Ask the SCSI Expert: visit the SCSI trade association's web site (www.scsita.org) to ask the SCSI Expert a question.

Q: I remember reading something years ago, which suggested that it might be possible to connect two SCSI cards (in two separate machines) together on a single external bus, sharing access to one or more disk devices.As far as I can remember, there were some possibilities by configuring unique SCSI Ids for all devices, including the adapter cards.

If this is true, could both machines be powered at the same time (and thus accessing the disk devices simultaneously) or could only one system be powered at a time?

If it is possible to have both systems powered at the same time, can I use such a setup as the quorum storage for NT Cluster Server?

A: SCSI has the ability to have two or more controllers on the same bus with unique Ids. There is a reservation system defined in SPC-2 (SCSI Primary Commands-2) that allows systems to have control over the disk to insure that files do not get overwritten.

This allows sharing of data between systems and redundant data access in case a server fails.

Q: Are there any issues with respect to the number of active drives present in a system, as FDISK only allows for one to be enabled, or this is a non-SCSI issue?

A: FDISK will work with Any number of drives. Combinations of IDE and SCSI will work too.

The drives have to work and have the correct Ids, and the low level performing of the media.

Q: Is it possible to have two SCSI adapters installed on one box? One adapter is built into the motherboard and the other is an Adaptec 2940 PCI.

A: Yes, it is common to have more than one SCSI controller in a box. Sometimes these are different types of SCSI controllers in a single box, such as single ended and LVD SCSI controllers. Multichannel SCSI Adapters are available, which Have up to four channels.

Q: Where can I find a list of device Ids so that I can hook up my hard drive and CD-ROM to SCSI?

A: SCSI are normally assigned by priority for performance. The Priority Table is in the SPI documents. The highest priority is 7 going down to zero and 15 going down to 8. The controller or Host Bus adapter is ID7; if the bus has more than one Host Bus Adapter it is assigned ID6, ID5, etc. The only one that is fixed is the controller, and the rest are optional. The general assignment is listed below, but is not always followed. Some buses only have 15 hard disks; this number depends on the specific configuration.

* ID 0 is normally the boot disk (some operating systems require the boot device to be ID 0.)

* ID 1 typically assigned to the second disk drive.

* ID 2 and 3 are for tape or removable media.

* ID 4 and 5 are for CD, CD-R, CD-RW, DVD RAM, and DVD-RW.

* ID 5 and 6 are for scanners.

* ID 7 is for the Controller of Host Bus adapter.

* ID 8 through 15 are for user disks (wide bus only).

Q: What are the maximum lengths of SCSI cabling for all the SCSI types?

A: The lengths depend on the interface. There is a table on the SCSI trade association web site (www.scsita.org) in the "About SCSI" page. Select "Terms and Terminology" which has the speed, the interface and the corresponding cable lengths.

For the relatively newer LVD SCSI standards such as Ultra2, Ultra3, Ultra160 and Ultra320 SCSI it is easy--point-to-point 25 meters, multidrop 12 meters.

www.scsita.org