SCSI Cables and Connectors: The Ultimate Guide

Introduction

SCSI, short for Small Computer System Interface, is a set of standards for physically connecting and transferring data between computers and peripheral devices. Developed in the 1980s, SCSI has been through many revisions and was once widely used on servers, workstations and Apple Macintosh computers. While no longer as prevalent as it once was, SCSI is still relevant in some legacy systems and specialized applications.

In this comprehensive guide, we‘ll dive deep into the world of SCSI cables and connectors. We‘ll explore the different types and specifications, discuss key considerations when choosing SCSI cables, and recommend some of the best options still available today. Whether you‘re supporting an aging server or simply curious about this classic technology, read on to become a SCSI cabling expert!

A Brief History of SCSI

The origins of SCSI can be traced back to 1979 and a company called Shugart Associates. They developed the SASI (Shugart Associates System Interface) to standardize the interface between computers and their disk drives. In 1981, SASI was renamed SCSI and further refined. The X3T9.2 committee of the American National Standards Institute (ANSI) would formalize it as a standard in 1986.

One of the key advantages of SCSI was its ability to connect multiple devices to a single computer. Up to 8 devices (including the host adapter) could be linked together in a daisy chain configuration. Later versions increased this to 16. This flexibility made SCSI popular for servers that needed to support many disk drives.

SCSI has gone through multiple generations over the years, with each new standard bringing increased performance:

Standard	Bus Width	Max Speed	Max Devices	Max Length
SCSI-1	8-bit	5 MB/s	8	6 meters
SCSI-2 (Fast)	8-bit	10 MB/s	8	3 meters
SCSI-3 (Ultra)	8/16-bit	20/40 MB/s	8/16	3 meters
Ultra2 SCSI	8/16-bit	40/80 MB/s	8/16	12 meters
Ultra3 (160)	16-bit	160 MB/s	16	12 meters
Ultra-320	16-bit	320 MB/s	16	12 meters
Ultra-640	16-bit	640 MB/s	16	12 meters

As you can see, SCSI has come a long way from the original 5 MB/s standard. However, by the early 2000s, SCSI was starting to be replaced by newer interfaces like SATA and SAS which offered higher speeds, lower costs, and didn‘t require termination.

SCSI Connectors Explained

One of the most visible aspects of SCSI are the many connector types used over the years. Let‘s take a closer look at the most common ones:

DB25

The DB25 connector was used on the very first SCSI-1 devices. It has a D-shaped shell with 25 pins/sockets in 2 rows. While it resembles the common parallel printer port, they are not interchangeable. The DB25 was quickly replaced by the 50-pin Centronics connector in later SCSI-1 devices.

50-pin IDC (Centronics)

The Centronics 50 connector became the standard for 8-bit SCSI devices. It uses a 50 conductor ribbon cable with insulation displacement pins. Two rows of 25 pins are surrounded by a metal shroud. This connector is still found on many 8-bit SCSI devices.

50-pin High Density (HD50)

As SCSI speeds increased, the bulky Centronics 50 connector was replaced by the High Density 50 (HD50) connector. It packs 50 pins into a much smaller D-shaped shell. HD50 uses either a 50-wire ribbon cable for internal devices, or a round shielded cable for external connections. This connector was very common on SCSI-2 and SCSI-3 devices.

68-pin High Density (HD68)

To support 16-bit wide SCSI buses, the 68-pin HD68 connector was introduced. It looks similar to HD50 but is slightly longer to fit the extra 18 pins. HD68 connectors are found on many Ultra2 and Ultra160 SCSI devices.

80-pin SCA (Single Connector Attachment)

The 80-pin SCA connector was used on some hot-swappable SCSI drives in the late 1990s. It combines power and data into a single compact connector. However, SCA never gained wide adoption and was soon made obsolete by SATA and SAS drives.

VHDCI (Very High Density Cable Interconnect)

The VHDCI connector further miniaturized SCSI into a 68-pin connector half the size of HD68. It was commonly used for connecting external Ultra160 and Ultra320 SCSI devices. VHDCI cables often have an HD68 connector on the other end for backward compatibility.

It‘s important to note that SCSI connectors come in both male (pins) and female (sockets) versions. When choosing cables, you need to ensure the genders match your devices. Internal ribbon cables typically have female connectors, while external cables have male connectors.

