3] External Disk Interface Connector Specification
3] ----------------------------------------------- 
 
 The 23 pin D type connector with sockets (DB23S) at the
 rear of the Amiga is nominally used to interface to MFM 
 devices.
  
3] External Disk Connector Pin Assignment (J7)
  
4] pin  name    dir   notes
4]--------------------------
  1   RDY*    I/O   If motor on, indicates disk installed and up to 
                       speed. If motor not on, Identification mode. 
                       (See below.)
  2   DKRD*   I     MFM input data to Amiga
  3   GND
  4   GND
  5   GND
  6   GND
  7   GND
  8   MTRXD*   OC   Motor on data, clocked into drive's motor on 
                       flip flop by the active transition of SELxB*.
                       Guaranteed setup time is 1.4 usec. Guaranteed 
                       hold time is 1.4 usec.
 9   SEL2B*   OC    Select drive 2
 10  DRESB*   OC    Amiga system reset.  Drives should reset their 
                       motor on flip flops and set their write 
                       protect flip flops.
 11  CHNG*   I/O    Note: Nominally used as an open collector input.
                       Drive's change flop is set at power up or when 
                       no disk is not installed.  Flop is reset when 
                       drive is selected and the head stepped, but 
                       only if a disk is installed.
 12  +5V            270 ma maximum; 410 ma surge. When below 3.75V, 
                       drives are required to reset their motor on 
                       flops, and set their write protect flops.
 13  SIDEB*   O     Side 1 if active, side 0 if inactive
 14  WPRO*   I/O    Asserted by selected, write protected disk.
 15  TK0*    I/O    Asserted by selected drive when read write head 
                       is positioned over track 0.
 16  DKWEB*   OC    Write gate (enable) to drive.
 17  DKWDB*   OC    MFM output data from Amiga.
 18  STEPB*   OC    Selected drive steps one cylinder in the 
                       direction indicated by DIRB.
 19  DIRB     OC   Direction to step the head.  Inactive to step 
                       towards center of disk (higher numbered 
                       tracks).
 20  SEL3B*   OC   Select drive 3.
 21  SEL1B*   OC   Select drive 1.
 22  INDEX*  I/O   Index is a pulse generated once per disk 
                      revolution between the end and beginning of 
                      cylinders.  The 8520 can be programmed to 
                      conditionally generate a level 6 interrupt 
                      to the 68000 whenever the INDEX* input goes 
                      active.
 23  +12V          160 ma maximum; 540 ma surge
 

3] External Disk Connector Identification Mode
4]--------------------------------------------------
 
 An identification mode is provided for reading a 32 bit serial
 identification data stream from an external device.  To initialize
 this mode, the motor must be turned on then off.  See pin 8,
 MTRXD* for a discussion of how to turn the motor on and off. The
 transition from motor on to motor off reinitializes the serial
 shift register. After initialization, the SELxB* signal should be 
 left in the inactive state. Now enter a loop where SELxB* is driven
 active, read serial input data on RDY* (pin 1), and drive SELxB* 
 inactive.  Repeat this loop a total of 32 times to read in 32 bits 
 of data.  The most significant bit is received first.
 

3] External Disk Connector Defined Identifications
4]--------------------------------------------------
 
      $0000 0000 - no drive present
      $FFFF FFFF - Amiga standard 3.25 diskette
      $5555 5555 - 48 TPI double density double sided
 
 
 As with other peripheral ID's, users should contact Commodore-Amiga
 (mailbox:  amiga) for ID assignment. The serial input data is 
 active low and must therefore be inverted to be consistant with the
 above table.
 
3] External Disk Connector Limitations
4]--------------------------------------------------
 
 1. The total cable length including daisy chaining must
    not exceed 1 meter.
 
 2. A maximum of 3 external devices may reside on this interface.
 
 3. Each device must provide a 1000 Ohm pull up resistor on those
    outputs driven by an open collector device on the Amiga
    (pins 8-10, 16-21).