Why use a communications bus?

2 min read

In the previous article, we looked at the primary type of communications bus currently used on modern cars, but why do we need one at all? Presumably, making communications links between ECU’s makes the vehicle more complicated and more expensive? So what has changed during the automotive evolution that makes a communications bus inevitable?

Wiring before the communications bus

In the ’60s, cars were far more straightforward, with just a few electronic devices. And it was in 1963, a British Standard for the colours of wires between different devices was first released. The BS-AU7 standard is based on the wiring of the 1934 Austin 7 vehicles, and hence the name. The standard harmonised the colour combinations needed to run every electrical component of a car. It gave every electrical connection between devices a unique colour. From the main battery lead (Brown) to the switch for the reverse lamp (Green with brown tracer).

The complete BS AU7 scorecard

In 1983 BS-AU7a was released with a whopping 133 wiring combinations serving 127 devices. While this might sound like a vast number, a single modern engine ECU can have more than 130 pins. Unfortunately, there aren’t enough wiring colour combinations in the world for modern vehicles. The number of possible combinations is not helped by the limited colours from which the wires PVC sheath can be manufactured.

The Wheels Alive website has an interesting article by Dave Moss on the origins of BS AU7.

Moving to micro controllers

Computers EVERYWHERE on a modern car!
Computers EVERYWHERE on a modern car!
Photo by Louis Reed on Unsplash

Perhaps more critically, cars in 1983 didn’t have a microcontroller except perhaps for some early GTi’s. Switches were just switches, and Microsoft Windows 3.11 was still another nine years from launch.

Fast forward some 30+ years, and there are microprocessors everywhere, and I don’t just mean in the car. Wireless technologies have now replaced copper telephone lines. Handheld devices have replaced bakelite rotary dial telephones with more processing power than the first NASA moon missions. As a result of these developments, fridges can go online, and my washing machine can email me when the washing cycle has been completed.

The automotive sector has gone a similar way. The infotainment system on the Freelander2 consists of 3 core devices, each containing multiple microcontrollers. Compared to a 2020 Range Rover, the Freelander2 is relatively unsophisticated, having fewer ECU’s and fancy features. At their most basic, modern cars are an incredibly complex way of moving computers around. I was once told that wiring directly would add a half tonne to a modern car. Just in the copper wire!

The communication bus replaces these wires with digital messages to reduce complexity and weight.


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