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The Official SCSI Cheat Sheet

Nobody wants 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!

Disclaimer: 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 scott@a2zcables.com and I will consider them.

Introduction
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, HVD

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.

 SE SCSI Specifications

 

LVD SCSI Specifications

 

HVD SCSI Specifications

 

SCSI Terminators

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 cabling distances. 

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 Situations” section.


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 connector.

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.

 

scsi high byte terminators

 

scsi cable external connectors

 scsi cable internal connectors

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Copyright © 2002 Øyvind Haugland
Sist endret:  13 januar 2019
 

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