Build Your Own SCSI 40MB Hard Disk

By Tim Standing, University of California at San Francisco

At the beginning of the summer, I decided to build my own Small Computer System Interface (SCSI) hard disk. I spent a month learning all I could about what made a hard disk fast and reliable. Then I ordered the parts and assembled it. It took me an additional three months of writing and debugging the software to make it work. I am writing this article so that you can learn from my experience and build your own hard disk. In the following pages, you'll find all the information that you will need to buy the parts and assemble a SCSI hard disk. My article in next months issue of MacTutor™ will describe a program for formatting your hard disk.

Since I knew next to nothing about electronics, the idea of building my own hard disk seemed foolhardy at first. However, I did know quite a bit about programming and thought that I could write the necessary software. I had installed a couple of hard disks in IBM PC's and knew that there was nothing to it but correctly connecting up the cables. I figured that assembling a hard disk for my Mac was probably just as easy and that I could put one together without understanding how any of those funny looking chips worked. I was right, I still don't know how any of them work. If you've made a few cables before, you too should be able to make yourself a hard disk. You should expect to spend about the same amount of time as I did assembling all the parts (about 6 hours).

This article is divided into two parts. First, I tell you what you need to know about controller boards and disk drives. Then, I give step-by-step instructions on assembly including a list of parts and suppliers.

Understanding Hard Disks

The SCSI interface is a standard way of communicating between microcomputers and peripherals (such as hard disks). The communication between the microcomputer and the peripheral is made using a parallel interface. A parallel interface sends each data byte (eight bits) down eight wires simultaneously, one for each bit. This is much faster than the other type of communication, serial, where all eight bits are sent down the same wire one after another. In addition, SCSI communication uses a bus. This means that all the devices are connected one together in a line, with the Mac at one end. When the Mac wants to talk to a hard disk on the bus, it first sends the hard disk's number (called an address) down the bus to see if the hard disk is there. Each device on the bus must therefore have a unique address so that two devices won't respond to the same command.

Your SCSI hard disk will be made of three parts: a disk drive, a controller board and a box with a power supply. The function of the box and power supply is to protect the electronics and to provide the necessary power. The disk drive itself stores the information that you write to it. So what does the controller board do? The controller board acts as an interpreter. It can determine which of the commands that come down the SCSI bus are addressed to it and can understand what the Mac wants it to do. It then translates these commands into something that the disk drive can understand. It tells the disk drive where to read and write from. It also checks the data for errors. If it finds one, it asks the disk drive to repeat the operation. Error-checking and correction is goes on independently without the Mac knowing it.

Choice of Disk Drive

I recommend a ST225 (20 Megabyte) disk drive from Seagate or one of the ST4000 series (also from Seagate). I've seen the ST225 advertised from a mail order house for as little as $329 (California Digital, 17700 Figueroa St., Carson, CA 90248; (213) 217-0500). The ST4000 series comes in several sizes ranging from 20 megabytes ($500) up to 90 megabytes ($1100) and is available from Mini Micro Supplies (see parts list for address and phone number).

I recommend that you buy a disk drive from Seagate because they have a good track record. Although they don't have the lowest prices, they do make good disk drives and stand behind them. If you have problems with your disk drive (like it makes a whining noise when you start it up or doesn't start at all), double-check everything to make sure that it's hooked up correctly. If it still doesn't work, call the technical support folks at the place you purchased it and explain the problem. Even the best disk drive manufactures have a 1 - 2 % failure rate mostly because the disk drives get dropped during shipping. [Note: This rate varies depending on who you talk to. Usually, once you get a good drive, it will remain a good drive. This applies to commercially assembled drives as well. -Ed]

All disk drives are not created equal. There are five things you should know about the disk drive you are thinking of buying. The first and most important is its formatted size. Disk drives range in formatted size from twenty megabytes up to several hundred megabytes. The size you choose depends on your needs and how much you want to spend.

The second thing you need to know about a disk drive is whether or not it is certified for Run Length Limited (RLL) encoding. By reading and writing at a higher density, RLL encoding allows you to get 50% more data on a given disk drive. Not all disk drives can operate at this higher density without making errors. RLL encoding also requires a special controller board. If you try to use an RLL controller board with a disk drive that is not certified for use with RLL, it may not work. I tried hooking up an RLL controller board to a Seagate ST 4051 disk drive (non-RLL certified) with no luck.

