Small Tools: Of Drives, Power Supplies and Power Systems
Classic Computer Magazine Archive START VOL. 3 NO. 12 / JULY 1989

Small Tools

Of Drives, Power Supplies
and Power Systems

Hard Disk Hardware Explained, Part II


Now we get to the hard disk mechanism itself. (Finally!) Generally, the hard disk contains still another microprocessor! The hard disk receives its instructions via the 34-pin cable and sends/receives data over the 20-pin cable. The microprocessor onboard does things like stepping the hard disk head, spinning up the drive to speed, maintaining the speed at 3,600 RPM and so forth.

The actual mechanisms come in many flavors. First, there are full-height and half-height 5 1/4-inch mechanisms. Then, there are 3 1/2-inch mechanisms--these are newer designs. The actual number of platters and heads inside determines the capacity of the drive; the stepper design determines the seek speed, but allof them spin at 3,600 RPM.

Atari's used several hard disk mechanisms that I've seen. They began with that old workhorse, the Seagate ST-225, a half-height 5 1/4 mechanism. They've also put a 3 1/2-inch Toshiba hard disk in an adapter bracket and I've heard reports of other mechanisms as well.

The ST-225 has an average access time of 65 milliseconds, which is about medium speed. It's easy to swap the ST225 for something like a Seagate ST-251, which is the same size--40-megabytes--and much faster: 40 milliseconds access time. If you want to hot-rod, go for the ST-251-1, which has a 28 milliseconds access time-appreciably faster.

The actual hard disk has several platters within it. It looks something like what you'd have if you glued two floppy disks together with a common hub. Each platter has a read/write head on its top and bottom surface. There are 17 sectors per track and 615 tracks across the hard disk. On some drives, you can actually see the "stepper" mechanism as it moves the head back and forth; on others, the whole works is a sealed unit. (Almost always, though, you can feel the hard drive stepping the head).

On power-up, the Adaptec controller wakes up instantly, says, "Gee, I'm powering up; let me wait for the hard disk to spin up." The hard disk spins up to 3,600 RPM, pulling a great deal of current from the power supply; this takes 10 to 30 seconds. Whenever the hard disk signals the Adaptec that it's awake and at speed, the Adaptec begins doing things to it. First, the Adaptec pulls the head to track 00, to verify that it can; then, it scans across the hard disk, looking for "bad track" information that's stored on the hard disk.

A rat's-eye view of the cabling in an ST hard disk system. Not
only do you have to contend with the SCSI, ST506 and power
cabling common to hard drives everywhere, you also have ACSI--
Atari's own DMA port communications standard.

You can't miss the sound of this process. It's important that you get to know it, for the spin-up sound ("vroooom") and the Adaptec wake-up cycle sound ("whick-whick-whick") should happen every time you power up the hard disk. (If you have a very quiet mechanism, it's harder to hear over the fan, but it's still there).

If your hard disk or controller or cables or power supply fails, the first warning you may have is that power-up will be different. Maybe the disk won't spin up; maybe it won't "whick-whick" as the Adaptec wakes it up. This is like an early warning light.

For instance, my ST-225 gives a good loud "shudder" while waking up. The ST-277R (60-megabyte with RLL formatting) is very quiet and wakes up extremely quickly. The 40-megabyte Miniscribe 6350's take forever to spin up, then give a loud "Snort" as the Adaptec wakes them up.

If you have two drives, the Adaptec lets them spin up individually, then checks them both out. It does this as each wakes up, so if they're different brands, all is well.

Important: if the checkout fails, the Adaptec often will shut down the drive. At this point, the Adaptec will sit and blink its onboard LED light according to an error code established by the company. It will also do this, if it can, if it diagnoses itself as having a problem. You can then look up this code in the ACB-4000 service manual and find out what's wrong. (Naturally, if the ACB is too far gone, it won't do anything.)

So, when checking out a hard disk, you should always be able to power it up, and hear/watch the Adaptec check it out. If that doesn't happen, you have a sick controller, hard disk, power supply or cabling; in the years I've been using hard drives, I have had all of these components go bad at one time or another. I would arbitrarily say, trust the cables least, the power supply next least, then the hard disk, then the controller and finally the ACSI-SCSI board.


Some hard disk manufacturers use something called RLL. RLL is a different way of recording data on the disk drive. Previously, the industry standard was MFM (Modified Frequency Modulation), or "double density," the same technique used on a floppy disk. RLL (Run Length Limited) lets you pack more data onto the hard disk, a sort of "double-double" density. In practice, it gives you about 1.5 times the amount of storage you presently have. For example, a 20-megabyte drive can become a 30-megabyte drive, just by changing the disk controller and reformatting.

Two popular RLL controllers are the Adaptec 4070 and the Omti 3527. Supra thinks highly of the OMTI controller because it uses a 1:1 interleave and and it gives you RLL capacity; this makes it unbelievably fast. I've only recently received the Omti 3527, because they're in short supply (probably because they're so popular). Street price of the Adaptec is around $100, while the OMTI costs around $140.

