The Official SCSI Cheat Sheet
to read an entire SCSI book just to solve a SCSI configuration problem, right?
Well, after helping people through hundreds of SCSI problems, I finally put
together this SCSI cabling "Cheat Sheet" to make life easier for others. If you
are baffled by terms such as SCSI 1, Ultra, SCSI 2, Wide, LVD and such,
this sheet is for you! However, I recommend forgetting the terms SCSI 1, 2, 3
and 5 because these terms complicate matters. Also, please read the entire
sheet before making any SCSI decisions. Have Fun!
The information on this page is for educational use only. If you damage or
destroy any data or hardware using any information provided on these pages, I
will not assume any responsibility. To the best of my knowledge, all data here
is accurate. If you feel any changes or updates are needed, please email them to
me at email@example.com and I
will consider them.
SCSI stands for “Small Computer Systems Interface” and is a computer bus
interface that allows SCSI devices (SCSI controllers, SCSI hard drives, SCSI
tape drives, SCSI scanners, etc.) to communicate along a SCSI chain. A SCSI
chain is composed of either internal or external SCSI cables (or both) that
attach SCSI devices together and requires SCSI terminators at each end of the
SCSI chain. Some SCSI devices have terminators built into them. SCSI cabling is
also confusing because SCSI standards and terminology have evolved and grown
more complicated over time.
The 3 Types of SCSI Signals - SE, LVD,
There are 3 types of SCSI signals,
and each must be terminated with the appropriate terminators. SE is terminated
with active, passive, FPT or LVD/SE terminators. LVD is terminated with LVD
terminators. HVD or "Differential" is terminated with HVD or "Differential"
terminators. You cannot mix termination types on the same SCSI chain. The only
exception to this is when an LVD/SE terminator or device is used in an LVD SCSI
chain OR in an SE SCSI chain.
SCSI terminators must be placed onto
or just beyond the last SCSI device on a SCSI chain. SCSI controllers usually
have automatically enabled/disabled termination or offer jumper pin caps that
can be placed onto pins to enable/disable termination. “High-byte” terminators
are not the same as terminators because they go somewhere in the middle of your
SCSI chain and terminate only "some" of the 68 or 80 wires in wide SCSI chain
and allow the other wires to continue to the end of your terminated SCSI chain.
See below for more details.
Passive Terminators -
Used in SCSI-1 Cabling (see SE SCSI chart above) when only one or two SCSI
devices are on your SCSI chain.
Active Terminators - Used in SE SCSI Cabling (see chart). Active
terminators are SE terminators.
FPT Terminators ( Forced Perfect Terminators ) - Often used
instead of active terminators in Narrow SE SCSI cabling (see chart) for long
HVD Terminators (High Voltage Differential Terminators) - Often
simply called "Differential" terminators, HVD terminators MUST BE USED with HVD
SCSI devices and ONLY with HVD SCSI devices.
Active Negation Terminators - These terminators offer better
termination than active terminators, however I recommend simply using an LVD/SE
terminator when your hardware calls for an active negation terminator because it
offers better termination and will be more useful in the future.
Feed-through Terminators (Pass-through Terminators ) - Used
when there is no place to attach a terminator at the end of a SCSI chain. Placed
between your SCSI cable and last SCSI device. Feed-Through terminators can be
passive, active, SE, LVD, etc.
LVD Terminators (Low Voltage Differential Terminators ) -
Required for LVD SCSI cabling (see LVD SCSI chart). LVD/SE terminators will
automatically work in either SE or LVD SCSI mode, however ALL devices on a SCSI
chain must be LVD to achieve LVD benefits (again, see the LVD SCSI chart). LVD
ONLY terminators will not work in SE mode and will shut down a SCSI chain.
Important Notes: To achieve Ultra2 LVD or Ultra3 LVD speeds (see LVD chart), you
must use a corresponding Ultra2 LVD or Ultra3 LVD terminator.