You should also know whether your disk drive automatically “parks its heads” when the power is turned off. “Parking the heads” means that the heads get locked in a special position when the disk drive is not being used so that they don't bounce around scratching the surface of the disk. This is especially important if you plan to carry your hard disk around a lot. Many disk drives require a specific command to park the heads. If the disk drive you use requires a specific command make sure the software you use includes it.

The fourth thing that you should find out is how the disk drive moves the heads across the disk. There are two ways of doing it. One is to use a stepper motor, like that found in the floppy disk drives on your Mac. Like the floppy drives, these are slow. The speed of a disk drive is measured by its average access time which is the time needed for the head to move 1/3 of the way across the disk. All disk drives which use stepper motors have average access times of between 65 and 90 milliseconds.

The other method of moving the heads across the disk is to use a voice coil. This is similar to the voice coil on a stereo speaker. These are much faster and also more expensive. The average access time for this type of disk drive ranges from 25 - 35 milliseconds. Usually these drives automatically park the heads when you turn off the power.

The average access time will not affect how fast it takes you to read a file or start a program when the hard disk is new, but it will after you've been using it for a while. This is because a relatively empty hard disk stores files in sectors (512 bytes each) that are next to each other. Reading a file on a hard disk like this does not require the head to move much. After the hard disk fills up, files end up with each sector located on a different part of the disk. Now the head must move a lot to read the file. Any operation that requires you to read a small file won't be affected by average access time (e.g. starting an application or reading a MacWrite document). However, operations that read many files or read large files are heavily influenced by the average access time of the disk drive. These types of operations include compiling, using a database, returning to the Finder, or opening a folder.

Finally, you need to find out how fast your disk drive will accept step pulses. A single step pulse tells the head to move one track over. The faster you can issue a step pulse, the faster the head will find the track that it is looking for. The interval between step pulses is called the step pulse timing. The step pulse timing has more of an affect for disk drives that use voice coils than for those that use stepper motors. When you purchase your disk drive, make sure that you find out what rate it will accept step pulses. The choices that you have using the controller card that I recommend are 3 milliseconds, 28 microseconds or 12 microseconds.

Older disk drives need 3 milliseconds between each step pulse. This is because they cannot count step pulses while the head is moving and have to wait for the head to reach the new track before they can accept the next pulse. A few years ago, disk drives became available that accept step pulses at 35 microsecond intervals. Step pulses issued at this speed require the disk drive to continue accepting and remembering step pulses while the head is still moving. Each step pulse is stored in a buffer; this is called buffered seeking. IBM adopted a step pulse timing of 35 microseconds for its PC AT computers. This is considered slow by current standards. Most disk drives that you can buy today will accept step pulses at 12 microsecond intervals This is true of all the new Seagate disk drives.

Choice of Controller Board

There are at least three controller boards on the market that will work on a SCSI bus. The one that I chose is made by Adaptec (model number ACB-4000A). It can control one or two disk drives and has many features that make it reliable and easy to use. Adaptec also makes a RLL version of this board called the ACB-4070. I know that the formatting program that I have written works with the ACB-4000A controller board, but I don't know if it will work with controller boards from other manufacturers.

The Adaptec controller board allows you to use any interleave from 1 to 16 (if you don't know what interleave is, you should skip ahead and read the section entitled “What is Interleave”). The driver that I have written performs best with an interleave of 3.

Choice of Box and Power Supply

Most of the boxes which you've seen advertised in the back pages of IBM PC magazines will work for this project. You need to decide whether you want room for one or two disk drives. I bought a box for just one disk drive. It is an ugly grey color and even looks sort of like an IBM PC. I wrote “IBM sucks” across the front of the box just so my Mac wouldn't think that it was about to be replaced.

When you buy a box, get one with a power supply. Make sure that you get a switching power supply as these are less prone to interference from your refrigerator or your landlord's vacuum cleaner. A switching power supply uses a sophisticated high frequency circuit rather than large capacitors to regulate output voltage. As a result, they are smaller, lighter, and create less heat.

What is Interleave

The interleave factor describes how many sectors on a track are skipped before reading the next one. This number is tuned to the speed of your software. If your software can't keep up with the disk drive operating with a given interleave, reading and writing will actually go slower. The smaller the number the faster you can read and write information. An interleave of 1 means that you read each sector in the order that it occurs on the track. An interleave of 2 means that you read one sector, skip one sector. An interleave of 3 means that you read one, skip 2, read one, skip 2, etc. By skipping two sectors, you give the Mac time to catch up with the incoming data.

