The Small Computer System Interface (SCSI, pronounced scuzzy) that every Mac from the Mac Plus on has supported (except for the iMac and the Blue & White Power Mac G3) is a mixed blessing. SCSI is a parallel interface, meaning that bits of data travel alongside each other rather than in tandem like they do in serial interfaces such as ADB or the Mac’s modem and printer ports.

When SCSI works nicely, it is a very speedy and efficient data transfer highway. When it doesn’t work nicely, it makes you want to tear your hair out.

Apple is moving from SCSI (also ADB and Serial) to USB and FireWire, and once the inconveniences of the transition period are over, I doubt that anybody will miss SCSI, which has been one of the few user-hostile, PC-like, aspects of life with the Mac. Messing up with SCSI connections is also one of the few ways a Mac-user can cause actual hardware damage to their Mac.

Fortunately, with patience and perseverance you get almost any SCSI setup to work properly, and once you do it will usually be trouble-free – until you add something new to your SCSI chain.

Your Identification Please

You can attach up to seven SCSI devices, including hard disk drives, CD-ROM drives, and scanners, to your computer by linking them together in a chain that starts at your computer’s SCSI port. Each device must be identified by its own SCSI ID number from 0 to 7. The Macintosh itself counts as one device and uses ID number 7 for itself. The internal hard drive’s ID is usually 0, and Apple sets its built-in CD-ROM drives to number 3. That leaves five “empty” numbers (1, 2, 4, 5, 6) available for other SCSI peripherals. A device with a higher number is given priority when two devices compete for access to the SCSI bus.

The SCSI Probe utility (see below) will identify what numbers are being used by which device. Some SCSI devices limit your choice of SCSI ID numbers. On the Zip drive, for example, 5 or 6 are your only available choices. However, most SCSI devices let you can choose any number from 1 to 6.

Sacrificing a Goat Might Help

Things sometimes work on SCSI chains and sometimes they don’t. Even after years of daily Mac use, I still hold my breath when I attach a new SCSI device – or even one that I’ve attached many times before. Will it mount? Maybe – maybe not. There are probably technical reasons for the times that it doesn’t, but they are seldom fathomable. Some have suggested that sacrificing a goat before hooking up a new SCSI device is a prudent precaution.

Even seasoned tech-veterans struggle with SCSI at times, and I figure that if a relatively technically competent person like myself has trouble, then ordinary Mac users must be getting really frustrated. Adopting FireWire and USB is one of Steve Jobs’ really good ideas.

However, SCSI will be around for a long time yet. Even the new Lombard PowerBooks will reportedly have SCSI ports, largely to support SCSI Disk Mode (of which more later).

Each SCSI device interfaces with the Mac through software called a driver. For instance, Apple’s Drive Setup or another drive formatting utility installs a hard disk driver on your hard drive when it is formatted. The drivers for scanners and CD-ROM drives are usually system extensions. Without the proper drivers in your System Folder, these SCSI devices won’t work. Make sure they’re there (drivers for third-party devices usually come on a CD-ROM or floppy disk bundled with the machine; keep this in a safe place for future reinstalls).

The Terrors and Tribulations of Termination

Problems Mac users experience with SCSI include hanging, frozen screens, crashes, and failure to recognize and/or mount a SCSI device. The reason is often due to improper SCSI termination. The cardinal rule of SCSI termination is that there are no hard and fast rules – suggestions yes, but not rules. One setup may require a pass-through terminator, while another ostensibly identical hookup will refuse to work unless the terminator is removed. It can be entirely idiosyncratic, or at least apparently so.

Each end of a SCSI chain (or bus) theoretically must be properly terminated to prevent signal “reflections” when signals reach the end of the line. Such bouncing reflections can make your Mac go weird. Terminators are devices that stop the bouncing signals.

The first SCSI device (usually your internal hard drive) and the last SCSI device in the chain (a scanner, Zip Drive, or whatever) must be terminated. Nothing in between should be terminated (except in rare cases in very long SCSI chains where a third terminator is sometimes used). However, some SCSI devices have internal termination – provision inside the box for termination, so you don’t even know if it’s terminated unless it says so in the manual. Some SCSI devices (e.g. Zip Drives) come with a terminator switch, so they can be placed either in the middle or at the end of a SCSI chain.

The official rule is that an external terminator (either in the empty second SCSI port or if there is no second port a “pass-through” terminator is sandwiched between the cable and the device’s single port) must be used if the last device is not internally terminated. In practise, my Microtek scanner works just fine with or without any terminator installed. As I said, suggestions, not rules, apply.

