Use legacy RCX sensors with EV3
Unfortunately, you cannot use the LEGO 8528 (NXT to RCX adapter) cable with sensors on the EV3. You can use it with motors though.

This is because the adapter cable only connects to pins 1 and 2 on the MINDSTORMS (RJ12) connector. On the NXT input (sensor) ports, pin 2 is GND.



On the EV3, however, pin 2 on the input port is no longer GND. It is used to tell the difference between NXT sensors and EV3 sensors.



Without a proper ground (GND), the sensors will not work correctly. This is why the videos for modifying the cables use pin 3, which is GND on the EV3.

Furthermore, for those not restricted by the rules of a competition that can make their own cable, you also need to know that the EV3 firmware lacks the ability to send power pulses needed by "active" RCX sensors. From the NXT hardware developer kit:



So, don't expect the RCX Light sensor or the RCX Rotation sensor to work with the EV3 even if you make your own cable because it lacks this feature.



As a side note, RXC motors will work with the LEGO 8528 cable and the EV3 because pins 1 and 2 on the output ports are for motor + and -. They work exactly the same as Power Functions motors as described here.