Article ID: 101352 - View products that this article applies to.
This article was previously published under Q101352
A terminator is a set of resistors placed at each end of a transmission line, such as the SCSI (small systems computer interface) bus. Terminators help to prevent signal reflections at the ends of the transmission line and ensures that the signal on the line quickly reaches its desired state. A correctly terminated SCSI bus supports faster and more reliable operation. Each end of the physical SCSI bus should have one SCSI terminator attached.
Physically, there are two types of terminators: internal and external. There are no electrical differences between these two terminator types. An external terminator looks like a SCSI bus connector without any cable attached to it. An internal terminator usually consists of two or three resistor SIPs (single in-line packages). On some new SCSI devices, the internal terminator is controlled by a switch or jumper on the device. For more information on internal termination in such devices, please refer to the device documentation.
CAUTION: Some external devices, such as SCSI disks, have internal terminators installed. The only method to detect these terminators involves opening the cabinet and inspecting the device. Remove internal terminators from external devices.
Electrically, there are two types of terminators: passive and active. Passive terminators were defined in the SCSI-I standard and are in widespread use today. A passive terminator is just a set of resisters. If you do not know what type terminator your device uses and it is not marked, it most likely uses a passive terminator. An active terminator includes active components, such as transistors, along with resistors. Active terminators were defined in the SCSI-II standard and are fairly new. Active terminators provide better termination response and support faster, more reliable bus operation. Some of the newer and faster controllers should have active terminators. The Adaptec aha154xC drive requires active terminators. Installing active terminators is a simple hardware modification that solves many SCSI bus problems.
To work properly, terminators require power from the SCSI bus. This power is referred to as Term Power. It is typically supplied by the host adapter and by the other devices on the SCSI bus. Ideally, Term Power should be supplied by the last device on each end of the SCSI bus. In any case, at least one device on the bus must supply Term Power. In most configurations, Term Power is not a problem. However, some of the older FD8xx adapters do not supply Term Power; these adapters were usually included with SCSI devices that supply Term Power. Term Power is normally controlled by a jumper or switch on the SCSI device. For more information on enabling Term Power, please refer to your hardware documentation. To ensure that Term Power is properly supplied, all devices connected to the SCSI bus should be powered on whenever the bus is in use.
The quality of the SCSI cables also affects the reliability and usability of a SCSI system. The SCSI-II standard defines the characteristics of good cables. They are 100% shielded round cables that contain 25 twisted pairs of wires. Each pair should have a characteristic impedance between 90 and 110 ohms. The wire gauge should be 26 or 28. Most SCSI cables do not meet these requirements. Adaptec recommends always using good SCSI cables with the aha154xC disk drive. If you connect more than three devices to a SCSI bus, it is a good idea to always use high quality SCSI cables.
Some SCSI configurations work in MS-DOS but do not work in Windows NT because Windows NT uses SCSI more aggressively. Typically, Windows NT transfers larger data blocks and uses faster data transfer modes than MS-DOS does. Also, because Windows NT supports more devices on a SCSI bus, termination issues become more important.
Also see knowledge base article:
TITLED : Problems attaching SCSI-2 devices to SCSI-3 adapter
The following vendors make SCSI cables:
Amphenol Interconnect Products* (607) 786-4221
Quintec Interconnect Systems (408) 272-8000
Icontec (408) 945-7766
* Amphenol also makes active terminators called "SCSI ALT. 2 active terminators."
The following publications provide more information on SCSI:
Title: Building Fast SCSI subsystems
Available from: Technology Focus Distributed Processing Technology
140 Candace Drive
Maitland, FL 32571
Phone: (407) 830-5522
Comment: Useful for people building large servers with SCSI.
Title: The SCSI Bench Reference (copyright 1989)
Author: Jeffrey D. Stai
Available from: ENDL Publications
14426 Black Walnut Court
Saratoga, CA 95070
Phone: (408) 867-6642
Comment: A technical guide for engineers and application support personnel who are working with SCSI.
Title: SCSI: Understanding the Small Computer System Interface
Author: John B. Lohmeyer (Chairman, X3T9.2) NCR Corporation
Publisher: Prentice-Hall, New Jersey
Title: Fast Track to SCSI: A Product Guide (copyright 1991)
Author: Fujitsu Microelectronics, Inc.
Publisher: Prentice-Hall, New Jersey
Comment: This is partially an introduction to SCSI and partially a Fujitsu SCSI product specification/catalog.
Article ID: 101352 - Last Review: February 20, 2007 - Revision: 2.3