diagnostic services that are 
supported on each subnet. 

A programmable MSSC device shall only be capable of being programmed from one subnet to which it is 
connected and the $A2 service shall only be supported on this subnet. This subnet that is used for 
programming should be the subnet with the higher baud rate capability. 
Note:
A programmable MSSC device is one that utilizes the SPS system (utility file concept) for 
reprogramming. Refer to Section 9 of this specification for more details. 

If a MSSC receives diagnostic requests on both subnets simultaneously, it must respond to each request 
within the P2
C
time, however depending on the service requested and the capabilities of the node one of 
the following must happen: 
1. For services which the MSSC node supports on both subnets, the MSSC node sends either: 

The positive response on each subnet within P2
C.

OR
the MSSC node sends the positive response on one subnet while sending a negative response $7F 
$RequestServiceId $78 message on the second subnet within P2
C
(indicating a positive response is 
forthcoming), followed by the positive response after transmitting the positive response on the first subnet. 
The exceptions are modes $20, $28, and $A5 where the $7F $RequestServiceId $78 is not allowed.