You'll often see ads in Atari magazines for "build your own" hard disks; I'm trying to familiarize you with the names (ACB-4070, for instance), so you'll know what you're reading about. Controllers aren't switchable between MFM and RLL. Be careful to know what you're buying. Adaptec 4000 is MFM; Adaptec 4070 and Omti 3527 are RLL.

As with many pluses, there's a minus. RLL makes the hard disk work a lot harder and is far less tolerant of minor disk errors than MFM. Hence, if you use RLL, you must use it with a disk mechanism that is "RLL Certified" or "RLL Compatible." Generally, this means the disk platters within the drives are "plated media," which is a little better than the usual media.

Now, the ST-225 and the Toshiba mechanism are not RLL certified. So don't try changing out Atari's ACB-4000 for a 4070 and expect to have a 30-megabyte drive. You'll have to change the drive as well. However, if you are planning on getting more storage and changing drives anyway, why not go RLL and get one-and-a-half of the drives you're buying for the price of the controller? Many rated drives cost only $20-$50 more than the standard MFM mechanism. For example, I use the Seagate ST (60-megabyte, $359 street price) RLL drive. Give RLL some thought.

One note Supra tells me about the very popular Seagate ST-238 (RLL and 39-megabyte) and ACB-4070 RLL Controller combination will fail intermittently if you don't shield them from each other; they're sensitive to electrical interference that the other generates.

Heard of ESDI? That's just another way to hook the hard disk to the controller; it's a different ST-506, you might say. No one I know is doing anything with ESDI in the ST world; it's pretty much restricted to IBM.

Again, there are various suppliers who will built a hard drive according to your specifications, such as Tech Specialties and Berkeley Microsystems. If you're truly brave, you can "roll your own." Supra and ICD will be happy to sell you the only hard-to-find part, the ACSI-SCSI converter, for around $140. You won't save a tremendous amount of money building your own, but it is a fun project.

The Power Supply

Finally, we come to the power supply.

The power supply takes in 110 VAC from your neighborhood power plant, and converts it to +5 and +12 volts. The ACSI-SCSI interface board requires 5 volts. The Adaptec ACB-4000 requires +5 and +12 volts, and uses the industry standard "keyed" 4-pin power plug; this prevents you from plugging it in backwards and blowing up the drive. The hard disks require the same and use the same plug.

You might look briefly at your power supply, but be darned careful not to touch it. You can then determine if you can add a second hard disk or a bigger hard disk without stressing the supply. The amount of current (amps) at + 5 and +12 volts available is listed on both the supply and the hard disk. (The interface boards don't really "drink" enough electricity to worry about).

For example, a full-height Miniscribe 40-meg drive takes 1.8 amps at 12 volts, and 0.9 amps at 5 volts. The power supply I'm using supplies 4 amps at 12 volts, and 10 amps at 5 volts, so I'm well within the safe range. (In fact, you can get into trouble if you don't load some power supplies enough! Many people use IBM clone 200-watt power supplies when building up their own hard disks, and only hang one half-height drive on it; this insufficiently loads the supply, which then hiccups on the power lines. Two drives, please, for a 150-watt supply; one drive is fine with an old 65-watt supply).

Last But Not Least

If you don't use any interface other than the Atari (which has a hardware bug), you can connect many, many physical drives to your ST. As I write this, I have six drives connected to my ST with about 240 megabytes of storage. If 240 megabytes seems excessive, remember that with Spectre 128, my ST is both a Mac and an ST. I have lots of software for both, plus backups and backups of the backups. (I've gotten paranoid after killing so many hard disks in so many creative ways.)

To do this, you need to learn about SCSI enough to use it, and if you want to do it, make a custom 50-pin cable with as many clamp-on 50-pin female connectors as you want to hook controllers up to. You can then hang up to eight Adaptec, Omti, or whichever controllers onto this cable, plug the end into the ACSI-SCSI board, and you're of and working.

Each controller, in turn, can handle two drives (some more, but two is usual), so you could put 16 drives on your ST. Believe it or not, if you use 100-megabyte drives, a billion-byte (gigabyte) system of Atari hardware is possible! Now, of course, you'll need separate power supplies and what-not, but it can be done. Apple even makes "stackable" SCSI drives using just this principle!

One excellent way to do this is to get an IBM clone case preferably with fliptop lid ($40 or so), and an IBM clone power supply. Put the hard disks where they usually go in an IBM and the controllers where the IBM's motherboard usually goes. Then, you weave the SCSI 50-pin cable through the chassis and everything is neat and secure. Most importantly, you then have the drives and controllers being cooled by the power supply's fan--and that makes them last a lot longer. It's fun to see the Atari sitting in front of what looks to be an IBM.

I mention this because having a multi-drive system greatly increases your immunity to failure. First, you have backup, alternate drives and controllers to swap in and out for tests. Second, you have a convenient place for a very quick back-up of your data. Third, you'll have gained the knowledge to hook this stuff up--and that's one of the best things of all to have.

Well, that's an overview of the disk drive hardware, with all sorts of experimenter's and debugger's notes.

See you next time!

START Contributing Editor David Small is the creator of the Magic Sac, Translator I and Spectre 128. Dan Moore is the author of Paper Clip for the 8-bit Atari and the never-released Paper Clip Elite for the ST.