Ask THE SCSI EXPERT - Technology Information

Computer Technology Review,  Oct, 1999  

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

Q: I have an external CD-Writer with a SCSI connection. However, my laptop does not have a SCSI port. Could I use a special cable to connect the CD-Writer to my laptop via either the COMs port or the USB port?

A: Simply put, the answer is no. You cannot connect the two with a cable. Serial lines are not fast enough to support SCSI. It would require a Parallel Port-to-SCSI converter (if it is still possible to find one) or an USB-to-SCSI converter. The data throughput would be very slow. The best method would be to purchase a PCMCIA card SCSI adapter.

Q: How can I determine if my computer (hardware interface) uses SCSI commands? What is the difference between IDE and SCSI? I am particularly interested in the difference between IDE and SCSI hard drives. What are the advantages of SCSI over IDE?

A: If you are running Windows 95 or Windows 98, go to the Control Panel, then go to System and look for a SCSI controller in the Device Manager. If you are running one drive on a PC with Windows 95/98, you won't notice the difference between SCSI and IDE. However, if you are running Windows NT, you will see a significant performance increase over ATA/IDE. Additionally, when there are multiple drives on the bus, SCSI provides much greater performance. For ATA/IDE, the maximum capacity is 2 drives per bus, while SCSI allows up to 15 drives per bus. With ATAIIDE, while the drives run the task, the processor is tied up. With SCSI, tasks can be queued with several tasks running simultaneously. SCSI really shows a performance advantage on servers and workstations, and allows you to connect devices outside of the box.

Q: Our server has a Wide-Ultra SCSI-3 interface. We want to connect a SCSI-2 CD-RW to the server Is it possible? If so, how?

A: Yes, a narrow device can be connected to a wide bus. There are about 10 different ways, depending on your system configuration and adapter card.

The 3 most common ways are:

1. Your adapter card may have a 50 pin connection already for an internal device; you can only use this for an internal CD-RW if you are using an internal or external cable. You cannot have three cables connected to the adapter.

2. You can use an internal 68 to 50 pin adapter and one of the internal 68 pin cable slots.

3. You can use an adapter that converts 68 pin to 50 pin and terminates the high byte. This works internally or externally.

Q: We have several SCSI cables with termination. Some of these cables have twisted wires between the connectors and some of them are straightforward cables. Can an untwisted cable be used for connecting LVD drives to a LVD host-adapter or do you need to have the twisted cable?

A: The SCSI specifications do not require twisted pairs in SCSI cables. These specifications, however, do strongly recommend twisted pairs. Use of twisted pairs, making sure that signal pairs are assigned to twisted pairs, and location of the SCSI signal pairs within the cable help reduce crosstalk and maintain cable impedance. These factors become increasingly important with longer cable lengths and with increasing SCSI bus speech. Cables without twisted pairs may work satisfactorily in short lengths, but if you have sporadic SCSI bus errors, try changing the cables to twisted pair cables.

Cables are arguably the most important factor in getting your SCSI subsystem to run reliably and at maximum data throughput. It is risky to try to "save money" by buying inexpensive SCSI cables.

COPYRIGHT 1999 West World Productions, Inc.
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