Holden:ALDL (8192 Baud)

Overview

ALDL or Assembly Line Diagnostic Link is the diagnostic interface created by GM and used by Holden on the Commodore models from the VN to the VZ. The VE is the first Commodore to exclusively use CAN bus for diagnostics and inter-module communication. The VN through to VS Commodores used GM's original 160 baud interface (compatible with a GM Tech 1 or Tech 2 scan tool), whilst the VT through to VZ Commodores used the newer 8192 baud interface (compatible with a Tech 2 scan tool). This article will focus on the 8192 baud interface. For information on the 160 baud interface, see ALDL (160 Baud).

Frame Structure

An ALDL frame consists of a message ID, length, an optional number of data bytes (up to 64 bytes), and a 2's compliment checksum. Any multi-byte fields are always big-endian unless otherwise noted. The table below illustrates the structure of an ALDL frame.

Length (bytes)	Field	Notes
1	Message ID	The value 0 is reserved and is not a valid ALDL message ID
1	Length	0x55 + n, where n is the number of data bytes (up to 64)
n	Data
1	Checksum	2's compliment checksum of all bytes in the frame (message ID, length, and data bytes)

Module IDs

The table below lists IDs for various modules across several Commodore models.

ID	Module
BD	BCM (VR/VS)
E4	Telematics Module (VY)
EA	ECC - Climate Control (VY)
EB	Radio (VY)
F0	ALDL Tester/Scan Tool
F1	BCM (VT/VX/VY)
F2	Instrument Cluster (VT/VX/VY)
F2	SRS (VR/VS)
F3	PIM (VY)
F4	PCM (VR/VS V8)
F5	PCM (VS V6/VT)
F7	PCM (VX/VY)
F9	ABS/TCS (VR/VS)
FA	SRS (VT)
FB	SRS (VX/VY)

Checksum Calculation

An ALDL checksum is calculated by adding every byte prior to the checksum field (ID byte, length byte, and data bytes), and calculating the 2's compliment of the last 8 bits in the sum. Verification works in a similar manner; all bytes of the frame are added (including the checksum byte). The last 8 bits of the sum will equal 0 if the sum is valid.

Modes

ALDL diagnostic requests work on the principle of modes. To issue a diagnostic request, an ALDL frame is sent with an ID corresponding to a module's diagnostic ID (see Module IDs), and the first data byte set to the mode number of the request. ALDL diagnostic requests will therefore always have a minimum of 1 data byte. A typical response to a diagnostic request will be an ALDL frame with the same ID as the original request, the first data byte set to the mode number of the request, and an optional number of extra data bytes forming the response. The table below lists the various ALDL modes.

Mode (decimal)	Description
00	Normal Mode
01	Live Data
02	Memory Block Read
03	Memory Scatter Read
04	Control Function
08	Disable Bus Chatter
09	Enable Bus Chatter
10	Clear DTCs

Mode 0

ALDL Mode 0, or Normal Mode reverts the effect of any previous ALDL requests (except requests with lasting effect such as a DTC clear), returning the addressed module to normal operation.

Mode 1

Each module maintains a set of tables containing live data as well as stored parameters. When a device issues a Mode 1 request to a module, it provides the number of the table being requested. The module responds by transmitting the requested table number, followed by the contents of the table. The exact contents and structure of the table depend on both the table in question, and the module. Tables are numbered sequentially, however most modules have less than 10 tables. Requests for table numbers higher than the total number of tables on a given module will not receive a responseTemplate:Dubious. The table below illustrates the full structure of a Mode 1 request.

Length (bytes)	Field	Value	Notes
1	ID	F1
1	Length	57
1	Mode	01
1	Table	n	Table number
1	Checksum

Mode 2

Mode 2 requests are used to retrieve a block of 64 bytes from the PCM's memory. Any 16-bit address within the processor's memory address space may be requestedTemplate:Cn. This means that ROM, RAM, calibration data and I/O registers may be retrieved with Mode 2 requests. Whilst Mode 2 requests memory in blocks of 64 bytes, the requested address does not need to be aligned on a 64 byte boundary. This may be useful for reducing the number of requests required for values in close proximity to one another. The below table illustrates the full structure of a Mode 2 request.

Length (bytes)	Field	Value	Notes
1	ID	F1
1	Length	58
1	Mode	02
2	Address	n	16-bit address in big endian format
1	Checksum

Mode 4

A Mode 4 request is used to control a module for testing purposes. Control can include anything from slewing or setting fixed output values, slewing or setting fixed input sensor values, altering the results of certain calculations and invoking certain module functions. Modules that respond to Mode 4 requests include the; PCM, BCM, instrument cluster and ABS/TCS modules. The structure of a Mode 4 request differs greatly depending on the module it is addressed to. The basic structure of a Mode 4 request is illustrated by the table below.

Length (bytes)	Field	Value	Notes
1	ID		ID of the module to be controlled
1	Length	55 + 1 + n
1	Mode	04
n	Control Structure		Varies depending on module
1	Checksum

Mode 8

Periodically, the BCM sends polling requests amongst other data on the bus. These requests poll the ECM for sensor data and operational parameters used by the instrument cluster. A Mode 8 request is used to temporarily disable these messages from being sent. This essentially silences the BCM and quietens the bus, allowing for another device such as a computer or scan tool to transmit with a lower chance of a packet collision. Unless another Mode 8 request is sent within a predetermined amount of timeTemplate:When, the BCM will automatically resume normal bus operation. The below table illustrates the full structure of a Mode 8 request.

Length (bytes)	Field	Value
1	ID	F1
1	Length	56
1	Mode	08
1	Checksum	B1

Mode 9

Similar to Mode 8, a Mode 9 request will re-enable BCM chatter. A Mode 9 request can be sent to enable bus chatter earlier, rather than waiting for the original Mode 8 request to time out. This may be useful (and potentially good practice) if a device only requires bus silence for a short period of time, as bus silence for longer periods of time can lead to Check Engine warnings on the clusterTemplate:Cn. The below table illustrates the full structure of a Mode 9 request.

Length (bytes)	Field	Value
1	ID	F1
1	Length	56
1	Mode	09
1	Checksum	B0

Mode 10

The majority of modules in the vehicle can have stored DTCs cleared with a Mode 10 request. These include, but are not limited to; the PCM, BCM, ABS/TCS, SRS modules as well as the instrument cluster. The below table illustrates the full structure of a Mode 10 request.

Length (bytes)	Field	Value	Notes
1	ID		ID of the module DTCs will be cleared from (see Module IDs).
1	Length	56
1	Mode	0A
1	Checksum