High Byte Terminators (50-Pin/68-Pin SCSI Adapters) – See my
“68-Pin to 50-Pin or 25-Pin Adapters” section and my “High-Byte Termination
Common SCSI Misunderstandings
Narrow SCSI -
Narrow SCSI means an 8-bit data bus and uses a 25-pin or 50-pin connector.
Wide SCSI - Wide SCSI means a 16-bit data bus and uses a 68 or 80-pin
Speed - No, LVD 68-pin and 80-pin devices cannot reach maximum speed when
they are adapted to 25-pin or 50-pin (SE SCSI) connectors anywhere in your SCSI
chain. However, 68-pin and 80-pin SE SCSI devices can perform at faster speeds
when placed BETWEEN your SCSI controller and your 25-pin or 50-pin SE SCSI
devices, but you will need a high-byte terminator/adapter (see section below).
Also, you cannot achieve LVD SCSI speeds if you have an active terminator (an SE
type of terminator) or SE SCSI device (non LVD) on your SCSI chain, sorry.
68-Pin to 50-Pin or 25-Pin Cables (Beware!) - 68-Pin to 50-Pin external
SCSI cables will work when the 25-pin or 50-pin side is towards the SCSI
controller and the 68-pin side is towards the SCSI device you want to attach
UNLESS the cable has a built-in high-byte terminator. To connect a 25-pin or
50-pin external SCSI device to either a 68-pin SCSI controller or to a 68-pin
SCSI device, you will need a 68-Pin/50-Pin SCSI adapter with a high-byte
terminator built in plus a 50-pin to 50-pin cable or 50-pin to 25-pin SCSI
cable. See my next section and my “High-Byte Termination Situations” diagram.
68-Pin to 50-Pin or 25-Pin Adapters (Beware!) -
When connecting 68-Pin devices onto a 25-pin
or 50-pin SCSI cable, you simply need a 68-pin to 50-pin adapter (not an adapter
with high-byte termination). However, often a 68-Pin/50-Pin SCSI adapter with
a built-in high-byte terminator will be required. SCSI adapters with
high-byte terminators are needed only when you are using a 68-pin
connector on your SCSI controller card and want to connect 25-pin or 50-pin
devices onto the end of your 68-pin SCSI chain. If you are using a 50-pin
connector on your SCSI controller, you do not have to worry about high-byte
termination. Externally, high-byte terminators are required when
attaching 50-pin SCSI devices to a 68-pin SCSI controller card or a 68-pin SCSI
device. Internally, high-byte terminators are required when attaching a
50-pin internal cable to a 68-pin SCSI controller card or when the last device
on a 68-pin internal cable is a 50-pin SCSI device. Internally, it is okay to
use a standard 50-pin/68-pin adapter without high-byte termination
when placed in the middle of your SCSI chain and your last device on the 68-pin
internal cable is a 68-pin device with termination. The point is this: A
high-byte terminator will terminate the 18 wires that will not continue on to
your 50-pin SCSI device(s). Note: 68 - 50 = 18 wires. See also my “High-Byte
Termination Situations” diagram.
80-Pin SCSI Adapters - SCA80
adapters (some include built-in terminators) are often used to connect an 80-pin
SCSI drive to either a 50-pin or 68-pin internal SCSI cable. BEFORE plugging an
80-pin SCSI adapter into your drive, FIRST connect your internal power connector
and 50-Pin or 68-Pin SCSI cable to your 80-pin adapter and then plug the adapter
into the 80-pin drive.
SCSI IDs - Each device on
your SCSI chain must have a unique SCSI ID on your SCSI chain. You can set up
the ID on each device by using tiny jumper pin caps that are usually provided on
SCSI devices. SCSI controllers are usually set to an ID of 7. For 25-pin or
50-pin SCSI devices, your IDs must be between 0 and 6. For 68-pin or 80-pin
devices, an ID between 0 and 17 must be used but NOT 7 because this is usually
reserved for the SCSI controller.
High-Byte Terminators - This is where people get most confused. Please
see my “High-Byte Terminator Situations” section below, my “68-Pin to 50-Pin or
25-Pin Adapters” section and my “68-Pin to 50-Pin or 25-Pin Cables” section.