It seems that the SCSI bus is one of the most misunderstood aspects of
connecting hard drives and other peripherals to the A4000 (or, for that
matter, any other Amiga model). This section of the guide is an attempt to
provide some simple examples of proper SCSI device connections. Please note
that in the following section, and in the Guide as a whole, I have used the
common term "controller" when referring to disk adapter boards, although
the more accurate description for both SCSI and IDE would be "host
Since understanding SCSI requires a background in the jargon, a few basic
definitions might be helpful:
This is the original standard, now also known as SCSI-1. The maximum
theoretical transfer rate is 5 megabytes per second, although most
combinations of drives and controllers do much less, usually less than
two megabytes per second. Total length of the SCSI bus cannot exceed six
An extension of the SCSI command set. Most CD-ROM drives that are double-
speed or faster are SCSI-2. Note that contrary to popular belief, this
doesn't go any faster than good old SCSI-1.
Here's where the speed was increased. Fast SCSI-2 has a maximum transfer
rate of 10 megabytes per second, synchronous. Again, this is theoretical,
and anything more than a third of that should be considered excellent.
Wide SCSI, Differential SCSI, and SCSI-3
SCSI transfers data over an 8-bit wide data path. A variation called Wide
SCSI uses a 16-bit wide data path via an additional cable, potentially
doubling transfer rates. SCSI-3 is essentially Fast Wide SCSI-2 using
only one cable. Another variation is differential SCSI, which uses
differential signal cables to provide a total bus length of up to 25
meters. None of these variations will be described in any detail here,
since there don't seem to be any Amiga implementations of controllers for
them. Adapters are available to connect these devices to normal SCSI
controllers, though, so it is possible to connect them to the Amiga.
SCSI devices are backwards-compatible. That is, you can connect a SCSI-1 or
SCSI-2 hard drive to a Fast SCSI-2 controller, or you can connect a Fast
SCSI-2 drive to a SCSI-1 or SCSI-2 controller. A Fast controller can't make
a SCSI-2 drive go any faster than SCSI-2, but it will work.
SCSI bus systems require an impedance-matching terminator circuit at each
end of the bus for reliable operation. Many people find termination to be
complex, but the subject can be simplified a great deal by remembering one
simple rule: the SCSI bus needs to be terminated at both ends, and *only*
at the ends.
The most common mistake in SCSI termination is assuming that the SCSI
controller itself doesn't count; in fact, it does count as a device, and
the termination rules apply to it just like other devices. Many Amiga
controllers have the termination resistors soldered into place, under the
assumption that only internal or only external SCSI devices will be
attached. If both internal and external devices are to be used, it is
necessary to remove these resistors. SIP sockets may be soldered in their
place to provide the greatest versatility, or you can just use external
Terminating resistors are usually SIP resistor packs; most are yellow,
blue, or black, and there may be one, two, or three of them. External
terminators look like a connector with no cable attached, and can be found
in Centronics 50-pin, DB25, and high-density 50 configurations. Some
devices, like newer hard drives or external CD-ROM drives, have a
single switch or jumper to enable termination.
All of the termination schemes described so far are known as "passive"
terminators. Electronically, they connect each signal pin to +5V through a
220 ohm resistor, and to ground through a 330 ohm resistor. This voltage
divider circuit provides impedance matching for the SCSI bus.
The alternative to a passive terminator is an "active" terminator, which
connects each of the SCSI signal pins through a 110 ohm resistor to a
precision +2.85V regulator (an LT1086CT, for example) which is powered by
+5V. Active terminators are superior to passive terminators simply because
they are active; unlike the fixed resistors in a passive terminator, the
active terminator's voltage regulator will track varying voltages and
properly terminate the SCSI bus. Active terminators can cure many problems
with unreliable SCSI devices; their only disadvantage is that they cost a
bit more (Dalco sells them for between thirty and forty dollars). Active
termination chips are made by Dallas Semiconductor and Texas Instruments.
Any combination of passive and active terminators may be used, although two
active terminators would be best. In practice, passive/passive or passive/
active are usually adequate.
