I have an 8U converted to 2.4 and need to use a buddy box with it. So will a FP-T5AU work as both have the round female connectors, and will I need to do any mode's to the 5AU ?
You must make sure the 5U as a student transmitter is turned off. Also, if the transmitter is a 5UAP (PCM) transmitter, it won't work with the 8U at all. The 8U trainer port is PPM, and a 5UAP (PCM) transmitter only has a PCM signal coming from its trainer port.