SCSI cables, terminators and adapters support legacy storage, server, scanner, laboratory and industrial systems that use parallel Small Computer System Interface connections. Wallco carries internal and external SCSI cable assemblies, physical interface adapters and terminators across multiple 25-, 50- and 68-contact connector formats used by narrow, wide and later high-density SCSI implementations.
A correct SCSI replacement requires more than matching the number of contacts. Parallel SCSI systems may use single-ended, low-voltage differential (LVD) or high-voltage differential (HVD) signaling, and those electrical interfaces are not universally interchangeable. Connector families also include DB, Centronics-style, high-density and very-high-density formats, so the equipment interface and signaling method should both be verified.
Termination is part of the SCSI bus design. Parallel SCSI buses are terminated at their physical ends, and the terminator must be compatible with the bus signaling. Cable length, device placement, bus width and termination all affect signal integrity, making careful identification important when repairing or extending legacy equipment.
Start by identifying the connector on both devices and counting contacts where possible. Legacy SCSI equipment has used DB25, 50-contact Centronics-style interfaces, 50-pin high-density connectors, 68-pin high-density connectors, VHDCI and other configurations. Similar descriptions such as SCSI-2 or SCSI-3 do not uniquely identify the physical connector.
Also determine whether the bus is narrow or wide and whether the application is internal or external. Narrow SCSI commonly uses an 8-bit data path, while wide SCSI uses a 16-bit data path and is commonly associated with 68-contact interfaces. Cable and adapter selection must preserve the connections required by the controller and device.
Parallel SCSI signaling can be single-ended, low-voltage differential or high-voltage differential. IBM documentation notes that HVD and single-ended SCSI are incompatible, while LVD systems have their own compatibility and bus-length considerations. Do not choose a cable or terminator solely because the connector fits; verify the electrical interface identified by the host adapter and connected equipment.
Some multimode LVD/SE equipment can operate with either LVD or single-ended devices, but adding a single-ended device can force the bus to operate under single-ended limitations. HVD equipment should not be connected to an LVD/SE bus unless the system documentation explicitly provides a compatible conversion method.
A parallel SCSI bus is terminated at both physical ends. The terminator may be built into a device or attached externally, depending on the system. T10 SCSI documentation specifies termination at both ends of a single-ended bus, and IBM examples show external terminators installed on the last enclosure in a device chain.
Use a terminator that matches the signaling type and connector. Improper termination, excessive cable length or incompatible devices can cause intermittent operation, transfer errors or complete loss of communication.
SCSI adapters are useful when compatible equipment uses different connector formats, such as transitions among DB25, Centronics 50, high-density 50 and high-density 68 interfaces. A passive adapter changes the physical interface; it does not make incompatible signaling standards electrically compatible. Confirm pin count, gender, signaling type and bus requirements before using an adapter in a live system.