Terminator power (+5V) is supposed to be supplied on pin 26 of the 50-pin
IDC header. But SCSI devices are not required to supply this power; many
have jumpers to enable or disable it. So it is possible to have a proper
termination setup, but no power provided to the terminators. As you might
expect, this will cause problems. Make sure that at least one device is
supplying termination power to the SCSI bus, preferably the controller,
since external devices may be turned off, which would deprive the rest of
the bus of termination power.
Internal SCSI devices are usually connected with 50-conductor ribbon cable.
50-pin IDC (Insulation Displacement Connector) headers are crimped onto the
cable for each device to be attached. "Stub" cables of no more than 3
centimeters off the main cable are allowed by the SCSI standard, but it's
better to avoid them altogether by running the cable direct from one device
to the next, with no branches off the main bus at all.
External SCSI device cables can use several connectors: Centronics 50-pin,
DB25, or high-density 50-pin (commonly, but imprecisely, referred to as
"SCSI-2", since many Fast SCSI-2 adapters use this type of connector).
Adapter cables may have any combination of these three basic types.
The SCSI standard states that the total length of the SCSI bus, including
internal and external cable, must not exceed six meters. For Fast SCSI-2,
the limit was reduced to three meters. In practice, some devices and cable
combinations may limit this severely, particularly cables with DB25
connectors (since Apple created the DB25 "pseudo-SCSI" cable by simply
discarding all those "extra" grounds that helped make SCSI capable of
running long distances in the first place). Conversely, some SCSI bus
implementations can go farther than the standard suggests.
SCSI Address Numbers
Each SCSI device (including the controller) has an address between 0 and
7 assigned to it by the user. These numbers are usually set as a binary
number with three jumpers. Controllers often have no jumpers, either
requiring software to change their address, or simply not being able to
change it at all. Standard Amiga controllers of either type default to a
SCSI address of 7.
The rules regulating addresses are pretty simple: each device must have
a unique address. (There is no physical "order" in which the addesses must
occur; you can use any order or combination of numbers, as long as there is
only one device with a given address.)
Since the Amiga scans the SCSI bus for bootable devices starting with
address 0 and proceeding to address 7, it is advised that you assign
address 0 to the boot hard drive, and set "HiID" to "On" for this drive in
the Rigid Disk Block (RDB). This will prevent the system for looking for
other hard drives with a higher boot priority, making for the quickest
booting possible, and preventing the system from trying to boot off of a
higher-numbered CD-ROM drive. (Check the Aminet disk/misc directory for
RDB utility programs.)
Logical Unit Numbers provide a way to access more than one device at a
given SCSI address. For example, some Adaptec SCSI-to-MFM adapter boards
like the 4000A could control two MFM hard drives. However, the 4000A board
used only a single SCSI address; to access each drive, a secondary number--
the LUN--was used: 0 for the first drive and 1 for the second. With modern
SCSI devices, LUNs are relatively rare, with the exception of CD-ROM
changers. These devices often use an LUN to select which CD is loaded.
Specific Troublesome Devices
There are a couple of devices out there that are almost guaranteed to be
troublemakers on a SCSI bus. Since this section of the Guide is also
distributed as the SCSI Examples document, these are included here even
though they don't necessarily apply to the A4000.
1. The NEC CDR-36 CD-ROM drive. This is a single-speed (150K/second)
external drive with a top-loading case. It may be helpful to disable
termination on this drive and use an external terminator; it may not.
If odd problems persist, check that pin 17 on the DB25 connector on the
cable is not grounded. If it is, disconnect that pin.
2. A3000 problems. The A3000's internal SCSI controller has a few minor
flaws that can be problematic.
A. The Western Digital 33C93 SCSI chip itself: revision 04 of this chip
has some bugs that usually show up when a CD-ROM or tape drive is
attached, and revision 08 fixes them.
B. The DB25 used as an external SCSI connector on the A3000 can cause
problems. Use only short, high-quality SCSI cables attached to this
connector, or run 50-pin ribbon cable from the internal connector.
