It was a Saturday morning, and I was staring at a problem that had been nagging me for weeks. I have this digital modeling guitar amp that I love to play, but the floorboard I'd been using for years (made by the same vendor), wouldn't play nice with it. There is a newer floorboard which is supposed to be gauranteed to work, but it's not all that different from the one I have. Before swapping out my gear, I really wanted to understand what was actually broken.
The new board is mostly the same, but it has these color-changing LED lights for all the foot switches. My theory was there may be some control signal (related to those lights) that my old board didn't understand. Any time those signals were received, my floorboard would lock up. The thought was: if I could filter these out, maybe it could all work.
Down the Rabbit Hole
I started with a little digging on the Internet. I discovered the amp and floorboard communicate over a custom protocol using RS485 over an 8P8C jack (aka "RJ45"). This isn't Ethernet, despite the connectors being the same.
First step: I needed to actually see what was happening on that cable without breaking anything. I grabbed a breadboard, some hookup wire, and started building an ethernet tap. Nothing complicated—just a way to split the signal so I could MITM my signal and measure it with a multimeter. If I could see the voltage levels, I could compare them against the specs and figure out if something was obviously wrong.
The readings were close to what the documentation said they should be. But a few pins seemed off. The specs I'd found online didn't quite match. It was pretty easy to correct my own schematic, make some notes, and keep going.
By mid-afternoon, I had a working tap and a better understanding of the pinout. But a multimeter only tells you so much. It shows you voltage at a single moment in time. It doesn't show you the data—the actual messages being sent back and forth. For that, I needed something smarter. I decided to use a Teensy 4.0 microcontroller. In theory, it should let me read the serial data and manipulate and/or report it as need.
However, RS485 uses differential signaling. Instead of a single wire carrying a signal, you have two wires whose difference in voltage represents the logical signal. One wire's data is inverted, transmitted, then inverted again. It's a clever design for noise immunity over long cables, but it also means a standard microcontroller can't just read it directly.
There are a few ways to deal with this, but the most straight forward was to order a small RS485 transceiver. It's basically a breakout board that converts differential signals into single serial lines. With this, I can connect the Teensy to a single pin and read the serial data fine.
The transceiver arrived by Monday afternoon. A few minutes of wiring, a quick test of the serial connection, and it was ready. I plugged the tap into the live cable between the amp and floorboard, fired up the Teensy, wrote a tiny bit of bootstrap code, and opened a serial monitor.
Now I could read the raw hex values, and started mapping the patterns. I could start seeing the control commands from the amp to the floorboard (and vice-versa). But I didn't see any corrupted data. The LED control signals were coming through fine. The voltage levels were good. The timing looked reasonable. Everything looked... normal.
I didn't see what I thought I would see. This was definitely a little frustrating. I'd spent the better part of a weekend building hardware taps and transceiver circuits. I'd learned more about RS485 than I'd expected to. But I didn't seem closer to solving this.
So... I sat back down on the net and started digging a bit more (at least this time, I had more specific ideas to search for). And then, in some ancient forum, as a bit of a side-note that no one seemed to care about, someone described having the same problem, with my same floorboard, and my same amp model. Their fix? Swap the ethernet cable...
I grabbed a shorter cable from my spares bin, plugged it in, and powered everything on. It just worked. No errors. No lockups. Just a functioning floorboard and a happy amp.
I probably should have started there.
The Anticlimactic Truth
I won't lie, there was a moment of deflation. All that work, all that learning, all those hours on a weekend that could have been spent playing guitar or doing literally anything else—and the answer was "use a shorter cable."
But here's what I actually gained from this: I do have a deeper hands-on understanding of RS485 now. I've got a working 8P8C tap and some quick Teensy code for serial debugging that I can reuse on future projects. I learned how to work with transceiver modules, and it was still fun to do it.
Not every project has to succeed in the way you imagined to be worthwhile. Sometimes the real victory is in understanding your tools better, building things that might help you later, and accepting that sometimes the answer is simpler than the problem deserves.
