General Motors entertainment and comfort (E&C) serial bus

Introduction

The purpose of this web site is to present information about the General Motors entertainment and serial (E&C) bus. The E&C bus was used in certain 1987 to 2005 GM vehicles to allow for data transmission between the radio, CD changer, OnStar module, and HVAC system.

My motivation in understanding this bus is to be able to develop electronic interfaces that will allow newer electronic devices to be installed in these older cars. For example, OnStar hardware for these cars no longer works because of the 1 January 2008 analog cellular shutdown. It would be useful if the old OnStar modules could be removed and a Bluetooth tranceiver installed in its place. In fact, this modification has already been done for vehicles that use the newer Class 2 serial bus.

As of 15 February 2009, I have just started working on this project. I am sure that it will take several months and the assistance of others. I will endeavor to share what I learn on this web page.

E&C serial bus characteristics

The Entertainment and Comfort (E&C) serial bus allows for low-speed communication between various modules in the car. It is an event-driven interface; that is, data is not sent unless one device needs to communicate with another. Often communication results from a human action, such as pressing a button on the radio.

In my car (a 2005 Buick Park Avenue), the E&C bus is connected to four devices:

1. The radio.
2. The CD changer in the trunk.
3. The Vehicle Integration Unit (VIU) for OnStar.
4. The Instrument Panel Integration Module (IPM).

The IPM, in this particular application, bridges HVAC commands from the E&C bus to the car's more modern Class 2 bus. There are E&C HVAC signals in this car because there are temperature controls on the steering wheel, and the radio interprets all steering wheel control commands.

The E&C bus is circuit 835 in GM's wiring diagrams, and it is carried on a dark green wire. It is wired in a “star topology” from a centrally located splice point. The circuit is accessible at the diagnostic connector; on OBD-II cars, it is pin 14.

The E&C bus is a compartively old-fashioned serial connection. It is a 1000 bps pulse width-modulated bus that is pulled low to communicate. The table below summarizes what I know about it. The photograph to the right shows oscilloscope traces of both the logical 0 pulse and the logical 1 pulse (both triggered the oscilloscope during the camera's exposure).

Figure 1. Photograph of oscilloscope traces that capture both logical 0 and logical 1 pulses in an E&C data transmission.

Reading E&C data with an RS232 serial port

It is possible to read E&C serial bus data with a PC RS232 serial port. RS232 serial data has a fixed bit width, and is fundamentally incompatible with PWM data transmission. Fortunately, the E&C serial bus has some characteristics that give us a workaround.

RS232 uses two voltage states to represent logical 0s or logical 1s. When there is no data transmission, RS232 idles in the logical 1 state. To indicate that a byte is about to be sent, a device sends a logical 0 called the “start bit.” Then seven or eight bits follow, and finally the devices sends a logical 1 called the “stop bit.”

Looking at the table and the oscilloscope trace above, we see that many similarities to RS232:

1. The idle state is “high”.
2. Every E&C bit starts low.
3. Every E&C bit ends high.
4. The duration of every E&C “bit window” is constant.

Let us imagine how an RS232 serial port would interpret E&C bits. At right, we divide the oscilloscope trace into the ten bits of an RS232 byte. We expect to read only two possible sequences: one that is binary 11111111 and another that is binary 00000111. RS232 bytes are LSB first, so these correspond to hexadecimal codes 0xFF and 0xE0, respectively.

Using the interface above, we can view the output with a terminal. The common setting of 9600-8N1 works well enough. We obtain the expected result: the terminal prints a sequence of 0xFF and 0xE0 characters whenever an E&C data packet is sent across the bus.

Some actual recorded E&C signals

What follows is the most recent development in this project. So far, I have just recorded the bit sequences that I can associate with obivious events. There may be significant complexity in some of these recordings. For example, one device sends out a packet, and then another device replies with another packet. To help diagnose which devices send which signals, I try recording the same event with some of the devices removed from the car. There are still a lot of recordings to be made and decoded.

Future work

Once I figure out how to “speak” E&C, I'll need to figure out how to make a microconroller that can interpret commands and issue appropriate replies. Personally, I have two goals for my car: (1) I would like to be able to initate fictitious OnStar sessions via a Bluetooth transceiver, and (2) I want to spoof the existence of a CD changer so that I can use the radio's auxiliary input.

References

To my knowlege, there are no public documents that describe the E&C serial bus and the messages that get sent on it. Some of what I have learned is from the following documents:

1. Tom Boynton, General Motors computerized vehicle control systems: A short history
2. Delphi Automotive Systems Corporation, E&C to RS-232 Gateway: Systems Analysis INterface Tool (SAINT): Users Guide
3. Christopher A. Lupini, Multiplex bus progression 2003, SAE 2003-01-0111