APPENDIX I

             EXTERNAL DISK CONNECTOR INTERFACE SPECIFICATION


GENERAL
The 23-pin female connector at the rear of the main computer unit is used
to interface to and control devices that generate and receive MFM data.
This interface can be reached either as a resource or under the control
of a driver. The following pages describe the interface in both cases.

                           - Appendix I 353 -


SUMMARY TABLE

Pin #  Name    Note

1      RDY-    I/O   IDand ready
2      DKRD-   I     MFM input
3      GRND    G     -
4      GRND    G     -
5      GRND    G     -
6      GRND    G     -
7      GRND    G     -
8      MTRXD-  0     Motor control.
9      SEL2B-  0*    Select drive 2
10     DRESB-  0     Reset
11     CHNG-   I/O   Msk changed
12     +Sv     PWR   540 mA average 870 mA surge
13     SIDEB-  0     Side 1 if low
14     WRPRO-  I/O   Write protect
15     TK0-    I/O   Track 0
16     DKWEB-  O     Write gate
17     DKWDB-  O     Write data
18     STEPB-  O     Step
19     DIRB    O     Direction (high is out)
20     SEL3B-  O*    Select drive 3
21     SELlB-  O*    Select drive 1
22     INDEX-  I/OO  Index
23     +12v    PWR   120 mA average 370 mA surge

KEY TO CLASS:

G     ground, note connector shield grounded.

I     input pulled up to 5v by 1K ohm.

I/O   input in driver, but bidirectional input (1k pullup)

O     output pulled though 1K to 5v

O*    output, separates resources.

PWR   available for extemal use, but currently used up by external drive.

                           - 354 Appendix I -


SIGNALS WHEN DRIVING A DISK
The following describes the interface under driver control.

SEL1B-, SEL2B-, SEL3B-
    Select lines for the three extemal disk drives active low.

TK
    A selected drive pulls this signal low whenever its read-write head
is on track 00.

RDY-
    When a disk drive's motor is on, this line indicates the selected
disk is installed and rotating at speed. The driver ignores this signal.
When the motor is off this is used as a ID data line. See below.

WPRO- (Pin #14)
    A selected drive pulls this signal low whenever it has a write-
protected diskette installed.

INDEX- (Pin #22)
    A selected drive pulses this signal low once for each revolution of
its motor.

SIDEB- (Pin #13)
    The system drives this signal to all disk drives-low for side 1, high
for side 0.

STEPB- (Pin #18)
    Pulsed to step the selected drive's head.

DIRB (Pin #19)
    The system drives this signal high or low to tell the selected drive
which way to step when the STEPB- pulse arrives. Low means step in (to
higher-numbered track); high means step out.

DKRD- (Pin #2)
    A selected drive will put out read data on this line.

DKWDB- (Pin #17)
    The system drives write data to all disks via this signal. The data
is only written when DKWEB- is active (low). Data is written only to
selected drives.

                           - Appendix I 355 -


DKWEB- (Pin #16)
    This signal causes a selected drive to start writing data (provided
by DKWDB-) onto the disk.

CHNG- (Pin #11)
    A selected drive will drive this signal low whenever its intemal
"disk change" latch is set.
This latch is set when the drive is first powered on, or whenever there
is no diskette in the drive. To reset the latch, the system must select
the drive, and step the head. Of course, the latch will not reset if
there is no diskette installed.

MTRXD- (Pin #8)
    This is the motor control line for all four disk drives. When the
system wants to tum on a disk drive motor, it first deselects the drive
(if selected), pulls MTRXD- low, and selects the drive. To turn the motor
off, the system deselects the drive, pulls MTRXD- high, and selects the
drive. The system will always set MTRXD- at least 1.4 microseconds before
it selects the drive, and will not change MTRXD- for at least 1.4
microseconds after selecting the drive. An external drives must have
logic equivalent to a D flip-flop, whose D input is the MTRXD signal, and
whose clock input is activated by the off-to-on (high-to-low) transition
of its SELxB- signal. As noted above, both the setup and hold times of
MTRXD-with respect to SELxB- will always be at least 1.4 microseconds.
The output of this flip-flop controls the disk drive motor. Thus, the
system can control all four motors using only one signal on the cable
(MIRXD-).

DRESB- (Pin #10)
    This signal is a buffered version of the system reset signal. Three
things can make it go active low):

o  System power-up (DRESB- will go low for approximately one second);

o  System CPU executes a RESET instruction (DRESB- will go low for
approximately 17 microseconds);

o  Hard reset from keyboard (lasts as long as keyboard reset is held
down).

External disk drives should respond to DRESB- by shuffing off their motor
flip-flops and write protecting themselves.

A level of 3.75v or below on the 5v+ requires extemal disks to write-
protect and reset the motor on line.

                           - 356 Appendix I -


Device I.D.
    This interface suppots a method of establisning the type of disk(s)
attached. The I.D. sequence is as follows.

        1. Drive MTRXD- low.

        2. Drive SELxB- low

        3. Drive SELxB- hign.

        4. Dnve MTRXD- high.

        5. D;ive SELxB- low

        6. Drive SELxB- high.

        7. Drive SELxB- low

        8. Read and save state of RDY.

        9. Drive SELxB- high.

Repeat steps 6 to 9, 15 times more.

Convert the 16 values of RDY- into a 16-bit word. The most significant
bit is the first value and so on. This 16-bit quantity is the device I.D.

The following I.D.s are defined:

0000 0000 0000 0000  Reserved
1111 1111 1111 1111  Amiga standard 3.25
1010 1010 1010 1010  Reserved
0101 0101 0101 0101  48 TPI double-density,double-sided
1000 0000 0000 0000  Reserved
0111 1111 1111 1111  Reserved
0000 1111 xxxx xxxx  Available for users
1111 0000 xxxx xxxx  Extension reserved
xxxx 0000 0000 0000  Reserved
xxxx 1111 1111 1111  Reserved
0011 0011 0011 0011  Reserved
1100 1100 1100 1100  Reserved

                           - Appendix I 357 -

End.