Most SCSI devices come with either an internal or an external terminator. A few upscale SCSI devices have active termination, which is very convenient circuitry that turns termination on or off automatically as needed. Before adding a new device to your SCSI chain, read the manual and/or ask your dealer about termination. Take anything either says with a grain of salt.

Some General Guidelines for Peaceful Co-Existence with SCSI

• SCSI cables should ideally be kept as short as possible, especially with PowerBooks – ideally three feet or less. Long SCSI cables are often troublemakers. Keeping an extra SCSI cable around isn’t a bad idea – they do go bad, especially with rough treatment. The tiny internal wires can fray, jacks can become damaged, and shielding can rupture. Generally, the thicker and heavier the cable insulation and shielding, and the more robust the connectors, the more reliable a SCSI cable will be.
• If you’re having problems, try changing the order of SCSI devices in the chain. Scanners often like to be the first device in the chain.
• Keep SCSI Probe, an invaluable little freeware utility, on your hard drive. SCSI Probe helps you mount stubborn SCSI devices and can help sleuth what the trouble is if they won’t mount. You can download a copy of SCSI Probe here.
• Make sure that no two devices in your SCSI chain are attempting to use the same SCSI ID number (use SCSI Probe to check).
• Use the wire clips or thumbscrews supplied on SCSI cable connectors (except PowerBook HDI-30 connectors), to fasten them securely
• When connecting SCSI equipment, always turn off power to all devices in the chain, including your computer. Sleep mode is not good enough here. Your PowerBook needs to be shut down. If you don’t heed this, you could possibly lose data and/or damage your hardware.
• Do not unmount a PowerBook’s SCSI icon by dragging it to the Trash. If you do, you could damage the directories of all mounted drives.
• Don’t use a terminator when connecting to another PowerBook in SCSI Disk Mode.

Connecting SCSI Devices to Your PowerBook

As you may have inferred from the comments immediately above, PowerBooks add some additional complications to the SCSI equation. For one thing, they use a non-standard SCSI connector – a compact square one called an HDI-30, which may have either 29 or 30 pins. Actually, there are two “standard” SCSI connectors: the Centronics 50 and the DB-25. These are functionally interchangeable. The Centronics 50 is big – about twice the size of a DB-25. Desktop Macs (except the iMac and Blue & White G3) are equipped with a DB-25 female plug. Most SCSI peripherals have female Centronics 50 connectors, although some (e.g. Zip Drives) use DB-25 connectors.

To use standard SCSI cables with your PowerBook, or to connect to standard SCSI devices, you will need an adapter or adapter cable. Both HDI-30-to-Centronics 50 and HDI-30-to-DB-25 adapters and cables are available.

Apple sells two HDI-30 adapter cable models:

1. From your PowerBook to a SCSI device: The 29-pin Apple HDI-30 SCSI System Cable (light gray – 19 inches long – ends in a male C-50 connector) – M2538**/A
2. For SCSI disk mode: The 30-pin Apple HDI-30 SCSI Disk Adapter Cable (dark gray – 10 inches long – ends in a female C-50 connector, which mates to a standard male C-50 cable that in turn ends in a male DB-25 connector that plugs into the other Mac.) M2539**/A

These cables are relatively expensive, and if you want both connect your PowerBook to external SCSI devices, and to use SCSI Disk mode as well (which makes your ‘Book think it’s an external hard drive connected to another Mac) you will need both cables.

A more economical solution is to buy a switchable HDI-30 SCSI Dock adapter, which has 30 pins but also incorporates a switch to turn the 30th pin off, thus disabling SCSI Disk Mode when desired. Models are available to connect with either Centronics 50 or DB-25 SCSI cables.

PowerBooks are not considered internally terminated (actually older models are if they have a SCSI internal hard drive, but their “small, built-in terminator” attached to their drive hasn’t the muscle to handle a large SCSI chain), and according to Apple they require two terminators (one pass-through one at the PowerBook end of the SCSI chain and one regular one) rather than one. (However, my PowerBook 5300 and PowerBook G3 Series are both happy connecting to my Zip drive with just its internal terminator switched on). On the other hand, a PowerBook Duo’s SCSI port behaves like a desktop Mac’s, normally requiring only a single terminator.

If you use a DB-25 adapter in SCSI Disk Mode, my experience is that no terminator is necessary either, although Apple says that one is. Trial and error is the only way to solve these conundrums for your particular setup.