SCSI Termination and Signals

To function properly, a SCSI bus needs the correct termination and signal voltages on each wire. Termination is the placement of resistors at each end of the bus to prevent signal reflections. Without proper termination, signals can echo back and forth causing data corruption.

On 8-bit SCSI buses, termination is done by placing a 220 ohm resistor on each data line to +5V and a 330 ohm to ground. The 16-bit buses used in later standards reduce this to 110 ohms. Many SCSI devices have built-in active terminators that can be enabled with a switch or jumper. External passive terminators that plug into the last device are also available.

Only the devices at each end of the chain should be terminated. For example, on a SCSI bus with 3 devices, the host adapter and the last device would be terminated, while the middle device is unterminated.

Proper termination is just one aspect of ensuring signal integrity. The SCSI standards also specify minimum and maximum voltages for each signal. For example, the 8-bit standards use standard TTL logic with a minimum output high voltage (Voh) of 2.7V. Later standards like Ultra2 and Ultra160 use low voltage differential (LVD) signaling with a 400mV voltage swing.

Meeting these tight electrical requirements is one reason why it‘s important to use high-quality SCSI cables and connectors. Low-grade cables can degrade signals and cause intermittent operation.

Choosing the Best SCSI Cables

With all the different SCSI standards and connectors out there, what factors should you consider when choosing cables? Here are my top 5 tips:

Connector Type – Make sure you get cables with the right connectors for your devices. Check if you need HD50, HD68, VHDCI, etc. and if the connectors need to be male or female. Using adapters is possible but can degrade signal quality.
Cable Length – Stick to the minimum cable length needed to make your connections. Longer cables are more susceptible to external interference and signal attenuation. Each SCSI standard specifies a maximum cumulative cable length (see table above). Going beyond these limits is asking for trouble.
SCSI Standard – Check what SCSI standard your host adapter and devices support. There‘s no point buying expensive Ultra320 cables for an old SCSI-2 system. At the same time, don‘t bottleneck your Ultra160 drives with a SCSI-1 cable not rated for those speeds.
Shielding – For best signal integrity and protection from electromagnetic interference (EMI), look for cables with high-quality shielding. Foil and braid shielding are commonly used. Some cables may advertise double or even triple shielding for added protection.
Brand Reputation – Not all SCSI cables are created equal. Stick with reputable brands like Adaptec, Amphenol and Tripp-Lite that use high-grade materials and have a track record of quality. Saving a few bucks on a no-name cable isn‘t worth the risk of intermittent problems down the road.

By following these guidelines and sticking with products from well-known vendors, you can avoid many of the pitfalls that give SCSI a reputation as a finicky interface. In the next section, we‘ll recommend specific cable models for various scenarios.

Best SCSI Cables in 2023

Despite the decline of SCSI in mainstream computing, there are still many options available from specialty retailers and online marketplaces. Here are my top picks for various use cases:

Best Internal Ultra320 Cable

Adaptec 2279100-R 0.5m 68-pin LVD U320 SCSI Cable

This Adaptec cable is ideal for connecting Ultra320 SCSI drives inside a server or workstation. It has 4 female HD68 connectors: 1 for the host adapter, 2 for drives, and 1 for termination. The short 0.5 meter length is perfect for installations with tight clearances. Adaptec‘s build quality is top-notch with 30 micron gold-plated contacts for long-term reliability.

Best External HD68 Cable

Tripp-Lite U336-01M 1m Ultra-320 SCSI HD68 Cable

When you need to connect an external Ultra320 SCSI device, this Tripp-Lite cable gets the job done. It has male HD68 connectors on each end and is double-shielded for excellent signal integrity. The flexible round cable is easier to manage than a ribbon cable. At 1 meter long, it‘s suitable for most external connections without excess length.

Best HD50 to Centronics 50 Cable

Amphenol CS-SCSI8C50 0.8m HD50 to Centronics 50 Cable

If you need to connect an older 50-pin Centronics SCSI device to a newer controller with HD50 connectors, this Amphenol cable is the solution. It has a female HD50 connector on one end and a female Centronics 50 (IDC) on the other. The 0.8 meter length provides flexibility in positioning devices. Foil and braid shielding maintains good signal quality despite the transition between connector types.

