SCSI Cables - HELP (The Official SCSI Cheat Sheet)
The Official SCSI Cheat Sheet
Newly Updated June, 2006!
Revised and further simplified thanks to all of your feedback - Thank you!
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, A2Z Cables Incorporated and/or I personally will not assume any responsibility. To the best of our knowledge, all data here is accurate.
I. Introduction
II. Types of SCSI: SE, LVD, HVD
III. SCSI Termination
IV. Frequently Asked Questions
V. Mixing 50-pin & 68-pin Devices - Confusion Simplified!
VI. Connector Pictures
Firstly, SCSI stands for “Small Computer Systems Interface” and is a data transmission interface that allows SCSI devices (controllers, terminators, drives, burners, scanners, etc.) to communicate along a SCSI chain. A SCSI chain is composed of either internal and/or external SCSI cables that connect SCSI devices together and requires SCSI terminators at each end. Note that some SCSI devices have terminators built into them, especially SCSI controllers, and are sometimes enabled or disabled using tiny jumper pins on the SCSI device. Each SCSI device on a SCSI chain must also have a unique SCSI ID which is usually set up using tiny jumper pins on each device but the number of the IDs have limitations (explained later in charts).
SE SCSI Includes all SCSI devices that do not say "LVD" or "HVD" or "Differential". SE SCSI must be terminated with an SE terminator which includes passive (old), active (very common), FPT (a deluxe active), active negation (uncommon) or LVD/SE (best), and each is backwards compatible to work with the one listed before it. When in doubt, buy an LVD/SE terminator and save yourself headaches. The maximum speed on an SE SCSI chain is 40MB/second, and cable quality (such as "LVD" grade) will NOT change this.
LVD SCSI The latest and greatest in SCSI connectivity. HOWEVER, your entire SCSI chain (controller, terminators & devices) must be LVD or LVD/SE to achieve LVD benefits of speed, length and number of devices. If just one device is not LVD the entire chain will default to SE SCSI limitations (see above). The same goes for Ultra160 LVD or Ultra320 LVD - all devices on the chain must be Ultra160 LVD or Ultra320 LVD respectively.
HVD SCSI Also known as "Differential" SCSI, this is the least common SCSI type but allows longer cabling distances. HVD SCSI requires that the entire chain (controller, devices and terminators) be HVD (Differential). HVD cannot mix with SE or LVD SCSI.
Now that you know what type of SCSI you will be working with, please see the appropriate chart below for specifications.
Okay, let's keep it simple. Here are a list of rules for properly terminating a SCSI chain:
1. SE SCSI: SE SCSI must be terminated with an SE terminator which includes passive (old), active (very common), FPT (deluxe active), active negation (uncommon) or LVD/SE (best). When in doubt, buy an LVD/SE terminator and save yourself headaches. LVD/SE terminators are backwards compatible to work as any of those listed here and can be used later on if you upgrade your chain or devices to LVD.
2. LVD SCSI: To achieve all the speed, distance and number of device benefits of LVD SCSI (see table above), your entire chain (controller, devices, terminators and cables) must be LVD or LVD/SE. If you have just one SE or HVD SCSI device, your chain will either shut down or run in SE mode (and any device IDs above 7 will become inaccessible). The same rule applies to Ultra2, Ultra3, Ultra160 and Ultra320 LVD: An LVD SCSI chain will only go as fast as the slowest LVD device on the SCSI chain (including terminators & cables)
3. HVD SCSI: Also known as "Differential" SCSI, HVD SCSI requires that the entire chain (controller, devices and terminators) be HVD (Differential). HVD cannot mix with SE or LVD SCSI.
Q: What is the difference between NARROW SCSI (50-pin or 25-pin)and WIDE SCSI (80-pin or 68-pin)?
A: Narrow SCSI uses fewer wires and a 1-byte (8-bit) bus, and Wide SCSI uses more wires and a 2-byte (16-bit) bus.
Q: What is a HIGH-BYTE TERMINATOR?
A: A high-byte terminator allows WIDE SCSI (see above) and NARROW SCSI (see above) to be connected together by terminating the extra wires found in WIDE SCSI. Please see our section about mixing 50-pin and 68-pin SCSI.
Q: MUST I have a HIGH-BYTE TERMINATOR?
