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Visit the SCSI Trade Association's web site ( to ask the SCSI Expert a question.

Q: Does an official SCSI compliance checklist for target devices exist? Does a standard compliance test procedure for target devices exist? If so, where can I find them?

A: There is no official compliance test. The large system houses have purchase specifications that often go well beyond the standards. These system houses have their own sets, of tests. In most cases, all of the mandatory sections must be implemented and most system houses require several of the optional features too.

Q: I need to change the SCSI initiator ID on my controller, because I'm putting two computers on the same SCSI bus for a cluster application. The drives are SCSI targets 1-6.

What is the ordering of SCSI targets in order of priority when using Wide SCSI? Do targets go up to 15, or do they stop at 14?

A: The SCSI IDs, in order of priority, are: 07,06, 05, 04,03, 02, 01, 00, 15, 14, 13, ..., 08. On a Narrow SCSI bus, you can only use 07 through 00. On a Wide SCSI bus, you can use IDs 15 through 00, but 07 is still the highest priority. The two controllers should be set to ID 6 and 7, the boot disk drive ID 0, the user disks ID 8-15, with ID 3 - 5 for legacy equipment; Tape, Jaz Drives ID 3, CDROM, CDRW, DVD-RAM ID 4, and scanners ID 5.

Q: What are some examples of Low Voltage differential devices, High Voltage differential Devices and Single Ended devices?

A: Low Voltage differential devices are all Ultra2 SCSI and Ultra160 SCSI disk drives. Adapters such as the Adaptec AHA2930U2, AHA-2940U2W, Adaptec SCSI Card 19160, 29160 and 39160 or equivalents are also Low Voltage differential.

High Voltage differential devices include some disk drives up to Ultra SCSI speeds, tape drives, RAID boxes, and adapters such as the AHA2944UW or equivalent.

Single Ended devices go up to Ultra speeds: CD-R, DVD-RAM, earlier disk drives, tape drives, scanners, Jaz, Zip and removable media drives. There are too many Single Ended adapters to count, two such examples being the Adaptec AVA-2906 and AHA-2940U or equivalent.

Q: Is it possible to use a minated LVD/SE Ultra160 SCSI twisted pair ribbon cable designed for internal Ultra160 LVD SCSI drives to connect an external Ultra160 LVD SCSI drive?

A: Yes, it is possible to use a ribbon cable to connect external drives. However the ribbon cable is unshielded, so you may have noise problems, depending on the environment in which they are used. External cables are shielded for FCC reasons, so they will not cause noise on other equipment. I recommend that you use as short a cable as possible and only on a temporary basis.

Q: I recently received error messages that said SCSI transport failed: reason timeout: retrying command' in my systems logfiles. Is the time out duration specified in a SCSI standard, or does it depend on the systems implementation (i.e. drivers)?

If it is defined by the SCSI standard, how long is this time out?

A: Time out depends on the drivers but it is normally a very long time. Time out usually means that there is a bus problem. Check that there is active termination on both ends of the bus and, that all other termination is disabled. Be sure to, check your cables and connectors, too. Mismatched cables can cause bus reflections that will miscount that can hand the bus. Any single ended bus with a 25 pin connector will often have problems and hand the bus.
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Title Annotation:Technology Information; Questions and Answers
Publication:Computer Technology Review
Geographic Code:1USA
Date:Feb 1, 2001
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