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        Port 3FC  -  Modem control register

        111x xxxx  Forced to 0
        xxx1 xxxx  0=normal, 1=loopback mode
        xxxx 1xxx  1=Interrupts to system bus (interrupt enable)
        xxxx x1xx  Unused
        xxxx xx1x  1=Activate rts
        xxxx xxx1  1=Activate dtr

        Loopback mode: When the UART is in loopback mode, all characters
        written to the Transmit-buffer is imidiately received in the
        Receive-buffer. This can be used for testing purposes. Note:
        Characters written to the transmit-buffer is not send. Nor is
        anything received from remote.

        Bit 2 and 3 is directly connected to the OUT1 and OUT2 pins on
        the UART. On the PC, OUT1 is not used.