Question: The 'C54x data sheet states that the DX signal is placed in high impedance state when not transmitting. Is this done automatically after completion of transmitting each frame or is the DX signal placed in high impedance state only when XRST*= 0?

Answer: DX goes automatically to hi-Z after the last bit in the last element in a frame is transmitted (Tdis(CLKX-DXZ)timing) even if the FPER (frame period) is longer than the total number of bits xmitted.