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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces and Configuration | Small Computer Systems Interface (SCSI) | SCSI Data Transfer Modes and Feature Sets ]

SCSI Transfer Mode and Feature Set Compatibility

I sometimes call the various types of SCSI "flavors"; well, if that's so, the way the interface is heading we'll soon be in Baskin Robbins territory. ;^) The sheer number of different kinds of SCSI can certainly make the interface seem overwhelming! How does a SCSI user make it all of this hardware work together? Fortunately, while the standards and feature sets can be quite confusing, the hardware is actually well-engineered, and the standards are designed to allow different hardware types to work together fairly readily.

It's important to remember that a key design goal of all SCSI standards is backwards compatibility. Few people want to buy new hardware that won't work with their older hardware. Therefore, in most cases, at least in theory, you can mix older, slower hardware with newer, faster hardware. You can, again in theory, put a brand-new Ultra160 SCSI hard disk on the same SCSI bus with a decade-old SCSI-1 host adapter (albeit with added hardware and suboptimal results.) This is generally true, but note the important qualifier: in theory. Since changes are always being made to the signaling and other aspects of the interface, there is no guarantee that any two very different pieces of SCSI hardware will work together.

There are no hard and fast rules regarding the compatibility of different SCSI transfer modes and feature sets, especially if they are very different in terms of key attributes. Here are some issues that you should keep in mind as you consider device compatibility:

  • Age:  The greater the difference in age between two devices, the greater the difficulties associated with getting them to work together. The extreme example I gave above of trying to get an Ultra160 drive to work with a SCSI-1 host adapter (or vice-versa) would probably not be much fun. :^) However, mixing Ultra160 and Ultra2 devices is fairly straightforward.
  • Drive and Host Speed Negotiation: You can use faster drives on slower host adapters or vice versa, but communication will only occur as fast as the slowest device can handle. For example, you can connect a Wide Ultra SCSI drive to an Ultra160 host adapter, but the drive will only run at a maximum of 40 MB/s throughput, not 160 MB/s.
  • Signaling: Mixing different types of signaling on the same bus can lead to problems ranging from slowdowns to disaster. :^) The older (high voltage) differential signaling is not electrically compatible with either single-ended or LVD devices, and should never be mixed with those types, or you risk disaster such as smoked hardware. Multimode LVD devices can be mixed with SE devices, but they won't function at Ultra2 or higher speeds if you do so.
  • Bus Width: You can mix wide and narrow devices on the same SCSI bus, but there are specific requirements in doing this, to ensure that the bus functions properly.
  • Packages: If you want to be sure that a particular SCSI implementation will work, buy a complete system or SCSI "package" (including a host adapter, drives, cables and terminators) from a reputable dealer.
  • Ask For Help: If you are considering a particular hardware combination, ask for advice on it before trying the setup. Unless the setup is extremely strange, someone may have already done what you are contemplating. A good place to try is the comp.periphs.scsi newsgroup.

Next: SCSI Protocols and Interface Features

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