Ask THE SCSI EXPERT.

Q: Are SCSI terminators for single-ended and differential SCSI devices interchangeable? Can single-ended cables be used for differential interface cables when connecting differential SCSI devices?

A: Differential can mean the old SCSI-2; differential, now known as HVD (High Voltage Differential) SCSI, is not voltage or pin-compatible with single-ended. HVD SCSI will be obsolete in SPI-3. The terminators will not work and devices will not work together, but it uses the same cables.

Most devices and terminators that are not LVD (Low Voltage Differential) will work only in what is termed multimode. They will work as single ended Ultra SCSI or Ultra2 SCSI. LVD SCSI is susceptible to crosstalk; twisted flat cables should be used instead of the flat ribbon cable.

* LVD and single ended - YES

* HVD arid single ended - NO

Q: What is the difference between single channel and quad channel SCSI? The only information that I've found so far is that quad is faster. How does quad channel SCSI work and why is it faster? Does channel refer to logical or physical channels?

A: There are single, dual, and quad channel adapters on the market. This means strictly the number of SCSI buses coming off of one controller. Quad has four SCSI buses and can move data at four times the rate of a single bus adapter, providing that the system bus and operating system can handle the I/O rate and the data rate. Look for the speed: Ultra wide (Single-ended, Fast-20), Ultra2 SCSI (LVD SCSI, Fast-40) or Ultra3 SCSI (LVD SCSI, Fast-80).

Q: Is it true that you're supposed to connect your fastest device last on the end of the chain?

A: This is not an issue on small services or PC/Workstations only on high or PC/Workstations, only performance systems normally running RAID (Redundant Array of Independent Disks). If it is a SCA-2 backplane, then the ID is set by the location of the drive. If it is not a SCA-2 backplane, then the priority goes by the ID of the drive.

If the device is fast enough to saturate the SCSI bus, it should be at a lower priority. Normally, the SCSI bus is fast enough to handle four devices running at maximum speed. If you are running systems where you are striping data across several (more than four) drives and the system does not use the fairness algorithm, then the fastest devices need a higher ID, allowing the slower devices access to the bus.

Q: What does SCA (SCSI) mean?

A: SCA is a special 80-pin hot plug connector for disk drives that plug into backplanes. The SCA-2 connector is normally used on RAID or servers.

The connector has the normal SCSI signal except for Termpwr. Added signals are the SCSI ID, which is hardwired by the slot position, power, and mated signals.

Q: Do the most popular operating systems require attached SCSI devices to have the queue algorithm modifier on a SCSI drive or RAID array set to restrict tag re-ordering? It seems that most vendors ship their disk drives with restricted tag re-ordering, but I have also been told that in a Unix environment it is not required. Could you comment on this?

A: Queue and RAID are SCSI options; they are not requirements. The controllers negotiate for the maximum options that the controller and device will operate.

Most newer drives support Queues. This is something that you should test for and require the level of queue that you need to support your controller requirements. Companies normally restrict the drives that are used in RAID systems to the correct performance level.