Best External VHDCI Cable

Adaptec ACK-68VHD-10 1m VHDCI to HD68 Cable

This Adaptec cable is a great choice for connecting an external VHDCI SCSI device to an internal HD68 connector. It has a male VHDCI connector on one end and a male HD68 on the other, with a rugged metal backshell for strain relief. The 1 meter length is ideal for most external connections. Double shielding provides excellent protection against EMI.

Of course, these are just a few examples of the many SCSI cables available. Your specific needs may vary. But by sticking with reputable brands and following the selection tips above, you can find the perfect cable for your application.

Installation and Troubleshooting

Installing SCSI devices can be a bit more involved than with more modern plug-and-play interfaces. Here are some tips to make the process go smoothly:

Make sure each device has a unique SCSI ID number set. This is usually done with jumpers or DIP switches on the device. Valid IDs are 0-7 for 8-bit buses and 0-15 for 16-bit. Avoid using ID 7 as it has special meaning for the host adapter.
Verify proper termination. Only the devices at each end of the SCSI chain should be terminated. Don‘t forget to terminate the host adapter (it counts as a device too). Some SCSI cards allow termination to be set via software.
Keep cables as short as possible, especially for fast standards like Ultra160 and above. Avoid sharp bends that could damage the wires.
Make sure all connectors are firmly seated. Ribbon cables can sometimes partially pop out if not fully latched. External cables should have jackscrews to keep them secure.
Don‘t connect or disconnect SCSI devices while the system is powered on. SCSI doesn‘t support hot-plugging and you may damage the devices.

Even with these precautions, problems can still occur. Some common SCSI trouble spots include:

Termination – Incorrect termination is a frequent culprit behind SCSI woes. Double-check which devices should be terminated.
Cable issues – Cables can fail due to physical damage, bent pins, or worn-out connectors. Try swapping suspect cables with known good ones.
Unique IDs – If two devices are set to the same ID, neither will work correctly. Make sure each device has its own unique ID.
Driver problems – In some cases, you may need to load a specific driver for your SCSI controller. Check with the manufacturer for the latest drivers.

If you‘re still having trouble, try removing devices from the SCSI chain one at a time to isolate the problem. You can also invest in a SCSI bus analyzer tool to diagnose tricky problems.

The Future of SCSI

It‘s clear that SCSI‘s heyday as a mainstream storage interface has passed. The faster, simpler, and more affordable SATA interface has become ubiquitous for internal drives. For external devices, USB, Thunderbolt, and eSATA offer superior speed and plug-and-play connectivity.

So is SCSI completely obsolete? Not quite. In certain niche applications, SCSI remains relevant. Some examples include:

Legacy systems – Older servers, workstations, and industrial computers may rely on SCSI peripherals. Keeping these systems running requires continued access to SCSI cables and connectors.
Tape drives – Many backup tape drives and libraries still use SCSI or Fibre Channel interfaces. With tape‘s continued role in enterprise data archiving, expect to see SCSI in these devices for some time.
Scientific instruments – Some high-end scientific equipment like gene sequencers and mass spectrometers use SCSI for its ability to scale to high drive counts.
Military and aviation – Older SCSI-based systems are still used in some military vehicles and aircraft. The long lifecycles in these industries mean SCSI will likely hang on longer here.

While SCSI may no longer be a cutting-edge technology, it still has a role to play in keeping legacy systems and specialty equipment productive. For those who need it, understanding SCSI cable and connector types remains a valuable skill.

Conclusion

We‘ve covered a lot of ground in this deep dive into SCSI cables and connectors. From the early days of 8-bit SCSI-1 to the blazing fast Ultra320, SCSI has left its mark on the history of computing. While newer interfaces have largely superseded it, SCSI remains an important technology to understand for those working with legacy systems and certain niche applications.

By learning the different SCSI standards, connector types, and cabling best practices, you can keep older SCSI-based systems running smoothly. Investing in high-quality cables from reputable vendors is one of the best ways to minimize problems.

I hope this guide has boosted your SCSI knowledge and given you the confidence to tackle your next SCSI project. Feel free to refer back to the connector pinouts, cable recommendations and troubleshooting tips. Stay curious and keep learning!