Introduction to Drive Interfaces

Charlie Ruggiero - 2001.01.29

In the Macintosh world, there are many different ways of connecting storage devices to computers. Early Macintosh computers started with SCSI, then Macs moved on to primarily supporting IDE. Macs now support FireWire and USB storage devices as well as IDE.

The most popular devices that the Macintosh supports are hard drives, CD and DVD drives, and other types of removable-media devices. Below is a list of devices and what type of connection they work best with and why:

• Hard Drives: IDE is cheap, but SCSI drives can be faster. For most people IDE is the best way to go due to the cost per gigabyte. FireWire hard drives are okay, except they tend to cost more (however prices are dropping) and may be slower than IDE and SCSI drives. USB hard drives are much too slow and not worth the money unless your computer only has USB connections.
• CD Burners: SCSI has been the most reliable for me. IDE holds a close second, but IDE drives must be installed inside the computer. FireWire burners still have problems to work through, and in my experience USB burners should not even be looked at. USB burners tend to be unreliable and slow. A 4x USB burner may not actually go at 4x. (Macs only support (As noted on MacFixIt on 12/12/00: "Apple's USB device driver . . . has a maximum data transfer rate of 680 KB/sec." and is therefor unsuitable for 4x CD burns.)
• Other Removable Media Drives: USB should be used, especially for Zip drives. FireWire is second. I pick both these because of the ability to hot-swap devices. SCSI and IDE both require the computer to be turned off before unplugging or plugging in devices. These days some USB and FireWire Drives can operate completely from port power, which reduces the number of cables you need.

Following are some basics of SCSI, IDE, FireWire, and USB as they pertain to hard drives.

SCSI Basics

The most important rule about SCSI is that you get the termination correct. Termination is what tells the computer where the beginning and the end of the SCSI chain are. As a general rule, the first and last device should be terminated, and this includes the internal hard drive (if it is SCSI.)

Computer[internal hd1] - [external hd1] - [external hd2]

In the above case, "internal hd1" and "external hd2" should be terminated. There are exceptions to this rule. Some SCSI cards already have termination on them and act as if they are the first device. In this case you should only terminate the last device. Unfortunately, these rules are broken sometimes (much like the English language.) Sometimes it is okay to terminate only the last SCSI device, especially if the SCSI chain is short. There are other instances that change the way you terminate in more advanced setups, but most of you will probably not need to know those rules.

The second most important rule is setting the correct device ID (or SCSI ID) for each drive. Two devices must never have the same number. For Mac users, and especially people with older Macs, the SCSI cards and built-in SCSI only supports 7 devices. You cannot use ID 7, because the Macintosh takes ID number 7 for itself. Most older Macs with internal SCSI drives use ID 0 for that drive. This leaves 5 IDs - 6 if you don't have an internal SCSI hard drive. (There are SCSI cards that support as many as 15 devices.) On some Macs the CD-ROM is also on the SCSI chain, and Apple almost always sets it at ID 3. Don't rely on this; you should check the back of the device or use a program such as SCSI Prove to see for sure.

The third most important rule about SCSI is making sure you have the correct cable. There are a ton of different SCSI connections. The most popular on the older Macs are:

• 50-pin internal: Usually black. Has 50 pins and used internally. Usually supports 5 to 20 MB a second.
• 25-pin external: Found on most Macs (Beige G3 and earlier.) Usually supports 5 to possibly 10 MB second.
• 50-pin external: Also called "Centronics" and usually found on the back of hard drives and CD-ROMs. There are cheap 25-pin to 50-pin Centronics cables. Usually supports 5 to 20 MB second.
• 30-pin external: Square and found on the back of PowerBooks with SCSI ports.

There are many others you find such as: 50-pin Narrow, 68-pin wide, and various combinations/speeds of all of these. This is an introduction to SCSI, so I will not delve into various cables and connectors on hard drives. I will say this, though, if you have a hard drive that uses a cable connection you do not have, there is an adapter or cable out there that will work with your system without buying a new SCSI card (at least 99% of the time).

Other quick tips about SCSI:

• Never, ever, unplug SCSI drives while your computer is on.
• Always check termination on new drives; they may have a switch and not require a physical terminator.
• Certain devices need stronger termination than others. Example would be my scanner cannot be the last SCSI device because the burner will crash halfway through making a disc. If I move the burner to the end of the chain everything works great.
• Just because a device is SCSI does not mean it will work with the Macintosh.
• Hard drives, SCSI cards, and cables that support things like SCSI-3UW will cost you a lot more than your plain old Centronics 50-pin cable. If you don't need it, don't buy it.
• SCSI drives are more expensive than IDE drives, so if you can, get IDE before SCSI.
• SCSI IDs are set either by a switch of some kind or jumpers. These can be located at the back of the device, or on the bottom of the device (especially in the case of older hard drives).

IDE

IDE was once used primarily in IBM compatible machines, but since 1997 has become a regular Macintosh feature. IDE does not have IDs like SCSI, they have two settings: Master and Slave. On Revision 2 Beige G3s and newer Macs, you can connect up to two IDE drives per cable, and there is only one IDE cable per connection on your motherboard. If you have two motherboard connections, you can have four IDE drives. On earlier Macs you can usually only have one IDE drive per connection. One drive has to be the Master; the second must be the Slave. You can't have two Slaves and one Master - just one of each.

IDE drives of all kinds must be internal, because there is no external connection standards or system for using drives outside of the computer. Say you have a Power Mac G4 and you want a CD-RW and DVD ROM drive at the same time. You should not buy an IDE CD-RW, because you will have to replace your DVD ROM drive.

There are three common IDE drive "speeds" today: ATA33, ATA66, and ATA100. ATA100 is brand new and rarely used. ATA66 is faster than ATA33, and it is compatible with ATA33 drives. ATA66 drives are almost always compatible with ATA33, so you really shouldn't have to worry about this.

IDE drives are almost always cheaper than SCSI drives. In fact, a FireWire or USB hard drive is usually just an IDE drive inside a case with a special converter that allows it to work with FireWire or USB.

FireWire

FireWire was developed by Apple a few years ago with intention of using it for peripherals such as cameras, scanners, keyboards, and so on. It was not used by Apple when it was first developed, but they licensed it to companies like Sony, who used the technology to easily transfer Digital Video (DV). This helped to prop up FireWire's popularity. Apple then started using FireWire for peripherals like they planned. This has created a market for FireWire hard drives.

The great feature of FireWire hard drives is that they are very easy to connect. There are no jumpers, special cables, or termination needed. These drives can be plugged in and unplugged without shutting down your Mac. Some also draw their power from the FireWire port, eliminating the need for a power supply or AC cord.

IDE drives are usually the guts of FireWire drives. Unfortunately, if the IDE-to-FireWire controller inside the case is not of good quality, the speed sometimes suffers.

USB

USB is similar to FireWire in that it is hot-swappable, there are no settings to make, and they have IDE drives as the actual drives. The way USB differs is that USB is much slower than FireWire. It's almost useless for everyday use. I would only use USB for backing up data, and even then I would only use it if my computer only had USB (such as the low-end iMacs).  

Charlie Ruggiero has used a lot of Macs, from Plus to G4, and even ran a BBS (remember those?) on a Plus. He works as Macintosh tech support and technology advisor for the College of Education at Michigan State University. He does a lot of hardware and software troubleshooting, as well as a great deal of video editing, capture, and streaming. Charlie is well versed in HyperCard, fairly knowledgeable in Future Basic, and has a good background in sound and video. He even has his own site, Edge of Heaven.