In order to use a smaller interleave number you must have faster software. The driver that I have written works best using an interleave of 3. This limitation comes from the way that the SCSI manager was written by Apple. If I have the time this winter, I'ld like to rewrite the SCSI manager to make transfers faster. Taking advantage of this will require reformatting your disk with a smaller interleave number.

Read Precompensation and Write Current Reduction

As the head moves towards the center of the disk, data are written closer together. The exact method of reading and writing has to be altered slightly to compensate for this higher density. The Read Precompensation number and the Write Current Reduction number determines the first cylinder which requires these different read and write signals. You will need these two numbers in order to correctly low level format your hard disk.

When you buy your disk drive, make sure that you find out what values to use for Read Precompensation and the Write Current Reduction. For a Seagate ST225, the Read Precompensation is 300. The Write Current Reduction is calculated by the disk drive itself, so you should use a value of 0. For the Seagate ST4000 series of disk drives, both of these numbers are calculated by the disk drive, so you should set both of them to 0. If you buy a Seagate disk drive and don't know what values to use, you can call Seagate Technical Support at (408) 438-5333.

Static Electricity

The chips used on the disk drive and controller board are very sensitive to static electricity. These chips contain very small transistors that can easily be burned out by small sparks. Therefore, both your controller board and disk drive will arrive in static shielding bags with huge warning labels all over them. If you are like me, you'll be totally intimidated by these labels and afraid to open the bags and take things out. Here's what you have to be careful with to avoid frying any of these chips.

Leave everything in its bag until you are ready to use it. This will protect the parts from all sources of static electricity. When you're ready, spread out some aluminum foil and place the parts on the foil. Touch only the sides of the controller board and the metal frame of the disk drive when handling them. Avoid touching the components or any metal on the controller board or the disk drive board (this includes the connectors). If you must touch the components or connectors to plug in a cable, you should first touch both hands to the foil and then to the metal frame of the disk drive. By doing this immediately before you touch any static sensitive parts, you will discharge all the static electricity that has built up on your hands. If you have to service your hard disk after you've put it all together, you should observe the same precautions. If you follow these simple rules, you shouldn't have any problems.

Termination

What is termination and what do you need to know about it? Termination is a method of reducing noise caused by signals bouncing off the ends of the wires. This type of noise is a problem on any high frequency data bus like a SCSI bus. It is called termination simply because it is something that is done to both ends of the bus. Termination is accomplished by taking resistors and placing them on all of the data wires. Two resistors are used for each of the eight data lines. One of these resistors goes between the data wire and ground, and the other goes between the data wire and + 5 volts.

Although Apple should include termination inside the Mac Plus, they don't. They tell you that only the device closest to the Mac and the device furthest away from the Mac should be terminated. If you only have one device (excluding the Mac), then you only need one terminator. If you have two or more devices you will need two terminators. Terminators can be bought from your local Apple dealer (Apple part number M2559).

Assembly Instructions

As I said in the beginning, it took me 6 hours to put all the hardware together. If you have assembled everything correctly, it will take no more than 30 minutes to do the formatting.

Tools Needed

Tool: What you need it for:

Regular Screwdriver Putting the parts together

Philips Screwdriver

Soldering Iron There are only 12 wires that need soldering.

Pliers For attaching connectors to ribbon cable.

File, Hacksaw, Drill For adding new slots for connectors on the back panel of the box that you use.

Volt Ohm Meter To check that cables are made correctly.

Magic Marker

Heat Shrink Tubing

Parts List

Quantity: Item: Description:

1 Adaptec, ACB-4000A Adaptec controller board.

1 Seagate, ST 4051 Hard disk drive (see the or ST 225 disk drive above section for suggestions).

1 Box with power supply

1 Apple, M2559 SCSI cable terminator.

1 Apple, M2556 SCSI System Cable. This connects your Mac to the hard disk.

1 DIP switch Buy a three station DIP switch. You will mount this on the back panel and use it for setting the SCSI address of your disk drive.

8 feet 50 pin Ribbon cable

1 AMP Female connector This is attached to the ground on the frame of the disk drive.

1 Spade Connector To attach the ground wire to the chassis ground.

1 20 pin IDE connector Edge connector for the disk drive.

2 34 pin IDE connector Edge connector for connecting the controller board to the disk drive.

1 50 pin IDC connector Female pin connector to attach to the controller board.

2 20 pin IDC connector Female pin connectors. One is used for the connection between the controller board and the disk drive. The other is used to connect the disk drive address lines to the DIP switch mounted on the back panel.

