It seems that the SCSI bus is one of the most misunderstood aspects of connecting SCSI 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 certain background in the jargon, a few basic definitions that 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. The total length of the SCSI bus cannot exceed six meters. A "ringing" effect is measurable on the
"grounds" ( unconnected ). There are measures to counteract this "ringing phenonomen.
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. It is probably a marketing ploy or term.
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.
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, it simply cannot happen, however it will work at normal SCSI 2 speeds.
SCSI bus systems require an impedance-matching terminator circuit at each end of the bus for reliable operation.
Many folks find termination to be complex, but the subject can be simplified a great deal by remembering one
very simple rule: the SCSI bus needs to be terminated at both ends and *only* at the ends. Never in the middle.
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 the
common Centronics 50-pin, DB25, and high-density 50 configurations. Some devices, like newer hard drives or
newer 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 (These sell for between thirty-Five and fifty dollars). Active termination chips are manufactured by both Dallas
Semiconductor and Ti (Texas Instruments).
Any combination of passive and active terminators may be used, although two active terminators would be prefered.
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 cm
(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. Generally SCSI-2 uses 50 individual wires.
The SCSI standard states that the total length of the SCSI bus, including internal and external cable, must not
exceed 6M (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 D25 connectors ( since Apple created the D25
"pseudo-SCSI" cable by simply discarding all those "extra" grounds that helped make SCSI capable of running long
distances in the first place). The earth wires in between each data wire facilitated a pseudo cross-talk shielding.
This "hack" of removing the extra 25 wires was removed from the equation by Apple and these removed wires were
actually earth return wires.
Conversely, some SCSI bus implementations can go farther than the standard suggests. This is by experimentation.
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, thus, 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 however, other address "mode" can be implements as more SCSI
devices are added to the system. Eight ( 8 ) is the maximum that can be connected. Numbering from 0 to 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) of the Amiga's hard drive. This is refererred to as similar to the "BAT" or Block Allocation Table
in an IBM compatible P.C's Hard drive. 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 software programs.)
The drive selection can naturally be selected in the "Start-Up Sequence" of any Amiga running a SCSI device.
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.
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 and shown as the SCSI Examples document, these are included
here even though they don't necessarily apply to the AMIGA A4000 Desktop (with SCSI card) and the A4000T.
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 or, 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. Bing that this a relatively "older" device now, this is worthy of noting anyhow.
2. A3000 problems. The A3000's internal SCSI controller has a few mino 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. Shielding on the MOBO chip helps.
B. The D25 used as an external SCSI connector on the Amiga 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
D. Many Amiga 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 Logic Probe.
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. Easier though, just look for the diode itself.
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)." GVP is short for Great Vally Products, from King Of Prussia, a town in the USA.
The use of SMD (Surface Mount Device) resistors complicates disabling termination on these boards. It will
be simplest for many users to re-organize the SCSI bus so that the controller is on one end.
These examples show connections to the A2091 controller (see Drives / A2091 Reference), but the connections
for other controllers will follow the same standard. Note the A2090 card had its own mysterious SCSI issues.
In Example 1, the 200MB 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
A2091 controller, internal 200MB 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 200MB drive.) Additionally, the newer drive will likely have the more desirable active termination on
its own small controlling motherboard. Partitioning your hard drive is fairly easy and for the most part of the life of
your hard drive, uneventful.
You cannot exceed a hard drive's partitioning once done. It can be altered, naturally from any size you choose,
however once you have a partition, if you want to alter it, then save your vauable data either onto another hard drive
or burn it to a 680MB CD. Repartitioning can be done once you have backed up all your valuable data. It is wise to
formart each partition you have cleaned out and set up as new partitions. Choose slow format, it's more thorough.
A2091 controller, internal 200MB SCSI-1 hard drive, internal 1.08GB Fast SCSI-2 hard drive and 4.2GB Fast SCSI
hard drive. Cable connections utilised are the standard 50-conductor ribbon. The old SCSI-1 200MB HD1 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.
(Notes: 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
A2091 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 ).
1.08GB Fast SCSI-2 Not Terminated Address 1, the CDROM is Terminated as Address 5
Added note: If you are not an AMIGA person, you may be looking at this page and thinking why are these hard
drives so ridiculously small ?. Simple answer, the Amiga has a very efficient operating system, that works within
the total capacity of 6, yes, six 880K floppy disks, roughly about 5.28 Megabytes on hard drive in comparison
to an IBM compatible's bloat ware, AMIGA has efficient coding, that is the Amiga Operating System !