A4091 Reference
                                by Bob Emery

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  The A4091 is a full-length Zorro-III DMA Fast SCSI-2 hard drive controller
  designed for the A4000. It is based on the NCR 53C710 chip. (Editor's note:
  this is the same Fast SCSI controller chip used in the Warp Engine.)

  Unlike some similar products, it is only a Fast SCSI-2 controller; it has
  no sockets for additional RAM. The A4091 package includes an active
  terminator and a custom ribbon cable, which is nicely folded to accommodate
  up to five internal drives, one on-board and a pair in each of the front
  and rear bays.

  The A4091 requires a revision 11 Buster, which Commodore supplied loose
  with the card. It works with A3640 3.1 and 3.2 revisions, but not 3.0.

  The board is theoretically capable of 10 Megabytes per second transfer
  rates.

  Board Layout
  _________________________________________________________________
 | ... ... LED      :::::::::::::::                                |___
 |                    50-Pin SCSI                                  |
 | ||| ||| |||         Connector                                   |
 | ||| ||| ||| |||                                                 |#
 |             |||      ||||||                                     |# External
 | ||| ||| ||| |||      ||||||                                     |# HiDensity
 | ||| ||| ||| |||      ||||||                                     |# SCSI-2
 |                      ||||||                                     |# Connector
 | |||     |||                                                     |#
 | ||| ||| |||                                                     |=sw 8
 |     |||                      . Power                            |=   :
 |                              : Connector                        |=   :
 |_________________________________________________________________|=sw 1
     ||||||||||||||||||||||||||||||                                |

  LED Connectors

          To DSPLY     To MB

          CN351        CN308
         -------------------------
         | o o o       o o o
         |
         | N R B       B R N
         | C e l       l e C
         |   d a       a d
         |     c       c
         |     k       k
         |


  Unplug the Front Panel LED cable from the connector on the motherboard and
  plug it into the CN351. To make the LED operate for IDE drives as well, use
  the extra cable supplied with the A4091 to connect CN308 to the 
  motherboard.

  Rear Connector View

  ---------------------------------------------------------------
 |                                                               |
 |             DIP Switch            External SCSI-2 Connector   |
 |         _________________                                     |
 |    ON  |                 |        1                       25  |
 |        | 1 2 3 4 5 6 7 8 |      {}:::::::::::::::::::::::::{} |
 |    OFF |_________________|        26                      50  |
 |                                                               |
  ---------------------------------------------------------------

  DIP Switch Settings, defaults are all OFF


  SCSI Address of Card

            0     1     2     3     4     5     6     7

    SW1    ON    OFF   ON    OFF   ON    OFF   ON    OFF
    SW2    ON    ON    OFF   OFF   ON    ON    OFF   OFF
    SW3    ON    ON    ON    ON    OFF   OFF   OFF   OFF


  SCSI Fast Bus

    SW4    OFF indicates that the SCSI Fast Bus feature is enabled.
           Set this switch to ON if none of your SCSI devices
           support SCSI Fast Bus.

  Short/Long Spinup

    SW5    OFF indicates that your system uses the standard spinup
           (booting) time. Set this switch to ON to request a longer
           booting period. If one of your SCSI devices has a long
           power-on cycle, the Amiga may not recognise it during the
           standard booting period.

  Synchronous Mode

    SW6    OFF indicates that the synchronous mode feature is enabled.
           Set this switch to ON to disable synchronous mode. Synchronous
           mode does not require acknowledgement for every byte transmitted,
           which can mean improved response time with most SCSI devices.

  External SCSI Termination

    SW7    OFF indicates that you do not have any external devices.
           This activates the terminator on the board since this is
           one end of the SCSI bus. Set this switch to ON when you
           install an external device. This disables the termination
           on the board since it is now in the middle of the SCSI bus
           (not at the end).

  Logical Unit (LUN) Enable

    SW8    OFF indicates that unit 0 is the only unit recognized. Set
           this switch to on to enable the system to recognize 1-6 as
           LUNs.