ask THE SCSI EXPERT - Technology Information

Computer Technology Review, Dec, 2000

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Q: We have an Ultra2 LVD SCSI controller and 15 UItra2 LVD SCSI 50GB or 73GB drives to connect together. I understand that Ultra2 SCSI can be connected by up to 12 meters of cable, but is it possible to go 12 meters with 15 drives attached, or am I required to shorten the cable for this many drives?

Also, we may need the cable to go from internal to external and back to internal again. This is to connect an external drive array to a server, which has a SCSI controller on the motherboard. Will the two splicing connectors in the middle affect this 12 meter length and, if so, by how much?

A: The SCSI specifications state that you may have up to 12 meters (40 feet) of cable in an Ultra2 LVD SCSI or Ultra160 SCSI system with up to 16 active IDs. It further specifies that you may have up to 25 meters (82 feet) of cable on a "point-to-point" application, meaning only one initiator and one target. As a rule with cabling, it is best to keep the total length to the minimum practical length.

The SCSI specifications also note that it is best to avoid grouping of peripherals. In other words, it is optimal to space the peripherals at equal intervals along the cable. This caveat is seldom observed, with the possible exception of internal ribbon cables. In any case, be sure that you have 12 inches or so of cabling between drives.

Each transition between backplane or twisted-flat cable and round shielded cable causes an impedance "lump" in the SCSI transmission line that can cause signal reflections. Every connector can have the same effect. Because connectors and cabling are unavoidable, reflections are unavoidable, but every effort should be made to reduce them by using a minimum number of connectors. Take a look at the back-plane or twisted-flat cable and round cable impedance specifications to try to match them as close as possible. Reflections can cause anything from multiple SCSI resend requests to bus shutdown. Incidentally, if you do have reflection problems, changing the total cable length of the bus segment by five or six feet may reduce the effects of the reflections.

Another factor to consider is to use one or two LVD-to LVD SCSI Expanders between the server and the drive enclosure, especially if the 12 meter cable length is a limitation for you. With one of these expanders, you add another 12 meters of cable to the total length. You might consider running a cable from the server to the drive enclosure and then place an LYD-to-LVD expander at the conversion to the backplane or twisted-flat cable. You can then space the drives over almost the entire length of the nearly 12 meters of twisted-flat cable that you can have in the bus segment inside the enclosure.

Using two of these expanders will allow a total maximum cable length of between 36 and 48 meters without slowing the data throughput.

Note: Flat cable refers to twisted and flat cable for LVD SCSI.

Q: Does a Fast SCSI CD-ROM slow down the speed of Ultra SCSI hard disks, if they are connected on the same cable?

A: For a Fast SCSI device on an Ultra SCSI bus, each device negotiates separately with the controller. On a server, the bus could reach saturation, where the time of transfers from the CD-ROM the SCSI bus is tied up.

When the SCSI bus is initialized, the host adapter polls each peripheral on the bus to determine its capabilities: its data throughput, whether it is wide or narrow, whether it is synchronous or asynchronous, and so on. Every SCSI device on the bus then stores this information so that it knows the speed it can use when exchanging data with any other SCSI device on the bus. In your case, the host can then "talk" to the Ultra HDD at Ultra speed and to the CD-ROM at Fast speed. The CD and the HDD will "talk" to each other at the slower speed of the CD-ROM.