Ask THE SCSI FIPET - SCSI Trade Association's web site - Industry Trend or Event

Computer Technology Review, Oct, 2000

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

Q: I would like to use the SCA-2 80-pin connector. I need to know the pin definition and what the difference is between 80-pin and 68-pin. Where can I get this information?

A: The. 80-pin SCA-2 connector is for hot plugging drives into a backplane. It is used for high reliability systems. The 80-pin connector has the power, the SCSI ID and Mated signals. It does not allow termination. The termination must be on the backplane.

There are adapters from companies that adapt 68-pin cables to 80-pin drives. Two should be the maximum ever used on one system. The bus loading of the adapter is about the same as a drive. The drive with the adapter is twice the capacitance allowed at that point in the cable.

The version of the adapter with the terminator should be used for the end device on the cable.

The pin out is in the SPI-2 (SCSI Parallel Interface - 2) standard, which is available on the SCSI committee (T10) web site www.t10.org.

If more than two drives are required, an expansion box with the 80-pin SCA-2 hot plug backplane should be used, not the individual drive adapters.

Q: Is it possible to run a SCSI switch between a peripheral device and two hosts? Note that I will be using 68-pin differential SCSI.

A: Yes, you can use an "A-B" SCSI switch to connect one peripheral (or a string of peripherals) to either of two hosts. When you switch from one to the other, it will probably be necessary to reset the SCSI bus.

Q: How do I correct the problem of my drive being forced into SE mode?

A: Any single ended device, terminator or adapter card can change a bus to single ended. Everything on the bus must be LVD in order to run LVD.

Q: Is it possible to connect two computers using SCSI adapters to exchange data between these two computers?

A: Usually most operating systems will not allow you to change the controller to a target mode. While most controllers will work in either target or initiator mode, the drivers and operating systems will not allow it.

Q: Could you give me a brief explanation of the differences between hard disk types, according to drive speeds and disk interfaces available in the current market?

A: The majority of the new drives are multimode, which will work in LVD SCSI mode or single ended mode. Multimode drives are Ultra2 SCSI or Ultra160 SCSI, and they do not provide termination. The termination must be provided in the cable system or backplane. Backplane systems use the SCA-2 connector with integrated power for hot plug capabilities.

If the multimode drives are connected to a single ended bus, the single ended rules apply. The speed will drop to 40MB/sec on a wide bus. The drives will work on a narrow bus if the high byte is terminated or pulled up; the speed is then 20MB/sec.

Ultra160 SCSI drives in multimode, which are the latest technology available, run 160MB/sec.

Ultra2 SCSI drives in multimode, which are the previous generation but still widely used and sold in the market place, run 80MB/sec.

Ultra SCSI drives in single ended mode (released two SCSI generations ago, and therefore are somewhat hard to find) runs at either 40MB/sec for wide or 20MB/sec for narrow.

The current generation of 7200 RPM drives has a sustained transfer rate of 27MB/sec. 10,000 RPM drives have a sustained transfer rate of 35MB/sec. 15,000 RPM drives have sustained transfer rate of 50MB/sec.