A terminator is a set of resistors placed at each end of atransmission line, such as the SCSI (small systems computer interface)bus. Terminators help to prevent signal reflections at the ends of thetransmission line and ensures that the signal on the line quicklyreaches its desired state. A correctly terminated SCSI bus supportsfaster and more reliable operation. Each end of the physical SCSI busshould have one SCSI terminator attached.
Physically, there are two types of terminators: internal and external.There are no electrical differences between these two terminatortypes. An external terminator looks like a SCSI bus connector withoutany cable attached to it. An internal terminator usually consists oftwo or three resistor SIPs (single in-line packages). On some new SCSIdevices, the internal terminator is controlled by a switch or jumperon the device. For more information on internal termination in suchdevices, 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 inwidespread 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 notmarked, it most likely uses a passive terminator. An active terminatorincludes active components, such as transistors, along with resistors.Active terminators were defined in the SCSI-II standard and are fairlynew. Active terminators provide better termination response andsupport faster, more reliable bus operation. Some of the newer andfaster controllers should have active terminators. The Adaptecaha154xC drive requires active terminators. Installing activeterminators is a simple hardware modification that solves many SCSIbus problems.
To work properly, terminators require power from the SCSI bus. Thispower is referred to as Term Power. It is typically supplied by thehost adapter and by the other devices on the SCSI bus. Ideally, TermPower should be supplied by the last device on each end of the SCSIbus. In any case, at least one device on the bus must supply TermPower. In most configurations, Term Power is not a problem. However,some of the older FD8xx adapters do not supply Term Power; theseadapters were usually included with SCSI devices that supply TermPower. Term Power is normally controlled by a jumper or switch on theSCSI device. For more information on enabling Term Power, please referto your hardware documentation. To ensure that Term Power is properlysupplied, all devices connected to the SCSI bus should be powered onwhenever the bus is in use.
The quality of the SCSI cables also affects the reliability andusability of a SCSI system. The SCSI-II standard defines thecharacteristics of good cables. They are 100% shielded round cablesthat contain 25 twisted pairs of wires. Each pair should have acharacteristic impedance between 90 and 110 ohms. The wire gaugeshould be 26 or 28. Most SCSI cables do not meet these requirements.Adaptec recommends always using good SCSI cables with the aha154xCdisk drive. If you connect more than three devices to a SCSI bus, itis a good idea to always use high quality SCSI cables.
Some SCSI configurations work in MS-DOS but do not work in Windows NTbecause Windows NT uses SCSI more aggressively. Typically, Windows NTtransfers larger data blocks and uses faster data transfer modes thanMS-DOS does. Also, because Windows NT supports more devices on a SCSIbus, 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 activeterminators."
The following publications provide more information on SCSI:
Title: Building Fast SCSI subsystems
Available from: Technology FocusDistributed 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.