DIACOM Information

DIACOM stands for Diagnostic Interface for Automotive Computers.

Diacom displays diagnostic information, available from a vehicle's ECU, on a PC screen. This process is shown in the diagram and highlights that the Diacom hardware connects to both the vehicles ALDL connector and the PC's parallel port. The Diacom software is controlled from the keyboard and displays results on the PC's screen. Not shown is Diacom software's ability to save and load information to and from disk.

Rinda's information on Diacom lets you download a demo. But, as you'll see, it's really an advertisement, but does show Diacom's user interface. Diacom's strength lies not in its relatively unsophisticated software and its very simple hardware, but in the database of vehicles that Rinda maintains and that Diacom therefore supports.

For many early ECUs, Diacom can only tell you very basic information about your vehicle, because that's all that's available from the ECM. For example, the Australian Holden VN Commodore uses a GM P4 series ECU (type number 1227808) and this ECU outputs just twenty (20) bytes of data. But that 20 bytes can be used to diagnose quite a number of faults present in the ECU itself, within a vehicle sensor, or with the vehicle's wiring. For an experienced mechanic using a "swap and retest" methodology, this can be enough extra information to enable accurate and speedy fault identification.

The problem, for the average weekend handyman, is that Diacom is not a cheap product to purchase.

 Diacom Hardware

The three transistors in the middle and to the right control the ALDL mode selected. The two transistors on the bottom right read 160 baud ALDL and other data. The transistor at the top and the middle leftmost are for reading (and writing) 8192 baud data streams. The bottom leftmost transistor enables the other transistors above it.

 Diacom Prototype

I have made up this circuit and can verify that it works on a Holden VN Commodore with software supplied by Rinda. Rather than use 2670 ohm resistors, I used 2.7k (also called 2k7) ohm resistors, and I also used BC547 transistors rather than the ones suggested (these are garden variety NPN transistors).

This is a picture of my one-off unit. The wires on the bottom go off to whatever ALDL connector you require for your vehicle (ie. the Car ALDL connector in the schematic).

The photo also shows a red LED, on the left, to indicate power (connect to pin H for some vehicles) and a green LED for ALDL data (connect to pin D or E depending on vehicle). These were added as a diagnostic aid and connect to the points above and GND, with a series 3k3 current limiting resistor.

Unfortunately, we can't answer any questions about how to build this circuit as this information is intended to enable you to fix an existing cable that was originally purchased from Rinda. But you can make your own 160 or 8192 baud interface.