What do I need to be careful of when interfacing between the single-chip CPU and the MU4-RS2IF?

The MU4-RS2IF interfaces at the RS-232C level. When interfacing with the single-chip CPU, to bring the operating voltage of the CPU to the RS-232C level, use an RS-232C transceiver IC such as MAX232. Be careful of the input-output relationship between the MU-4 terminal and MU2-RS2 terminal when you connect them. In addition, be sure to control the MODE terminal of the MU-4. The MODE terminal is connected to the RS-232C DTR line.