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In the previous edition of Sad Macs, this Fix-It was devoted exclusively to SCSI connections. With SCSI fading from the Mac scene, I've expanded the Fix-It to include other connection technologies. Still, SCSI devices are likely to remain in use for many more years. And they are still a common source of problems. It's a good place to start out.

SCSI Basics

SCSI chain Only one device can be connected directly to a SCSI port, such as the port on the back of many Mac models. However each external SCSI device is typically equipped with two SCSI ports. So, for example, if you connect an external hard drive to a Macintosh, this will still leave one SCSI port empty on your external drive. A second SCSI device can be connected to this port. This arrangement can continue, creating a daisy chain of SCSI devices. If all devices are connected correctly, the Macintosh will recognize all of them, even though only one device is actually plugged into the Mac.

SCSI ID numbers Each SCSI device has an ID number. The Macintosh uses these numbers to differentiate one SCSI device from another. ID numbers can range from 0 to 7. ID 7 is reserved for the Macintosh (which, despite its number, is technically not part of the SCSI chain). ID 0 is reserved for internal hard drives (which are considered part of the SCSI chain).

Thus, you can attach up to seven SCSI devices to one Macintosh (however, see "TECHNICALLY SPEAKING — More Than One SCSI Bus; More Than One Type of Bus" for an important exception to this). With an internal hard drive, you can connect six external SCSI devices, with ID numbers from 1 to 6.

Some Macintoshes can have more than one internal SCSI device (such as both an internal hard drive and a CD-ROM drive). Each device will have its own ID number (internal CD-ROM drives, for example, should have their ID number set to 3).

You assign the ID number to each external device. Usually, you do this by pressing a button located somewhere on the device. Each time you press the button, you cycle to another ID number, which should be indicated in a display next to the button. The number currently in the display is the ID number for that device. These ID numbers do not have to be assigned in the order that the devices are connected. For example, the first external device in a chain could have an ID of 4, and the second device could have an ID of 2.

All other things being equal, assign ID numbers based on how often you use a device, giving higher ID numbers (where six is higher than five, for example) to those external devices that you use more often. The device with the higher number is given priority when two devices are simultaneously competing for access to the SCSI bus.

The most important rule to remember is that each device must have a different ID number. To repeat: No two devices can have the same ID number (on the same SCSI bus). Otherwise, problems will certainly result.

TECHNICALLY SPEAKING — More Than One SCSI Bus; More Than One Type of Bus

Multiple buses Depending on the Mac model you have, your Mac may have more than one SCSI bus. In particular, many PCI-based desktop Mac models have two buses. This allows for more than seven SCSI devices to be attached to a Mac. The external port connects to one bus, while most (although not necessarily all) internal SCSI devices use the other bus. Each bus can hold up to six devices beyond the Mac itself (the Mac is listed as ID 7 on all buses). The internal SCSI bus is typically bus 0, with other buses numbered incrementally. The internal bus, although having a 7 device limit, may actually be more limited - due to the constraints of how many devices can fit inside the box.

Having multiple buses means, for example, that you can have two devices with an ID of 3, one on bus 0 and the other on bus 1. Normally this is not a problem. However some hardware has problems unless it has an ID that is not assigned to any other device on any bus. UMAX scanners have been plagued with this problem.

Note: With multiple buses, each device actually has a "long name" ID that identifies it uniquely. For example: ID 1.3.0 means Bus 1, Device 3, logical unit 0. Allowing multiple logical units for one device is rarely implemented on the Mac; don't worry much about it. If only one unit is used, it will be assigned ID 0.

For a device to work properly with a multiple bus Mac, the device must be "SCSI Manager 4.3 compliant." This refers to the Mac OS software that controls the SCSI communication. If a device is not compliant, you may (for example) have trouble assigning the same ID number to different devices even if they are on different buses.

Fast, Ultra, Narrow, Wide There are multiple variations on the SCSI standard. Each new variation is associated with increasing speed of data transfer. For example, there is Fast SCSI (which is faster than regular SCSI) and Ultra SCSI (which is faster still). There is also Wide SCSI, which is faster than Narrow SCSI. Wide vs. narrow refers to the transfer bandwidth (it's like measuring the diameter of a water hose). The Fast and Ultra types refer to the speed of the transfer (it's like the water pressure determining how fast water is pushed through the hose). Ultra-Wide is as fast as it gets—except that there are also multiple variations of a given designation (where Ultra-2, for example, is faster than ordinary Ultra).

If your Mac has more than one SCSI bus, they may be of different types. In particular, the internal bus is often faster than the external bus. To take advantage of the faster SCSI drives, you need to hook them up to a SCSI port that can carry the data at the same speed. For example, connecting a Fast/Wide drive (which can transfer data at 20MB/sec) to a port that can only carry data at 5MB/sec will defeat the advantage of the fast drive. This is one reason for getting a PCI card with a SCSI port. The card can be designed to handle faster drives than the port built in to the Mac (see also: "Take Note: Adding a SCSI Card," later in this Fix-It).

Note: A SCSI card, if you add one, is typically assigned its own bus; on Macs that do not have any built-in SCSI port, the card will likely be assigned bus 0.

SCSI Termination SCSI termination may sound like what you want to do to a drive that has just crashed for the third time this week, but it is nothing of the kind. But SCSI termination is a messy topic, so let me disentangle it for you.

I am the first to admit that I don't really know the nuts and bolts of what termination is all about. Fortunately, to fix most termination problems, you don't have to know very much.

Essentially, termination tells the Mac where a SCSI bus begins and ends, preventing signals from getting mixed up by reaching the end of a chain and bouncing back again. It is also important for keeping signal strength at an appropriate level and maintaining transmission speed at all locations across your cables. The longer your chain, the more likely it is that termination problems will appear.

That said, the main thing you need to know is that a typical SCSI chain needs to be terminated at both ends of the chain. Most Macintoshes with internal drives are considered terminated at the Macintosh end. In those cases, all you need to do is make sure the opposite (external device) end of the chain is terminated. The most common method to do this is to get a special SCSI plug called a terminator. It looks like an ordinary SCSI plug with no cable attached. Simply plug it in to the second (empty) port on the last SCSI device in a chain, and you are done.

If you have no external SCSI devices attached, you can skip this whole discussion. If you have two or more external SCSI devices attached, you will likely need to be concerned about termination issues.



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