The future of Serial Data Communications will be the Local Area Network (LAN) System. Sure Class 2 and the different forms of UART have been very reliable, but they are not going to do the job with the new electronics on the horizon.
How it works
The first thing, if you are not familiar with serial data, please check out our article on the subject by clicking here. The LAN system is loosely based on the Controller Area Network (CAN) that has been used on the Catera and CTS as well as the Duramax Diesel. GMLAN is a reliable, cost effective, flexible, and modular way to handle information sharing between different Electronic Control Units (ECU’s) in the vehicle through the means of a family of Serial Communication Buses. They also use the use The Controller Area Network (CAN) Protocol.
GMLAN Supports 3 Separate Buses
 | Low Speed |
| Mid Speed - (NOT slated for use in any models.) See more about Fiber Optics to see what else is being used |
| High Speed |
There are Gateways between the different speed buses.
GMLAN Low Speed Bus
| Typically Used For Operator Controlled Functions Where The System Response Time Requirements Are On The Order Of 100 - 200 msec. |
| Uses Single Wire Physical Media (Also Referred To As Single Wire CAN Or SWCAN) |
| 33.33kbps in normal mode, 83.33kbps in high speed programming mode |
| A 0 of data is represented by 0 volts and 1 by 5 volts. When the module needs to be 12 volt initial spike to turn on modules that go off line (see figure 1 |
Figure 1
GMLAN Mid Speed Bus
| Intended To Be Used For Infotainment Applications (e.g Graphics Display) Where The System Response Time Demands Large Quantities Of Data To Be Transmitted In A Relatively Short Period Of Time. Navigation systems, DVD players, Radios and other more sophisticated entertainment systems. |
| The Mid Speed Bus has a nominal speed of approximately 95.2kbps |
| As of the writing of this article, the Mid Speed bus is NOT slated for use in any models. |
GMLAN High Speed Bus
This figure is a scope pattern from
the
GMLAN high speed data bus.
|
Typically Used For Sharing Real Time Data (e.g. Driver Commanded Torque, Actual Engine Torque, Steering Angle, etc.) |
|
Primarily Powertrain And Chassis Devices as well as Anti Lock Brakes |
|
Uses Two Wire Physical Media |
GMLAN Subnets May Contain Gateways To Other Subnets
This illustration is a rough sketch of the way the LAN system and its "Gateway Modules" may be laid out. A Gateway Module is any module that can talk on two different data buses. In this picture G1 can be a gateway between the Low and High Speed buses. It can talk on the bus with module G2 and also G3. However if G2 and G3 wanted to share information they could not do it directly.
Diagnosis
If you can remember back to when the Class 2 Serial Data Bus first came out, the predominant scan tool of choice was the Tech 1. Although state of the art at the time it was not fast enough for interpreting the data. So GM came out with the VIM (Vehicle Interface Module) which was plugged into the Tech 1 to do the translating. Now that we are on the verge of the GM LAN System, we are going to need something to help the Tech 2 get our data right. The tool to the rescue will be the Controller Area Network Diagnostic Interface or CANDI. Much like the VIM, the CANDI will plug into the Tech 2 and make it so the scan tool will talk on the data bus. With the CANDI module, the Tech 2 can still be used as the tool of choice for the next 5 to 10 years!
Mik Stubing Lead Instructor GMTCNY
