There is no change in RTS output. It is always Low. Is this OK?
When the RTS terminal is Low, it indicates that the MU-4 can receive data. With one-way communication, if the internal buffer gets full when sending data cyclically using the @DT command, RTS switches to High, and data cannot be received. Monitor this terminal with the user CPU, and stop transmitting data when RTS is High.
When sending data from the CPU with two-way communication, it is necessary to follow a data transmission procedure where for each packet sent with the @DT command, the *DT response returned from the MU-4 is confirmed. In other words, the program should be written so that the internal buffer does not become full.