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Why use a communications bus?

2 min read

We previously looked at the main type of communications bus used on modern cars, but why do we need one at all? What has changed during the automotive evolution that makes a communications bus inevitable?

Before the communications bus

Back in the ’60s, when cars were simple, the wiring colour standard, BS-AU7, was defined. It harmonised the colour combinations needed to run every electrical component of a car. This meant a single wire to each device had a specific colour. From the main battery lead (Brown) to the switch for the reverse lamp (Green with brown tracer).

In 1983 BS-AU7a was released with a whopping 111 wiring combinations serving 127 devices, already more devices than wire colours. This sounds like a huge number, but a single modern engine ECU can have more than 130 pins. There aren’t enough wiring colour combinations in the world for modern vehicles. Not helped by there being limited colours the wires sheath can be.

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 9 years from launch.

Fast forward some 30+ years, and there are microprocessors everywhere, and I don’t just mean in the car. Copper telephones were replaced with handheld wireless devices, each with more processing power than the moon missions. As a result of these developments, fridges can go online, and my washing machine can email me when the cycle is finished.

The automotive sector has gone a similar way. The infotainment system on the Freelander2 consists of 3 core devices, each containing multiple microcontrollers. The Freelander2 is a relatively unsophisticated system compared to a 2020 Range Rover. At their most basic, 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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