Classic Computer Magazine Archive START VOL. 3 NO. 11 / JUNE 1989

Small Tools

Of Cables And Controllers


So, what's in this box called a "hard disk" anyway?

Usually, when I ask questions like that, a phillips-head screwdriver isn't far away; I dig in and find out. If you feel comfortable with hardware and know about things like static electricity, I'd feel free. (Warning! This will violate your factory warranty!) One thing to be especially careful of are the heat sinks in the power supply board. They'll have a DANGER! tag on them, and for a darned good reason; they have 300 volts on them! Touching them will guarantee you at least a good jolt. I speak from experience; when my hard disk gave up on trying to kill me just with software errors, it tried to get me with hardware.

atariharddrive.jpg A hard disk for an
Atari consists of an
verter, a disk con-
troller, the hard disk
mechanism itself, a
power supply and
cables. There's a lot
to know about each
they can all fail in
unspeakable ways!

Because of the complexity of this subject, it will take two columns to complete this discussion of hard drive hardware. This month, I'll focus on cabling, converters and disk controllers. Along the way, in these columns, I'll take a few short detours to help orientate you further. For example, I'll mention the SCSI bus as part of the hard disk, but also point out that it's the same SCSI that Apple uses on the Mac. This way, you'll be familiar with the jargon. I've found that once someone has the overview--the orientation--then the usually narrow techie documents make more sense.

You Start With the Cable. . .

First, let's look at the cable from the ST to the hard disk. It has male DB-l9 connectors on either end and 19 conductors inside.

Helpful Note: I have had two hard disk cables go bad so far. They usually go "intermittent" first, a maddening state where your hard disk works, but gives errors that are impossible to track down. If you're getting these errors, then it may be a warning to you that your cable is deteriorating.

If your hard disk is giving flaky, unreliable performance, try switching cables. (This usually means a trip to the dealer or borrowing a cable from a friend. And for heaven's sakes, if you find a bad cable, mark it "BAD!" before you throw it in your junk box. A year from now, you might not remember it was bad. (This is good practice for all bad cables, disks, PC boards, etc.)

This is made triply worse by the 18-inch cable lengths that Atari supplies. This short length practically guarantees some sharp bends in the cables; a few bends later, the internal conductors break. Again, I've seen this twice.

When I opened up one Atari cable to fix it, I found numerous cold solder joints, a sure sign of bad construction. I have also seen cables get so twisted up that the connectors inside the plug pull towards one another and short out; again, not good. Therefore, investing in a spare hard disk cable wouldn't be a bad idea at all. ICD has a particularly nice long cable I use and happily recommend.

Electrically, this cable is on what is known as the ACSI bus. This is Atari's own custom interface used by no one else in the industry. It does bear a passing relationship, however, to SCSI, (often pronounced "scuzzy") which is an industry standard.

Let's follow the cable into your drive. It plugs into a 19-pin female connector on the case.

A few manufacturers (ICD for one) now supply a 19-pin daisy-chain twin female connector on their ACSI plug-in board. You can then plug another cable from the first hard disk to a second hard disk, much like daisy-chaining floppy disk drives. This is both good and bad. The good is that it lets you add another hard disk; the bad is that unless you know what you're doing, the second drive won't work at all. The second drive must be internally configured (wired) to respond to a different SCSI device number; otherwise, the two drives will both respond to-any command, confusing things terribly.

Also, the ACSI design is very sensitive overall and may keep a second unit from working at all, depending on when your machine was built, cable impedance, length, distance from a local radio station (I kid you not), and so on. Atari laser printers may aggravate this situation as well.


From the 19-pin female connector on the case, the signals enter the ACSI to SCSI interface board. This board simply converts the ACSI signals into the industry-standard SCSI signals. Generally, the SCSI signals emerge on a 50-pin ribbon cable from this board.