A: The short answer is "Not always", however it is recommended for proper error-free data transfer. Nevertheless, it is sometimes okay to do the following: 1. Internally, you can usually attach up to 3 50-pin devices onto the middle of a wide (68-wire) SCSI ribbon cable using a 50-pin/68-pin adapter WITHOUT high-byte termination. 2. Externally, on a SCSI chain with only a couple devices, you can sometimes attach 68-pin SCSI devices with 50-pin or 25-pin devices using a standard cable or adapter WITHOUT high-byte termination. HOWEVER, this is not recommended and many 68-pin devices will REQUIRE high-byte termination. So, when in doubt you should use an adapter with high-byte termination.
Q: What can you tell me about SCSI SPEEDS?
A: Generally, your SCSI chain will operate at the speed of your slowest SCSI device or terminator on the chain. Ultra2, Ultra3, Ultra160 or Ultra320 LVD requires that all devices meet the same level (or higher) of LVD respectively. One Exception: When mixing narrow and wide SCSI (see above), attaching the narrow SCSI devices onto the end of a wide SCSI chain using an adapter with high-byte termination will allow wide devices to achieve wide SE SCSI speeds and narrow to achieve narrow SE SCSI speeds (see SE SCSI chart above).
Q: What is ACTIVE NEGATION TERMINATION?
A: In SE SCSI, "active negation" terminators were specified for a small number of SE SCSI devices, however LVD/SE terminators are a recommended replacement and more useful for future upgrades and configurations.
Q: What is a FEED-THROUGH TERMINATOR (aka "PASS-THROUGH TERMINATOR")?
A: A feed-through or pass-through terminator allows you to connect AND terminate a SCSI device onto the end of your SCSI chain. In other words, SCSI data flows "through" the terminator, into the SCSI device, and back out to be terminated as the end of the SCSI chain.
Below are several diagrams (internal and external) showing how to properly connect 50-pin and 68-pin SCSI devices together. Here are some general rules to follow:
1. Terms to Know: Let's keep it simple: "Wide" SCSI means 68-pins or 80-pins and a 2-byte bus width. "Narrow" SCSI means 50-pins or 25-pins and a 1-byte bus width.
2. Grouping: Generally, it is best to keep wide SCSI devices grouped together and narrow SCSI devices grouped together. You can then make a wide-to-narrow or narrow-to-wide "bridge" to join them.
3. High-Byte Terminators: Again, "narrow" means 50-pin or 25-pin SCSI devices (with a 1-byte bus), and "wide" means 68-pin or 80-pin SCSI devices (with a 2-byte bus). Although a simple 50-pin/68-pin adapter can sometimes be used to connect narrow and wide SCSI devices, it is not recommended. Although you might get your devices to "talk" to each other, the recommended way to connect narrow and wide SCSI devices is by using a high-byte terminator - Adapters that include termination for the extra wires/pins found on the 68-pin or 80-pin side. Nevertheless, it is usually okay to do the following: 1. Internally, you can usually attach up to 3 50-pin devices onto the middle of a wide (68-wire) SCSI ribbon cable using a 50-pin/68-pin adapter WITHOUT high-byte termination. 2. Externally, on a SCSI chain with only a couple devices, you can sometimes attach 68-pin SCSI devices with 50-pin or 25-pin devices using a standard cable or adapter WITHOUT high-byte termination. HOWEVER, this is not recommended and many 68-pin devices will REQUIRE high-byte termination. So, when in doubt you should use an adapter with high-byte termination.
4. Maximizing Data Speed: Unless done properly, mixing narrow SCSI and wide SCSI on the same chain will force ALL devices to slow down to narrow SCSI speeds. When using a narrow SCSI controller, there is no way to avoid slowing your chain down to narrow SCSI speeds. HOWEVER, when using a wide SCSI controller, it is possible to have your wide SCSI devices operate at wide SCSI speeds and narrow SCSI devices operate at narrow SCSI speeds. The secret is to keep the wide SCSI devices grouped in a chain with a wide SCSI contoller and the narrow SCSI device(s) attached to the end of the wide chain and separated by a high-byte terminator/adapter (a wide-to-narrow "bridge"). Important Note: Although you can achieve "wide" SCSI speeds when using a high-byte terminator, it is not possible to achieve "LVD" speeds when mixing narrow and wide SCSI. As a rule, when mixing LVD SCSI with narrow and/or wide SE SCSI, all devices on the chain will operate in SE SCSI mode (see SE SCSI table and LVD SCSI table above).
5. SCSI ID's: When using a narrow SCSI controller (50-pin or 25-pin), you must set all SCSI ID's to be lower than 7 since narrow SCSI cannot see ID's higher than 7. (7 is usually the controller.)
Newly Updated June, 2006!
Revised and further simplified thanks to all of your feedback - Thank you!
