The SCSI Page

(I decided that I had to again update this page, after finding out from my host's statistics, that this is my most accessed page, more than four times my home page, and knowing that it was really broken.)

IMHO, go SCSI or go home. SCSI offers way better compatibility and performance over any IDE hack . And you can own SCSI for just a little extra over IDE.

SCSI Terms and Definitions

The SCSI Standards

SCSI Hardware

The SCSI II Ribbon Cable

SCSI Products

What is SCSI?

SCSI, an acronym for Small Computer Systems Interface, is simply a standard for connecting devices to a computer. The simplest setup is commonly a SCSI hard drive connected to a SCSI host controller with a SCSI bus, most commonly a 50 conductor ribbon cable. It is very easy to add up to seven SCSI devices to the single cable on a single controller.

SCSI I was the original 1986 SCSI standard that only supported few devices at a maximum 5 MB/s transfer rate. While you may not be able to buy SCSI I controllers anymore, SCSI I devices still abound, and are useable on new controllers. That is just one SCSI advantage, backward compatibility.

SCSI II also normally supports seven devices, but on a 10 MB/s bus. These devices can be any type imaginable, including hard drives, tape drives, CD ROMs. scanners, printers, etc. The bus is intelligent, and runs itself without any CPU involvement, thus requiring much less overhead than ATA and IDE crap.

SCSI III is the newest standard with many more extensions, including Fibre and Ultra SCSI.

For much more information, check out the text SCSI FAQ or in HTML.

Also, valuable information is available in the (now un-maintained) Linux SCSI howto.

For even more information, get the full SCSI I standard or the SCSI II standard, both which are in text and Info zipped, or the SCSI III overview (417KB) in PDF format.

If you want the latest on SCSI III, you will need to get the files from the ftp site. Note that there is much more information for SCSI III:

Hardware:

Read Gary Field's Rulebook.

The NCR 810 PCI SCSI II controllers are now owned and made by LSI It is just a single chip, and is available on the ASUS SC200 (C$70), and also a controller from LSI (NCR) (PDF). Note that the 810 does not have a BIOS, and needs a motherboard with built in NCR BIOS such as ASUS and Gigabyte.

Visit Adaptec, who build SCSI controllers. My Hippo has a 2842, which is an AIC7770 based 32 bit VLB SCSI controller, but I have replaced it with an old AMI, as I need the extra 16 MB cache. I also have a 2940 narrow, which uses the 2870 chipset, in my PPro.

Check out the Quantum White Papers, dealing mostly with new technology.

Do you know about Ultra SCSI ? This essentially doubles the SCSI bus bandwidth, with the same cabling and devices on the same old bus. All that happens is the the bus frequency is doubled, although the cable can only be half as long. With the new Seagate Ultra SCSI Cheetah, and the IBM Ultrastar drives, transferring at over 40 MB/s, you will need the extra bandwidth.

Find out about the super fast Fibre SCSI from Quantum, or Seagate.

A report about the new Magnetoresistive Head Technology (MR) and also the synergistic relationship between MR and PRML read heads on hard drives.

Do you have an Ultrastor controller, and you want to use a removable drive?

The SCSI II ribbon cable:

I researched this, and the SCSI II specification calls for a maximum 6 m cable with a 100 mm maximum stub length and a minimum 300 mm length between stubs. Termination must be at the SCSI cable ends.

An answer that I received was that the stub to stub length should be three times the stub length (that's where the .3 m and .1 m from above come from), so I measured my devices, and found that 30 mm was the realistic maximum stub length. so that a minimum 90 mm between stubs should suffice for most situations. This helps reduce all the excess cable that would be present with 300 mm spacing.