SCSI Cables, Connectors and Terminators

SCSI is a parallel bus, not a point-to-point link, and that single fact explains most of what goes wrong with it. Every device shares the same set of signal wires, the bus must be terminated at both physical ends, and each device needs its own address. Get any of those three wrong and the symptom is usually the same: the controller fails to see a drive, or the whole bus drops to the slowest device’s speed, or a scan hangs at boot.

The cable is the part people replace first, and often the wrong part. SCSI cable types are defined by width (how many data bits travel at once), by connector, and by the electrical signalling the bus uses. Those three things have to match the controller, the devices and the terminator.

Narrow and wide: how many bits, how many IDs

Narrow SCSI is an 8-bit bus. It uses 50-pin connectors and supports up to 8 IDs including the controller, so seven devices. Wide SCSI is a 16-bit bus, uses 68-pin connectors and supports up to 16 IDs, so fifteen devices. A wide controller can usually run narrow devices through an adapter, but the adapter has to handle termination correctly or the bus becomes unreliable.

The HD68 connector is the one most people meet on a wide controller or an external enclosure. It is a high-density 68-pin plug, and it is not the same as the 68-pin VHDCI connector used on some controller cards, which is smaller and needs its own cable. Buying the wrong one is a common and expensive mistake.

Speeds and the generations behind them

SCSI speed is quoted in megabytes per second, not gigabits, because it is a parallel bus clocked in step with the data. The generations ran Fast SCSI 10 MB/s, Fast Wide 20 MB/s, Ultra Wide 40 MB/s, Ultra2 Wide 80 MB/s, Ultra160 160 MB/s and Ultra320 320 MB/s. A cable rated for Ultra320 will run slower devices, but a cable rated only for Ultra Wide will not carry Ultra320 reliably, and the bus will negotiate down or produce errors.

SE, LVD and HVD: the rule that protects your hardware

Single-ended (SE) SCSI drives each signal against a common ground return. It is the oldest and shortest-reach form. Low voltage differential (LVD) drives each signal as a pair of opposing voltages, which cancels noise and allows longer buses. High voltage differential (HVD) does the same at much higher voltage swing, and it is the one that matters most here.

Never connect HVD devices to an SE or LVD bus. The voltage levels are incompatible and the attempt can destroy the SE or LVD transceivers. HVD equipment is rare, usually older and often marked clearly, but if a device’s signalling type is unknown, do not guess. LVD buses run up to 12 m (39 ft) with several devices, or 25 m (82 ft) point to point. SE buses are much shorter, around 3 m for Fast SCSI. If a bus is longer than its signalling allows, the usual result is intermittent errors under load, not a clean failure.

Termination: both ends, and only there

The SCSI bus is terminated at both physical ends and nowhere else. That means a terminator at the controller end and a terminator at the far end of the cable chain. If a device in the middle has its terminator enabled, the bus is terminated in three places and will misbehave. If neither end is terminated, the signals reflect and the bus is unreliable.

Active terminators are used on LVD and Ultra buses. They regulate the termination voltage rather than relying on a simple resistor, which matters as speeds rise. Passive terminators belong on older, slower SE buses only. A missing terminator often shows up as a device that appears during the controller’s scan but fails when data is written.

Cable length and device count

Length limits depend on the signalling type and how many devices are on the bus. LVD buses reach 12 m (39 ft) with several devices, or 25 m (82 ft) point to point. SE buses are much shorter, around 3 m for Fast SCSI. Adding devices shortens the usable length, and adding cable stubs off the main run makes it worse. Keep the chain as short and as straight as the installation allows.

What goes wrong, and what will not work

Mixing signalling types is the failure that damages hardware. HVD into SE or LVD can destroy transceivers, and there is no adapter that makes it safe.

Wrong terminator type is the most common fault. A passive terminator on an LVD bus, or an active terminator on an old SE bus, will cause errors that look like bad cables. Terminating the middle of the chain instead of the ends causes the same symptoms.

Duplicate SCSI IDs stop the bus working. Every device, including the controller, needs a unique ID, and the controller is usually ID 7 on narrow buses.

Using a narrow cable on a wide bus, or the reverse, without the correct adapter, either leaves half the data lines unconnected or shorts them. The adapter must also pass termination through correctly.

A cable rated below the bus speed will negotiate down or produce errors. Ultra320 cable on an Ultra160 bus is fine; the reverse is not.

If the equipment is being replaced rather than repaired, the modern equivalents are covered in SAS vs SATA cables and connectors, where the signalling is point-to-point and termination is not a user concern.

Practical checks before you buy

Confirm the bus width and connector on the controller. Confirm the signalling type (SE, LVD or HVD) on every device. Confirm the cable’s rated speed. Confirm where the two ends of the chain are, and that only those two points are terminated. If a device has a terminator jumper or switch, set it deliberately rather than leaving it at whatever it was when the drive was pulled from another machine.

For older serial equipment alongside SCSI, the pinouts are different and the voltage levels are not interchangeable; see RS-232 serial pinouts: DB9 and DB25 for the connector wiring on that side.