Ask THE SCSI EXPERT.

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

Q: What are the specifications related to LVD? Where can I obtain all information on the LVD SCSI specifications?

A: The, specifications related to LVD (Ultra2 SCSI-SPI-2 and Ultra l60 SCSI-SPI-3) SCSI are available from ANSI: www.ANSI.org, or the draft copies are on the SCSI Committee (T10) website: www.tlO.org. There are several documents related to LVD, but you should be primarily interested in the SPI-2, SPI-3, SPI-4 (draft only), and EPI documents.

Q: What is the difference between Fast SCSI and SCSI-2?

A: These two terms are synonymous in the way they are most frequently used. Fast SCSI (up to 10 Megatransfers per second, which is 10MB/sec for narrow SCSI or 20MB/sec for wide SCSI) is defined in the SCSI-2 specifications.

Sometimes the term "SCSI-2" is used to identify the 50 pin high density connector that was also introduced in the SCSI-2 specifications. Note: the wide bus was not defined until SCSI-3 SPI.

Q: What are the benefits of LVD?

A: The short answer is that LVD (Ultra2) SCSI is twice as fast as Ultra SCSI and the total SCSI cable length for LVD is up to 12 meters rather than the 1.5 meters of single ended Ultra SCSI.

Also, the lower voltage used for LVD SCSI allows the drivers to be on the SCSI ASIC. Another benefit is that, with multimode LVD (which almost all LVD devices are), manufacturers need make only one model that can be connected to either single ended or LVD SCSI buses.

These two things result in lower cost to the manufacturer, which is typically passed on to the consumer.

You did not ask specifically about Ultral60 SCSI, the current generation of LVD SCSI, or Ultra32O SCSI, which is now starting to appear on the market. Ultra32O SCSI is twice that of Ultral6O SCSI data throughput, and four times that of Ultra2 SCSI. Both Ultral60 SCSI and Ultra32O SCSI offer a number of new, very desirable features.

Q: Why doesn't a SCSI device have the option to enable DMA?

A: SCSI devices are inherently block transfers; the controllers always use DMA.

Q: Is SCSI backwards compatible? For example, is an, Ultra3 SCSI hard disk compatible with an Ultra Wide SCSI adapter card? We have an Adaptec card, AHA-2940W Is it compatibte with Ultra2 SCSI disks?

A: Ultra2 SCSI, Ultral60 SCSI, or Ultra32O SCSI devices are backward compatible with the; single ended (SE) SCSI bus as long as the interface is the multimode LVD (also called LVD/MSE) interface. When you connect a multimode LVD device to a SE bus, it switches to the SE mode with its limitations on data throughput and cable length. I have yet to see an LVD device that is not multimode.

The AHA-2940W is a Wide (16-bit), single ended SCSI host adapter so you can connect Ultra2 SCSI, Ultral6O SCSI, or Ultra320 SCSI multimode devices to it.