This question came from figuring out how to drive an SPI chip over a few meters distance. Normally, you’re not supposed to do that: I2C and SPI in particular are designed for use on a PCB, i.e. distances in the order of dozens of centimeters at best. Fortunately, in this case speed was not an issue, so what if we simply take the clock signal way down, to say 10 KHz (50 µs on + 50 µs off). Can we create a solid SPI setup across a few meters of wiring?
To try this out, I decided to take a reel with some 70 to 80 meters of multi-core wire:
Being untwisted and unshielded, this is probably as bad as it gets for signalling purposes. Then I started with a clean 10 KHz square wave of 0..5V from the signal generator:
Here’s what happens to the signal on the signal generator side (i.e. same probe placement), when simply connecting the open wires of the above cable to signal and ground:
This is caused by the load and the reflection of the signal on the end of that long cable. Think of it as an echo of that voltage change, propagating back and forth and colliding with the signal – causing this fairly substantial change in signal rise time and shape..
But to really see the effects, we have to also look at the signal at the other end, where it’s going to be picked up and used. I made the following setup, with three different probes:
(sorry for the confusion: “B” and “M” were mixed up in this drawing)
- YELLOW probe on the incoming signal generator end
- MAGENTA probe on the outgoing signal
- BLUE probe on the outgoing ground connection
- RED is the MAGENTA minus the BLUE level (as math function)
All the vertical scales are set to the same 1V per division from now on:
Note that there are some really strange effects in there: the magenta output signal is “ringing”, as is to be expected with such a long coil of wire and lots of stray capacitance, inductance, and resistance. But the blue “ground” level is also ringing, almost as much in fact! And the yellow input is funny, too. Let’s zoom in a bit on that rising edge:
What does this mean for signal propagation? Well, as you can see, everything rattles and shakes in situations like these. It really makes a difference for example as to how the end is connected to power. If you use the (blue) output ground as reference, then the signal will appear as the difference (i.e. the red line), which has less extreme ringing than if you use the (magenta) output referenced to the incoming ground pin.
None of these effects are problematic at this 10 KHz signalling rate. If you need to pass signals like these, it definitely pays to A) keep the signalling rate low, B) reduce the steepness of the edges, and C) add a fairly substantial load resistance at the end of the wire (between signal and ground, say 330..1000 Ω). The reasons and details of all this will have to wait for some future posts.
What I do want to mention, is that you can actually see the speed of light in action here. Let’s zoom even further into that rising edge (sorry, blue and magenta are reversed now):
It takes over 400 ns for the yellow rising edge to reach the other end. In vacuum, light travels some 120 meter in that amount of time – not so very different from the 80 meter or so of cable the electricity has to traverse. Pretty amazing what you can do these days, with nothing more than a signal generator, a modern oscilloscope, and a coil of wire!