3. Interface I:

   The Interface I is quite complicated. It uses three different I/O ports,
and contains logic to page and unpage an 8K ROM if new commands are used. The
ROM is paged if the processor executes the instruction at ROM address 0008 or
1708 hexadecimal, the error and close# routines. It is inactivated when the
Z80 executes the RET at address 0700.

a) Port E7:
   I/O port E7 is used to send or receive data to and from the microdrive.
Accessing this port will halt the Z80 until the Interface I has collected 8
bits from the microdrive head; therefore, it the microdrive motor isn't
running, or there is no formatted cartridge in the microdrive, the Spectrum
hangs. This is the famous 'IN 0 crash'.

b) Port EF:

       Bit    7   6    5    4    3    2    1     0
            +---------------------------------------+
        READ|   |   |    |busy| dtr |gap| sync|write|
            |   |   |    |    |     |   |     |prot.|
            |---+---+----+----+-----+---+-----+-----|
       WRITE|   |   |wait| cts|erase|r/w|comms|comms|
            |   |   |    |    |     |   | clk | data|
            +---------------------------------------+

Bits DTR and CTS are used by the RS232 interface.  The WAIT bit is used by the
Network to synchronise, GAP, SYNC, WR_PROT, ERASE, R/_W, COMMS CLK and COMMS
DATA are used by the microdrive system. If the microdrive is not being used,
the COMMS DATA output selects the function of bit 0 of out-port F7:


       Bit      7    6   5   4   3   2   1       0
            +------------------------------------------+
        READ|txdata|   |   |   |   |   |   |    net    |
            |      |   |   |   |   |   |   |   input   |
            |------+---+---+---+---+---+---+-----------|
       WRITE|      |   |   |   |   |   |   |net output/|
            |      |   |   |   |   |   |   |   rxdata  |
            +------------------------------------------+

TXDATA and RXDATA are the input and output of the RS232 port. COMMS DATA
determines whether bit 0 of F7 is output for the RS232 or the network.