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, A2Z Cables Incorporated and/or I personally will not assume any responsibility. To the best of our knowledge, all data here is accurate.
Table of Contents
I. Introduction
II. Types of SCSI: SE, LVD, HVD
III. SCSI Termination
IV. Frequently Asked Questions
V. Mixing 50-pin & 68-pin Devices - Confusion Simplified!
VI. Connector Pictures
Introduction
Firstly, SCSI stands for “Small Computer Systems Interface” and is a data transmission interface that allows SCSI devices (controllers, terminators, drives, burners, scanners, etc.) to communicate along a SCSI chain. A SCSI chain is composed of either internal and/or external SCSI cables that connect SCSI devices together and requires SCSI terminators at each end. Note that some SCSI devices have terminators built into them, especially SCSI controllers, and are sometimes enabled or disabled using tiny jumper pins on the SCSI device. Each SCSI device on a SCSI chain must also have a unique SCSI ID which is usually set up using tiny jumper pins on each device but the number of the IDs have limitations (explained later in charts).
The Three Types of SCSI - SE, LVD, HVD
SE SCSI Includes all SCSI devices that do not say "LVD" or "HVD" or "Differential". SE SCSI must be terminated with an SE terminator which includes passive (old), active (very common), FPT (a deluxe active), active negation (uncommon) or LVD/SE (best), and each is backwards compatible to work with the one listed before it. When in doubt, buy an LVD/SE terminator and save yourself headaches. The maximum speed on an SE SCSI chain is 40MB/second, and cable quality (such as "LVD" grade) will NOT change this.
LVD SCSI The latest and greatest in SCSI connectivity. HOWEVER, your entire SCSI chain (controller, terminators & devices) must be LVD or LVD/SE to achieve LVD benefits of speed, length and number of devices. If just one device is not LVD the entire chain will default to SE SCSI limitations (see above). The same goes for Ultra160 LVD or Ultra320 LVD - all devices on the chain must be Ultra160 LVD or Ultra320 LVD respectively.
HVD SCSI Also known as "Differential" SCSI, this is the least common SCSI type but allows longer cabling distances. HVD SCSI requires that the entire chain (controller, devices and terminators) be HVD (Differential). HVD cannot mix with SE or LVD SCSI.
Now that you know what type of SCSI you will be working with, please see the appropriate chart below for specifications.
SCSI Termination
Okay, let's keep it simple. Here are a list of rules for properly terminating a SCSI chain:
1. SE SCSI: SE SCSI must be terminated with an SE terminator which includes passive (old), active (very common), FPT (deluxe active), active negation (uncommon) or LVD/SE (best). When in doubt, buy an LVD/SE terminator and save yourself headaches. LVD/SE terminators are backwards compatible to work as any of those listed here and can be used later on if you upgrade your chain or devices to LVD.
2. LVD SCSI: To achieve all the speed, distance and number of device benefits of LVD SCSI (see table above), your entire chain (controller, devices, terminators and cables) must be LVD or LVD/SE. If you have just one SE or HVD SCSI device, your chain will either shut down or run in SE mode (and any device IDs above 7 will become inaccessible). The same rule applies to Ultra2, Ultra3, Ultra160 and Ultra320 LVD: An LVD SCSI chain will only go as fast as the slowest LVD device on the SCSI chain (including terminators & cables)
3. HVD SCSI: Also known as "Differential" SCSI, HVD SCSI requires that the entire chain (controller, devices and terminators) be HVD (Differential). HVD cannot mix with SE or LVD SCSI.
Frequently Asked Questions & Common Misunderstandings
Q: What is the difference between NARROW SCSI (50-pin or 25-pin)and WIDE SCSI (80-pin or 68-pin)?
A: Narrow SCSI uses fewer wires and a 1-byte (8-bit) bus, and Wide SCSI uses more wires and a 2-byte (16-bit) bus.
Q: What is a HIGH-BYTE TERMINATOR?
A: A high-byte terminator allows WIDE SCSI (see above) and NARROW SCSI (see above) to be connected together by terminating the extra wires found in WIDE SCSI. Please see our section about mixing 50-pin and 68-pin SCSI.
Q: MUST I have a HIGH-BYTE TERMINATOR?
