Analogue and Digital Signals

Remove the bodywork and lift the tank on modern bikes and you're bound to find a plethora of wiring without which the bike simply won't run. But what does it do?

As one of those signals, I feel duty bound to explain myself. The first thing to understand is that signals are simple. We have to be by nature. The complicated bit is understanding what information we relay, but you don't need to for now.

In the simplest sense there are analogue and digital signals. And, just like television, this relates to how the information is transmitted, rather than the content. So, let's look at one of each sort. The throttle position sensor on your bike is almost certainly and analogue signal - and works from 0 to 5 Volt. That means depending on how open the throttle is, the signal wire will have anywhere between 0 to 5 Volt on it.
So the ECU knows that when it sees 0 Volt the throttle is closed, when it sees 2.5 Volt it's 50 percent open and finally when it sees 5 Volt it is 100 percent open.

However, it would be difficult to measure wheel speed in this way. What would you do? Have 0 Volt as not moving and 5 Volt as 290km/h? It can be done, but not easily? A far more practical solution is to use a digital signal. A digital signal is literally an on or off signal. Assuming we're still working with 0 to 5 Volts you might say that 0 Volt means off, and 5 Volt means on. The problem of course is what happens at 2.5 Volt? Is it on?

To overcome this, digital signals are treated differently. Instead of saying 0 Volt is off and 5 Volt is on, we say anything below, say 1.5 Volt, is off, and anything above, say 3 Volt, is on. So anything between 1.5 to 3 Volt is ignored.

Getting back to wheel speed, you'll notice most sensors line up with a bolt, or some other form of trigger. When this trigger passes in front of the sensor it switches from 0 Volt to 5 Volt, and then back again, or vice-versa. The ECU sees this signal and waits for the next. If there's only one trigger on the wheel and it takes one second for the next pulse to arrive, the ECU knows the wheel is turning at one revolution per second.

With a little programming the ECU multiples this by a wheel circumference and displays a speed on your speedo. Simple, but clever. And this is how your bike knows what's going on.
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