[Gmecm] Class2 DIY tools?

[Phillip Kuhn]

Well that would explain the reason for the tech 2
being the only tool that could communicate, although
after actually looking at Service Information the
catera used 4 different types of data communications
being: class 2, keyword 82, keyword 2000,and CAN. All
were used for different purposes.Oddly enough it used
CAN but there was no adapter required to actually
communicate with the vehicle (unlike the vehicles that
utilize CAN nowadays) due to the fact that it used the
class 2 line to actually communicate with the scan
tool. I never did like the catera or ever did I have a
reason to look at the description for the data
communications, but I see now why it was so wierd to
work with. Luckily I came to cadillac as they were
phasing out the catera and my work on them has been
"reduced" due to that fact and to the expense of their
parts. Below is a description of the communications
used in a catera which I think might be useful to
those unfortunate enough to have to work on one.

Data Link Communications Operation
The serial data circuit is the means by which the
microprocessor controlled modules in the vehicle
communicate with each other and the scan tool. There
are four different types of protocols on this vehicle.

Class 2 
Controlled Area Network (CAN) 
Keyword 82 
Keyword 2000 

Class 2 Data Line
U.S Federal regulations require that all automobile
manufacturers establish a common communications
system. General Motors utilizes the Class 2
communications system.

Class 2 serial data is transferred at a rate of 10,400
bits per second, and the voltage is toggled between
zero and 7 volts. When communication is not occurring
on the data line the voltage is zero or ground and is
switched to 7 volts when active.

Class 2 data is also pulse width modulated. Each bit
of information can have one of two lengths: long or
short. The pulse width modulation of Class 2 data
allows better utilization of the data line.

The messages carried on Class 2 data streams are also
prioritized. This means if two devices try to
communicate on the data line at the same time, only
the higher priority message will continue. The device
with the lower priority message must wait.

The modules on the Class 2 data line are:

Body Control Module (BCM) 
Cellular Telephone 
Radio Receiver 
Remote Control Door Lock Receiver (1st design) 
Sound Processor Interface 

Controlled Area Network (CAN)
The Controlled Area Network (CAN) serial data line is
high speed serial data bus used to communicate
information between the Electronic Brake and Traction
Control Module (EBTCM), the Engine Control Module
(ECM) and the Transmission Control Module (TCM).
Typical data-transmission speeds range from about 125
kilobits per second to 1megabits per second, and must
be high enough to ensure that the required real-time
response is maintained.

The addressing scheme employed with CAN assigns a
label to every message, with each message receiving a
unique "identifier". The identifier classifies the
content of the message (such as engine speed). Each
module processes only those messages whose identifiers
are stored in the module's acceptance list. This is
CAN's form of message filtering.

The identifier labels both the data content and the
priority of the message being sent. Each module can
begin transmitting its most important data as soon as
the bus is unoccupied. When more than one module
starts to transmit simultaneously, the message with
the highest priority is assigned first access. A
module responds to failure to gain access by
automatically switching to receive mode, the module
then repeats the transmission attempt as soon as the
bus is free again.

Keyword 82 and Keyword 2000 Data Line
The Keyword protocols utilize a single wire
bi-directional data line between the modules and the
scan tool. With this type of communication, a voltage
level of less than 20 percent of B+ is considered a
logic 0. A voltage level greater than 80 percent of B+
is considered a logic 1.

The message structure is a request and response
arrangement that has some similarities to the UART
system.

The modules on the Keyword 82 data line are:

Automatic Level Control Sensor 
Memory Seat Module 
Instrument Cluster 
Heater and A/C Control 
SIR Module 
Power Steering Control Module 
Sun Roof Actuator 
Theft Deterrent Module (Immobilizer) 

The control modules on the Keyword 2000 data line are:

Engine Control Module (ECM 
Transmission Control Module (TCM) 
Electronic Brake/Traction Control Module 



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