An LED Matrix RJ45 Cable Tester

Ethernet connections are a beautiful thing. Robust, standardized, and mechanically sound, you simply insert an RJ45 plug into the corresponding jack and it stays there until released. It can then provide data transfer rates well beyond WiFi, without fooling about with passwords and the like. The down side is, of course, that you have to physically run these connections, and while the connectors are consistent, there are actually a variety of cable standards available.

To test things out, Ted Yapo created an RJ45 cable tester with an 8x8 LED matrix that shows the connection pattern. It's a ingeniously simple concept, with the X-axis of the matrix indicating one side of the connection, while the Y-axis shows the other. So with a straight cable, the LED in position 1, 1 (starting on the upper-left) is lit up, then 2,2; 3,3; etc. also illuminate, up to the 8,8 LED.

A crossover cable, where Tx and Rx pairs are switched, produces a swapped arrangement, while an ancient 10/100 cable shows only four LEDs. A rollover cable produces a mirror image of a straight-through cable.

The tester uses a PIC16F1705 scanning and Yapo notes that he could have used a 555 and 74HC4017 for microcontroller-less fun as well. Interestingly, at high enough scan rates, you can see unexpected LEDs illuminate, indicating coupling between different conductors.

It’s a beautiful device and a lot more build info is available on the project write-up if you want to construct your own. As Yapo puts it, “I don't need this, but couldn't help building it.” Perhaps you’re thinking the same thing!