The Modem Control Register is a Read/Write Register.

Bits 0 and 1 are ment for controlling their relevant data lines. E.g. setting bit 1active sets active the request to send line.

Bits 2 and 3 are responsible for Aux Output 1 and 2. Aux Output 2 can be connected with the external circuit controlling UART-CPU interrupt process. Aux Output 1 is usually disconnected but is still used in some boards to switch crystals between 1.8432MHZ and 4MHZ frequencies used for MIDI.

Bit 4 activates the loopback mode. In Loopback mode the transmitter serial output is placed into marking state. The receiver serial input is disconnected. The transmitter out is looped back to the receiver in. DSR, CTS, RI & DCD are disconnected. DTR, RTS, OUT1 & OUT2 are connected to the modem control inputs. The modem control output pins are then set inactive. In this mode any data placed in the transmitter registers for output is received by the receiver circuitry on the same chip and is available at the receiver buffer. This can be used to test the UARTs operation.

Bits 5,6 and 7 are reserved.