CopyrightDown to the Technical Support belowCopyright
This web site was designed to be best viewed in 1280 x 1024 or wider using    MOZILLA's   FIREFOX
Our S C S I Technical Support / Technical Assistance Page, devised over the years by Unitech Electronics Pty. Ltd.
Phone: +61 2 9820 - 3555 sales@unitechelectronics.comFAX: +61 2 9603 - 8685
Updated September 5th 2011
The Unitech Electronics recognised Trade Mark and LogoWRITE DOWN THE PART NUMBER AND CLICK HERE FOR ORDERING PAGEProudly Displaying The Australian Flag
Take advantage of the huge benefits of shopping by credit card, safe, convenient and reliable.

Credit Card minimum order is AUD$ 20.00      USD$ 20.00 APPROX excluding freight and GST

<- Back to the Main Index page<- Back to the Amiga Technical information pageTo the bottom of this pageBack to the Amiga Items information page NEXT ->

Another Unitech Electronics Service !

The information contained herein is to be used as a guide only. Notice: This page will not be updated

SCSI Examples
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 adapter".


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 meters. A "ringing" effect is measurable on the "grounds" ( unconnected ).


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.

Fast SCSI-2

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 terminators.

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.

Termination Power
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.

Cable Configurations

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 address's 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 "Hi ID" 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 slow 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 mother board controller termination, and follow the guidelines laid out in the Termination section above.

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 mother board revision) is reversed inside the A3000. Un-solder and replace it (this should be a 1N34 type, although a 1N5817 should work and might be more suitable). The mother board 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 that was 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 explains:

"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.

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 same standard.

In Example 1, the 200M hard drive is used as the boot drive, and the "Hi ID" flag is set to "On" in this drive's Rigid Disk Block. (The Hi ID flag may be called by another name, like Last drive or High drive.) For example 2, the 1.08GB drive is used as the boot drive, and the Hi ID flag is set in that drive's RDB. In examples 3 and 4 the 4.2GB is used as the boot drive using the Hi ID Flag to register at the booting hard drive in the sequence of drives

Click on the " FURTHER INFO " gadget ( BELOW ) to view the correct wiring of 50Way SCSI

To the scsi cable diagram page

Example 1: ( below )

2091 controller, internal 200M SCSI-1 hard drive.
Cable connections are 50-conductor ribbon.

SCSI 1 Pix

2091 and 200M SCSI-1 Terminated as Address 7

Example 2: ( below )

2091 controller, internal 200M SCSI-1 hard drive, internal 1.08GB Fast SCSI-2 hard drive. Cable connections are 50-conductor ribbon. The SCSI-1 drive has been re numbered as address 7, and the new Fast SCSI-2 drive is now set at address 1 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 termination on-board.

SCSI 2 Pix

2091 and 200M SCSI-1 Terminated as Address 7 and 1.08GB SCSI-1 Terminated as Address 1

Example 3: ( below )

2091 controller, internal 200MB SCSI-1 hard drive, internal 1.08GB Fast SCSI-2 hard drive and 4.2GB Fast SCSI-2 hard drive. Cable connections utilised are the standard 50-conductor ribbon. The old SCSI-1 drive has been re numbered as address 7, and the newer 1.08 GB hard drive is set as address 1, also the new 4.2GB 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 drives will likely have the more desirable active termination on-board.

SCSI 3 Pix

2091 and 200M SCSI-1 Terminated as Address 7 and 1.08GB SCSI-1 Terminated as Address 1

Example 4: ( below )

2091 controller, internal 200M SCSI-1 hard drive,  internal 1.08GB Fast SCSI-2 hard drive,  external SCSI-2 CD-ROM
drive.   The cable from the CD-ROM drive to the A2091 is the  standard 50-wire IDC ribbon cable, the terminator
on the CD-ROM drive is now active.   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 200 MB drive ).

SCSI 4 Pix

A2091,connected with to an internal 200MB SCSI-1 Address 7 not terminated and internal
1.08GB Fast SCSI-2 Not Terminated Address 1, CDROM is Terminated as Address 5

OK ... Confused ?? ... And who said SCSI was an exact science !.. :-).ED.webmaster

Added note: If you are not an AMIGA person, you may be looking at this page and thinking why are
these hard drive so ridiculously small ?.  Simple answer,  the Amiga has a very efficient operating
system,  that works within six 880K floppy disks,  roughly less that 6 Megabytes in comparison to
an IBM compatible's bloat ware,  AMIGA has efficient coding,  that is the Amiga Operating System !

Disclaimer: The information contained herein on this web site is believed to be accurate.
No responsibilities are taken for typographical errors or omissions or errors of any kind.

This layout is Copyright Unitech Electronics Pty. Ltd. 1989 - 2002


To the Free circuits page

To the Amiga parts - Amiga chips - Amiga ram - Amiga Software

This website, its graphics concept and its design and layout are Copyright 1985 - 2011 to
Unitech Electronics Pty. Ltd. Est. 1978.      E. & O. E.      We ship globally from our Sydney, Australia office

<- Back to the main index page<- Go back to the Amiga Technical information pageTo the top of this pageBack to the Amiga Items information page NEXT ->