Question: Does the C request line change to a high impedance state after it acknowledges the bus request?

Answer: Yes it does. But be very careful using the C4x comports this way. Suggest that he download and look at the file M4X_2.EXE which describes how to build an comport interface and control a C4x device from a PC's printer port. The concept is based on the fact that the C4x token control mechanisms are difficult to describe in a FPGA but not too difficult if a smart controller (CPU of some kind) can emulate the token control in software.