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 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. However, it can be useful to provide memory available to the hard drive controller on the 2091, allowing for better speed as explained above.)

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.

The version 7 ROM fixes the following bugs in the 6.6 ROM
- >1GB HD support
- DMA reselection bug fixed
- disconnect/reconnect performance improved a lot
- Sync messages are handled
- Error handling improved
- Sync problems with Seagate/Syquest fixed
- Removable media recognition fixed
- Slow boot jumper now does a 15 second wait plus 300ms scan

Thanks to Greg Scott (National Amiga), Eric Bailey, Dave Haynie, Warren Block and Sebastian Kinner