A: The short answer is "Not always", however it is recommended for proper error-free data transfer. Nevertheless, it is sometimes okay to do the following: 1. Internally, you can usually attach up to 3 50-pin devices onto the middle of a wide (68-wire) SCSI ribbon cable using a 50-pin/68-pin adapter WITHOUT high-byte termination. 2. Externally, on a SCSI chain with only a couple devices, you can sometimes attach 68-pin SCSI devices with 50-pin or 25-pin devices using a standard cable or adapter WITHOUT high-byte termination. HOWEVER, this is not recommended and many 68-pin devices will REQUIRE high-byte termination. So, when in doubt you should use an adapter with high-byte termination.
Q: What can you tell me about SCSI SPEEDS?
A: Generally, your SCSI chain will operate at the speed of your slowest SCSI device or terminator on the chain. Ultra2, Ultra3, Ultra160 or Ultra320 LVD requires that all devices meet the same level (or higher) of LVD respectively. One Exception: When mixing narrow and wide SCSI (see above), attaching the narrow SCSI devices onto the end of a wide SCSI chain using an adapter with high-byte termination will allow wide devices to achieve wide SE SCSI speeds and narrow to achieve narrow SE SCSI speeds (see SE SCSI chart above).
Q: What is ACTIVE NEGATION TERMINATION?
A: In SE SCSI, "active negation" terminators were specified for a small number of SE SCSI devices, however LVD/SE terminators are a recommended replacement and more useful for future upgrades and configurations.
Q: What is a FEED-THROUGH TERMINATOR (aka "PASS-THROUGH TERMINATOR")?
A: A feed-through or pass-through terminator allows you to connect AND terminate a SCSI device onto the end of your SCSI chain. In other words, SCSI data flows "through" the terminator, into the SCSI device, and back out to be terminated as the end of the SCSI chain.
HOW TO PROPERLY MIX 50-pin & 68-pin SCSI
Below are several diagrams (internal and external) showing how to properly connect 50-pin and 68-pin SCSI devices together. Here are some general rules to follow:
1. Terms to Know: Let's keep it simple: "Wide" SCSI means 68-pins or 80-pins and a 2-byte bus width. "Narrow" SCSI means 50-pins or 25-pins and a 1-byte bus width.
2. Grouping: Generally, it is best to keep wide SCSI devices grouped together and narrow SCSI devices grouped together. You can then make a wide-to-narrow or narrow-to-wide "bridge" to join them.
3. High-Byte Terminators: Again, "narrow" means 50-pin or 25-pin SCSI devices (with a 1-byte bus), and "wide" means 68-pin or 80-pin SCSI devices (with a 2-byte bus). Although a simple 50-pin/68-pin adapter can sometimes be used to connect narrow and wide SCSI devices, it is not recommended. Although you might get your devices to "talk" to each other, the recommended way to connect narrow and wide SCSI devices is by using a high-byte terminator - Adapters that include termination for the extra wires/pins found on the 68-pin or 80-pin side. Nevertheless, it is usually okay to do the following: 1. Internally, you can usually attach up to 3 50-pin devices onto the middle of a wide (68-wire) SCSI ribbon cable using a 50-pin/68-pin adapter WITHOUT high-byte termination. 2. Externally, on a SCSI chain with only a couple devices, you can sometimes attach 68-pin SCSI devices with 50-pin or 25-pin devices using a standard cable or adapter WITHOUT high-byte termination. HOWEVER, this is not recommended and many 68-pin devices will REQUIRE high-byte termination. So, when in doubt you should use an adapter with high-byte termination.
4. Maximizing Data Speed: Unless done properly, mixing narrow SCSI and wide SCSI on the same chain will force ALL devices to slow down to narrow SCSI speeds. When using a narrow SCSI controller, there is no way to avoid slowing your chain down to narrow SCSI speeds. HOWEVER, when using a wide SCSI controller, it is possible to have your wide SCSI devices operate at wide SCSI speeds and narrow SCSI devices operate at narrow SCSI speeds. The secret is to keep the wide SCSI devices grouped in a chain with a wide SCSI contoller and the narrow SCSI device(s) attached to the end of the wide chain and separated by a high-byte terminator/adapter (a wide-to-narrow "bridge"). Important Note: Although you can achieve "wide" SCSI speeds when using a high-byte terminator, it is not possible to achieve "LVD" speeds when mixing narrow and wide SCSI. As a rule, when mixing LVD SCSI with narrow and/or wide SE SCSI, all devices on the chain will operate in SE SCSI mode (see SE SCSI table and LVD SCSI table above).
5. SCSI ID's: When using a narrow SCSI controller (50-pin or 25-pin), you must set all SCSI ID's to be lower than 7 since narrow SCSI cannot see ID's higher than 7. (7 is usually the controller.)
Mixing Wide & Narrow SCSI Examples:
SCSI Connectors
