Messages on the Entertainment and Comfort serial bus

Stuart Schmitt, 29 April 2016
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Introduction

In the table below, all observed E&C messages are documented, along with best guesses about their meanings. A format for writing E&C messages is defined to indicate variable quantities and the number of bits used for constant numbers. Sequences of individual bits are bracketed and named with letters (for example, [ABCD] or [XXXXYYYY], where repeated letters indicate a value that spans multiple bits. The notation /n indicates the bit count. If it is absent, a “default” value is implied (2 bits for the priority, 6 bits for the address, and 8 bits for the data bytes). For example, the message

1-40-63-[XYZ]

indicates that there are three 1-bit variables X, Y, and Z. In this particular example, X = 1 means the audio system is on, Y = 1 means an OnStar session is active, and Z = 1 means the vehicle’s accessory power is on. Eight bits are used to communicate the value 63. With X = 1, Y = 0, and Z = 1, this example becomes equivalent to the 1-40-63-5 example shown in bus description page. For another example, the message

3-57-46/7-[XX]

contains a 2-bit variable XX. This particular message is an instruction to load one of two sets of personalized radio settings, numbered either 1 or 2. The first 7 bits of the data are the constant 46 that defines this command. Thus, this message can take two forms depending on which memory setting is being recalled:

1 11 100111 0111010 1 1    simplified to 3-57-174 for memory #1
1 11 100111 0111010 010 1  simplified to 3-57-46-1 for memory #2.

Note that in the first case the 0 in the second position of XX is omitted because it is unnecessary, and that in the second case there is an extra 0 appended to arrive at an even number of bits.

In many cases, variables are encoded in binary-coded decimal (BCD), usually sent least-significant-digit first.

Table of observed E&C messages