SCSI is the industry standard for general communication between CPU's, hard disks, and other high-speed devices. It's really pretty neat and very quick on its feet. The Apple SCSI connector on the back of the Mac is a DB25 with only 25 pins, just like a modem cable; how (you might ask) can Apple use 25 pins when the SCSI standard is 50 pins? Well, of the 50 pins, 24 are ground, and a few others aren't too important! With a little work, you can convert an adapter cable from the Apple 25-pin SCSI to a standard 50-pin SCSI. The innards of an Atari drive can then be made to work with a Mac--that's how compatible SCSI is across machines!

The 25-pin spare connector on a Supra drive is an Apple SCSI connector for daisy-chaining drives. WARNING: This is NOT a place to plug in a modem or printer. Just because the plug fits (DB-25) doesn't mean that you should; it will almost certainly destroy the Supra if you do.

The Disk Controller

Back to the SCSI ribbon cable coming from the ACSI-SCSI board.

The 50-pin ribbon cable carrying the converted commands and data plugs into the disk controller. This is a standalone computer with plenty of smarts. The one I have, for instance, uses an 8085 microprocessor; that's one of the many processors supplied in CP/M machines.

The disk controller is a dedicated computer which processes disk commands sent to it over the SCSI wires (bus). Most commonly, it reads or writes 512-byte sectors to the hard disk and passes the data from or to the ST. Sometimes it's also asked to do things like format the hard disks, or park the disk heads.

The board Atari usually uses is the very popular Adaptec ACB-4000 controller. This is a pretty good controller; cheap, reliable, nothing too fancy.

NOTE: The newest Atari designs have merged the ACSI and hard disk controller onto one PC board, eliminating the separate 50-pin cable. This doesn't change this discussion, really; just consider the two merged on the Megafile drives.

From the controller there are two cables to control the hard disk, one 34-pin and the other 20-pin.

ST 506--Or, On to the Disk

A hard disk known as the ST-506 (a Seagate 5-megabyte unit) long ago defined the signals in the 34-pin and 20-pin cables, so they're called an ST-506 compatible interface. If you look in an IBM add-ons book, you'll see that most of the hard disks are ST-506 compatible; that means you could plug most of them into this controller, given the correct software. Electrically, they should work okay and since both Supra and ICD supply software that let you specify your controller and hard disk specs, you can then format almost any ST-506 disk and make it go. I recommend ICD and Supra because I have personal experience with them; other people have said good things about other manufacturers whose products I haven't tried.

Many, many people are swapping out their 20-megabyte hard disk mechanism and replacing it with a 40 megabyte half-height mechanism. In this way, your Atari (or other) hard disk suddenly becomes a 40-megabyte drive.

Another very interesting alternative is to add a second hard disk to the ACB-4000 controller Atari uses. The controller can run two drives by using just a second set of ST-506 cables. I have two 40-meg drives running from a single controller. The setup is very reliable and it provides enough storage for most of my purposes. The second drive's partitions then become more drive letters to add (the first drive is C,D,E,F and the second is G,H,I,J). With the street price of 40-meg mechanisms at about $399, this is a very attractive and cost-effective way of adding storage. See my article in the Winter 1987 issue of START, "Megabytes not Megabucks," for a more detailed description of how to do this.

Note: Seagate's ST-4096, full-height, 80-megabyte, 28 millisecond (fast!) drive is now down to about $515. This is an incredible steal; 40-megabyte drives usually cost about $400. Check out the ads in Computer Shopper Magazine, but remember, full-height mechanisms won't fit in many chasses.

The best 40-megabyte mechanism I know of for the price that would fit directly into your ST chassis (e.g., half-height) is the Seagate ST-251-1; the difference between the ST-251-1 and the plain ST-251 is a little more cost, but a lot faster average access time. The performance makes an astounding difference; I recommend it.

Coming Attractions

Next column, I'll take a closer look at the disk drive mechanism itself and the power supply. I'll also give you a few more tips on trouble-shooting and even more suggestions for making the most of your hard drive. See you next time!

David Small is a START Contributing Editor and the creator of Magic Sac and Spectre 128. Dan Moore is best known in the Atari world as the creator of Paper Clip for the 8-bit machines.