Using Your PowerBook in SCSI Disk Mode as a Hard Disk

The HDI-30 port’s 30th pin activates SCSI Disk Mode, which lets you connect your PowerBook to another computer (desktop or another ‘Book) as a hard disk. The PowerBook internal drive will appear on the other computer’s desktop as a hard disk icon, and you can transfer information between the computers by simply dragging files. SCSI Disk Mode is also useful for installing software on a PowerBook that has no CD-ROM drive via a Mac that does. However, if the other computer is PowerPC and the PowerBook is a 680×0, or vice versa, you must drag the installer onto the PowerBook hard drive and install it from there. If you try to do this from the CD-ROM, it will install a version appropriate to the host machine’s processor type.

Also, if your PowerBook’s hard drive uses the Mac OS Extended format (also known as HFS+, and all G3 Macs are shipped with HFS+ formatting), in order to use your PowerBook in SCSI disk mode, the computer you are connecting to must be using Mac OS 8.1 or later. If the computer is using an earlier version of the Mac OS, you won’t be able to see the files on your PowerBook’s hard disk.

Before making the SCSI connection, you need to use the PowerBook Setup control panel to assign an unused SCSI ID number to the PowerBook. I usually pick the number 2. You must also turn off password protection before using SCSI Disk Mode.

Some desktop Macs, notable the LC/Performa 500 series and at least some of the Performa 5000 and 6000 series do not power their SCSI bus when the machine is not booted, so the PowerBook doesn’t recognize the (consequently unpowered) 30th pin on the HDI-30 connector and doesn’t go into SCSI Disk Mode.

The workaround I use with my own LC 520 is to put the PowerBook at the end of a SCSI chain (you may or may not need a pass-through terminator) with my scanner in the middle. I power up the scanner first, which powers the SCSI bus, then the PowerBook, and finally the LC. This works. Any peripheral SCSI device that powers the SCSI chain should work.

I’ve heard that somebody makes a gizmo specifically for this purpose, but can’t recall the name. Another thing that may or may not work is hitting the power keys on both the PowerBook and the desktop Mac simultaneously. My son gets this quick and dirty method to work occasionally.

To quit SCSI Disk Mode, first shut down the computer your PowerBook is connected to. Then turn off the PowerBook. Finally, turn off any other SCSI devices in the chain before making any disconnections.

Don’t keep your PowerBook closed when it’s running in SCSI Disk mode, as it will overheat unless the keyboard can vent.

You must then either remove the SCSI Disk adapter or cable or throw the switch on a switchable SCSI Dock adapter in order to return the PowerBook to normal mode.

Note that the PowerBook 140, 145, 145b, and 170 all lack the necessary ROM code for SCSI Disk Mode.

By the way, never try connecting two Macs together with just a plain SCSI cable (i.e. non-SCSI Disk Mode). It won’t work and will likely cause serious hardware damage. Neither can you connect two Macs to the same SCSI peripheral device (e.g. a scanner, Zip Drive, etc.) simultaneously without disaster.

Troubleshooting SCSI – The Highlights

If you’re experiencing what you think is a SCSI-related problem, disconnect the SCSI cable from the back of your computer and try rebooting. If the problem goes away, you are reasonably assured that some SCSI gremlin is the culprit.

If it’s still there, the problem is likely with your computer’s software or hardware – try rebooting again off your Disk Tools floppy or the software CD that came with the computer. If that works, you have software trouble – possibly an extension conflict or corrupted system or a damaged hard disk driver. Dealing with these things is beyond the scope of this article.

If the problem seems to be SCSI related, check for proper termination (experiment with more and fewer terminators, etc.), conflicting SCSI ID addresses (e.g. did the ID switch on the back of the Zip Drive get accidentally moved – check with SCSI Probe), and cabling problems. Here is where your spare SCSI cable will come in handy. Make sure that everything is plugged in and switched on and that all SCSI connectors are firmly seated. Make sure the proper software drivers are installed (try reinstalling fresh copies of the drivers).

The Mac’s Future Happily Doesn’t Include SCSI

There are several high-performance variations of SCSI, such as SCSI-2, SCSI-2 Fast, SCSI-2 Wide, SCSI-2 Fast/Wide, SCSI-3 Ultra Wide. These faster versions of SCSI are not supported by PowerBooks and require expansion cards in desktop Macs. We can safely (and mercifully) ignore them for the purposes of this article.

All are in the process of being made obsolete by FireWire, which is fast, hot-pluggable, and can support more devices in a daisy chain than any normal user will ever have to worry about. Lombard will reportedly support FireWire, and in the fullness of time, we will all be able to forget all about SCSI.

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© 1999 Charles W. Moore. Originally published at http://www.geocities.com/Athens/Aegean/9318/scsi.html