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Component Order in a SCSI Chain

SCSI: Science, Art, or Voodoo?

Sometimes getting a SCSI chain working properly seems to be more hit and miss than science. We know the last device should be terminated, the chain should not exceed a certain length, and every device must have a unique ID.

But even that isn't enough to consistently build a busy SCSI chain and have it work reliably. The actual order of devices matters, as trial and error prove.

Keith Bumgarner of MacInformed <http://www.macinformed.com/> sent some very helpful, but not widely known, information.

As a physicist/systems engineer who now, oddly enough, devotes full-time to Macintosh computing, I know it is well-documented that device buffer size is integral to determining both relative ID and positioning on a Macintosh SCSI chain.

Briefly, the Mac's hardware reset signal (pin 40) affirms SCSI devices on any bus in the protocol as 0, 6, 5, 4, 3, 2, 1. Given the physics of RF, the signal travels unimpeded until it hits a capacitance change (this is termination) and then routes back on itself. It must do so, initially, through the most stable SCSI device, this being the one with the largest throughput buffer. Buffer sizes, in almost 98% of the cases, can be categorized by the type of device. Fixed media hard drives typically have the largest buffers, followed by Winchester-type removables (SyQuest, Jaz), optical removables, streaming tape, CD-ROM and derivatives (with the exception of the CD-R unit with a larger stated hardware buffer), and then scanners and printers, which in addition to very small throughput buffers, generate high levels of the kiss-of-death to all RF devices: random electrical noise.

All this means, as Apple engineers started acknowledged about 3 years ago, is that hard drives should have higher ID numbers and live physically farthest from the Mac on the SCSI chain, and scanners should have lower ID numbers and live physically next to the Mac, if possible. Also, given the Mac's aggressive SCSI standpoint (hardware reset vs. Unit Attention), you cannot turn SCSI devices on and off without creating directory code errors on your media. These are low-level directory errors which affect the device partition, and, ultimately, the SCSI driver. They are, unfortunately, not the type of directory problems detectable by tools like Norton Utilities, but they can be determined by using some more obscure SCSI tools and measuring instruments.

Further correspondence results in the following suggested order for devices in a SCSI chain based on typical buffer size. This should vary based on actual buffer sizes.

computer or SCSI card, ID = 0
scanner
printer
CD-ROM or DVD player (Apple sets ID = 3)
tape drives
CD-R, DVD-RAM (check actual buffer size, lots of variety here)
optical drives
removable media hard drives (SyQuest, Jaz, Orb, etc.)
fixed media hard drives, higher capacity usually = larger buffer

The Iomega Zip drive presents a special case, mostly because it comes with an unusual SCSI cable with DB-25 connectors on each end, making it easiest to place as the first device in a SCSI chain. Since onboard termination of the exteranl 100 MB Zip drive is inadequate for a long SCSI chain, this is another reason to avoid placing a Zip drive at the end of a multi-item SCSI chain.

This is also a very god argument against using a scanner or printer that insists on being the last device in the SCSI chain.

Of course, you can never have all the above items in a typical Mac SCSI chain, since the SCSI-1 protocol Apple uses for most ports only supports seven devices.

Here's what a loaded SCSI chain might look like, based on this advice. This assumes the computer already has an internal hard drive and CD-ROM or DVD player.

scanner
Zip drive (unfortunately, it must be set to SCSI ID 5 or 6)
tape drive
CD-R burner, DVD-RAM, or 640 MB 3.5" optical drive
Jaz drive
partitioned 9GB hard drive for mastering CDs/DVDs

If you're having SCSI problems, this advice and a little research on actual buffer sizes may help you build a stable SCSI chain.

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