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
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
| DRAM _________ ::Memory ... |___
| |||| | Odd ROM | ::Size LED |
| |||| |_________| :: |
| _________ :: :: |#
| |||| |Even ROM | :: :: 50-Pin SCSI |# External
| |||| |_________| Options:: Connector |# SCSI
| :: |# Port
| |||| :: |# (DB25)
| |||| :: |#
| :: |
| |||| . Power |
| |||| : Connector |
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.
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.
Converted on 02 Jun 1997 with RexxDoesAmigaGuide2HTML 2.1 by Michael Ranner.