C. Termination. Various A3000s seem to have come with no terminators,
soldered-in SIPs, or even sockets. Check the motherboard controller
termination, and follow the guidelines laid out in the Termination
D. Many A3000s had a manufacturing flaw which resulted in terminator
power not being supplied at the external SCSI connector. The easiest way
to test this is with an external terminator with an LED indicator.
Otherwise, you'll need to check pin 25 of the A3000 DB25 SCSI connector
for +5V (the shield around the connector provides an easy ground test
point). If no voltage is supplied on pin 25, diode D800 (or D801, this
may vary depending on motherboard revision) is reversed inside the
A3000. Unsolder and replace it (this should be a 1N34 type, although a
1N5817 should work and might be more suitable). The motherboard silk
screen is likely to be wrong as well, so ignore it.
3. Some GVP controllers. For a while, it seemed like all the email I
received was from owners of GVP controllers. For many of these people,
upgrading to the Guru ROM solved their problems. Disabling termination on
some of these boards is also non-trivial; Guru ROM author Ralph Babel
"Most GVP cards use only two 10-pin SIP terminators _plus_ two extra
resistors (SMD, except for the very first revisions of the Series-II hard
card) for the parity line for a total of 17 terminated lines (they leave
out the RST line)."
The use of SMD (Surface Mount Device) resistors complicates disabling
termination on these boards. It will be simplest for many users to
reorganize the SCSI bus so that the controller is on one end.
GVP's current technical support number is 215-633-7711.
Example SCSI Bus Setups
These examples show connections to the A2091 controller (see Drives/
A2091 Reference), but the connections for other controllers will follow the
In Example 1, the 200M hard drive is used as the boot drive, and the "HiID"
flag is set to "On" in this drive's Rigid Disk Block. (The HiID flag may be
called by another name, like LastDrive or HighDrive.) For examples 2 and 3,
the 540M drive is used as the boot drive, and the HiID flag is set in that
Example 1: 2091 controller, internal 200M SCSI-1 hard drive. Cable
connections are 50-conductor ribbon.
| 2091 | | 200M SCSI-1 |
| Terminated |------| Terminated |
| Address 7 | | Address 0 |
Example 2: 2091 controller, internal 200M SCSI-1 hard drive, internal 540M
Fast SCSI-2 hard drive. Cable connections are 50-conductor
ribbon. The SCSI-1 drive has been renumbered as address 1, and
the new Fast SCSI-2 drive is now set at address 0 and used as a
boot drive, to provide better performance on the system
partitions. (Even though it will only be accessed at SCSI-1
rates, it is a newer drive, and will probably have significantly
better transfer rates than the older 200M drive.) Additionally,
the newer drive will likely have the more desirable active
________________ ________________ ________________
| 2091 | | 200M SCSI-1 | |540M Fast SCSI-2|
| Terminated |------| Not Terminated |------| Terminated |
| Address 7 | | Address 1 | | Address 0 |
|________________| |________________| |________________|
Example 3: 2091 controller, internal 200M SCSI-1 hard drive, internal 540M
Fast SCSI-2 hard drive, external SCSI-2 CD-ROM drive. The cable
from the CD-ROM drive to the A2091 is a Centronics 50-pin to
DB25 adapter cable, and the internal cables are 50-conductor
ribbon. An active terminator is attached to the last available
external SCSI connector on the CD-ROM drive. Note that the
terminating resistors on the A2091 have been removed so that the
SCSI bus is terminated only at the ends (the CD-ROM and the 540M
| CD-ROM SCSI-2 |
| Terminated |---. External cable connected to A2091
| Address 4 | | external SCSI DB25 connector
____________|___ ________________ _______________
| 2091 | | 200M SCSI-1 | |540 Fast SCSI-2|
| Not Terminated |------| Not Terminated |------| Terminated |
| Address 7 | | Address 1 | | Address 0 |
|________________| |________________| |_______________|
Converted on 02 Jun 1997 with RexxDoesAmigaGuide2HTML 2.1 by Michael Ranner.