Why do we need make-specific cables to read car ECUs?

Question

I find it bizarre that the automotive industry has standardized the use of the OBDII protocol, yet most cars require make-specific cables in order to access the complete set of diagnostics and utilities.

A few examples:

• VWs require a VAG-COM cable
• BMWs need INPA/EDIABAS-compatible cables
• Volvos require VADIS-compatible cables

I can understand the software being different across different makes. But given that all cables hook up to the OBDII port, why do they need to be custom? What is lacking in a generic OBDII cable?

Answer 1

The cable is more than just a cable, it has a microcontroller that handles the car's diagnostic protocol.

The software on your computer talks to the cable with a proprietary protocol specific to the cable's manufacturer, and the cable itself translates that to the car's protocol equivalent.

I've actually wondered why we can't have a single cable for all cars and handle the car's protocol on the computer directly (the cable being a simple converter that relays frames received on the car side to the computer's side and vice-versa), and here's a reply I got from Ross-Tech (the VCDS developer) :

A "dumb" network to K or CAN converter is not a practical device; diagnostic protocols are too demanding of real-time responses from he device that's interrogating them.

Apparently the latency induced by having the CAN packet going to the computer, being processed there and going back is too much, that's why it's needed to handle the car protocol directly on the cable.

Implementing OBD-II is simple and that protocol is standard and most likely open. Implementing manufacturer-specific protocols is hard because there is very little documentation about them and the only way to know that protocol is to reverse-engineer the firmwares of the ECUs or to capture the communication between the car and the manufacturer's official diagnostic device. So it is hard, time-consuming and expensive to do that for the protocol used by each manufacturer, and then implement all of that directly on the cable's microcontroller.

Still, there are universal cables, they're just extremely expensive (most likely for the reasons I mentioned above) and I doubt they have all the features that the manufacturer-specific tools have (it's not only about the diagnostic, the cables you mentioned can configure the car, for example add new keys or tweak the steering assist).

Answer 2

There are 6 pins in the OBD-II connector which the manufacturers can use at their discretion. In modern vehicles there are usually 2 or more communication buses. Only one bus is for engine and emissions, which is connected to the standard diagnostic pins on the OBD-II connector.

The manufacturer typically connects another bus to the OBD-II connector with the "manufacturer discretion" pins.

A vehicle diagnostics computer will only get engine, emissions, and a few other systems worth of information from the standard pinout, but with a cable and additional hardware the diagnostic computer can access the secondary (or even a third bus) as well and obtain/modify information affecting several other systems in the vehicle.

Each manufacturer uses these pins differently, though, so while it's possible to build a cable that can handle every known pinout, it doesn't account for changes in the future, and it's more expensive than simply having several cables that work with different manufacturers and lines.