A2091 Reference

-----------------------------------------------------------------------------

  The A2091 is a full-length Zorro-II DMA SCSI hard drive controller that was
  originally introduced with the A2000HD. Because of the A2091's high
  availability, it is often found in A4000s, even though it performs very
  slowly in them.

  The A2091 can't use DMA to transfer data to 32-bit Fast RAM, and if
  Zorro-II DMA memory is not available, the driver falls back to programmed
  I/O transfers. In either case, this makes for extremely slow disk transfers
  (at worst, 50K/second; at best, no more than 1M/second). There are
  utilities in the "hard" directory of Aminet that may help alleviate this
  problem. (Side note: with some of these, adding some memory on the A2091
  may help by providing Zorro-II DMA-accessible memory for buffering.)

  ROM revisions are a common problem with the A2091; 6.6 or later ROMs are
  desirable, with 7.0 being the last version (strongly recommended, and
  necessary for 68040 machines). Most A2091s have revision 04 of the Western
  Digital 33C93 SCSI controller chip; replacing this with the 08 version may
  clear up SCSI bus problems. (Despite common belief, the "PROTO" marking is
  insignificant on either version of the WD chip; version 04 or version 08
  are the only significant values.)

  Sixteen sockets allow for the addition of up to 2M of 16-bit Fast RAM,
  using CMOS 256K x 4 DRAMs (44C256) rated at 120 ns or faster. (This is not
  normally very useful on the A4000, since a SIMM added to the motherboard
  RAM sockets is simpler to obtain and install, and will operate *much* more
  quickly.)

  The hard drive power connector on the board is not a reliable way to power
  a card-mounted hard drive. Use the connectors attached directly to the
  power supply instead.

  Usually looked on as a cheap (sometimes very cheap) way to access SCSI
  peripherals from the A4000, the A2091 is certainly better than no SCSI
  controller at all.

  A little-known and mostly-unused function of the A2091 is a built-in XT-IDE
  controller. The IDE connector and LED are not installed, but visible on the
  board to the left and upper-right of the SCSI chip. Unfortunately, this
  port will only work with 8-bit (XT) IDE hard drives, not the common 16-bit
  AT-IDE hard drives. This option was apparently mostly used for the 20M 
  drives supplied with the A590, the A500 hard disk option which shared the
  2091 design.

  Board Layout
  _________________________________________________________________
 | DRAM   _________  ::Memory       ...                            |___
 | ||||  | Odd ROM | ::Size         LED                            |
 | ||||  |_________|          ::                                   |
 |        _________        :: ::                                   |#
 | ||||  |Even ROM |       :: :: 50-Pin SCSI                       |# External
 | ||||  |_________|   Options:: Connector                         |# SCSI
 |                            ::                                   |# Port
 | ||||                       ::                                   |# (DB25)
 | ||||                       ::                                   |#
 |                            ::                                   |
 | ||||                        . Power                             |
 | ||||                        : Connector                         |
 |_________________________________________________________________|
     |||||||||||||||||||||||||||                                   |

  Jumpers

    JP1: Memory Size
           0K: Set for no memory.
         512K: Set for 512K of RAM.
           1M: Set for 1M of RAM.
           2M: Set for 2M of RAM.

    JP2: Autoboot Enable
         AUTO: Set to enable autobooting from the A2091.
          DIS: Set to disable autobooting from the A2091.

    JP3: Interrupt Select
         INT6: Select interrupt 6.
         INT2: Select interrupt 2.

    JP5: Options
         Option 1: LUN Enable. When jumpered, enables scanning for Logical
                   Unit Numbers numbers at each physical SCSI address.
                   Default value: Off.
         Option 2: Time-Out Length. When jumpered, enables longer time-out
                   for slow-starting drives. Default: Off. (Note: When on,
                   this jumper disables parity during the message in phase.)
         Option 3: Reserved. Factory default: Off.

    JP201: Factory use only.

  For pin-out information on the internal 50-pin header and external DB25
  SCSI connectors, see Drives/SCSI Pin-Outs.