2 Amphenol, #57F-50 50 pin SCSI connector. These are connectors for the back panel and include latches.

1 foot Heat shrink tubing

1 1 1/4 inch fuse holder For back panel for termination power.

1 1 1/4 inch fuse 500 milliamp, 250 volt fuse for termination power.

4 6 X 32 X 1/2 inch screws They are used for mounting the controller board to the frame of the disk drive.

4 6 X 32 X 3/4 inch screws For mounting the SCSI connectors to the back panel.

4 6 X 32 nuts Also for mounting SCSI connectors.

8 Splined lock washers For mounting SCSI connectors.

4 1/4 inch standoffs. Used for mounting the controller board to the frame of the disk drive.

8 Nylon washers These are to prevent your screws or standoffs from grounding any of the components on the board.

Where to Get Parts

I purchased the controller board from Wyle Laboratories (408) 727-2500. Their price for the Adaptec ACB-4000A is $139. They have good service and great prices.

The box with power with supply and the Seagate disk drive (model # ST4051) were both purchased from Mini-Micro Supply Inc., 1977 O'Toole Ave. Suite B108, San Jose, CA 95131, (408) 435-1977. They charged me $120 for the enclosure (item # E001-P) and $650 for the disk drive. The disk drive may seem expensive until you realize it is 43 megabytes and has 40 millisecond average access time.

The two SCSI connectors for the back panel (Amphenol #57F-50) were ordered from Cetec (408) 748-8550 and were $14 each.

All the other electrical supplies I purchased from Zack's Electronics at 1444 Market St. in San Francisco (415) 626-1444. You could probably go to any electronics supply house and get them or order them from Cetec when you order the other connectors. All of the parts listed above cost me just under $50.

The Apple parts I bought from my local Apple dealer. The screws and nylon washers I bought from my local hardware store.

Before You Start

It is a good idea to wait to buy all the connectors and cables that you need until you have already received the controller board and the disk drive. That way you can see exactly what types of connectors you need.

Look over all the board and the disk drive and make sure that you can recognize all of the connectors that I have illustrated in the diagrams (see fig. #1 ). Notice that all the connectors are labelled as to which is pin #1. You will find the pin numbers on both male and female connectors molded into the plastic of the connector itself. It may take a while to find them as they are often less than 1/16 of an inch high. In addition, the pin numbers are written in white on the boards to which the connectors are attached. I have shown where pin #1 is for each connector in all my figures. In addition, the IDC and IDE connectors also have a triangle on the side nearest pin #1. I usually mark this with a black magic marker when I make up the cables so that I can easily keep track of pin #1 at each end of the cable.

None of the connectors used to connect to ribbon cable require soldering. They are pressed together with the cable between the two halves. You will need a pair of pliers or a small vice to apply enough pressure to get the two halves to mate properly. If you have never used this type of connector before, you should use a volt ohm meter to check that you've gotten good connections at both ends. If you make a mistake with one of the connectors, do not take it apart and reuse it. These connectors are not designed to be reused and will not make a good connection a second time.

The ribbon cable has a red stripe down one side to help you attach connectors correctly. Always attach the connectors to the ribbon cable so that the red wire is pin #1. Assembling the cables carefully and checking them with a volt ohm meter will pay off in the long run. This is where 90% of my problems came from.

In addition to 50 wire ribbon cable, you will need 34 wire, 20 wire and 6 wire ribbon cable. Rather than buying four different sizes, you can take a length of the 50 wire ribbon cable and cut it between two wires with a razor blade to get the desired width you need. When you do this, be careful not to expose any of the wires by cutting through the insulation.

Step By Step Instructions

1). Remove the fan, the power switch, the power supply, the fuse holder and all of the connectors from the box. Mark all wires and screws as you take them off so that you can be sure to replace them correctly.

2). Now you need to add several holes to the back panel of your box (see fig. 2). They will be used for mounting the two SCSI connectors, the extra fuse holder and the DIP switch. The fuse holder you are adding is used for the +5 volt power that is needed for terminating the SCSI bus. The three station DIP switch is used for setting the SCSI address of your hard disk. You will set it as a three digit binary number using these switches. Don't set your disk to number 7 as this is the number that your Mac uses.

Start by making an opening for the two SCSI connectors. The center to center distance between these two connectors must be at least 3/4 of an inch. This is because the cables that Apple sells for the SCSI bus all come with rubber hoods and require extra clearance. (Before you start, remember that you still have to be able to mount the fan on the back panel.) I used a hack saw and a couple of files to make this opening. You must also drill and shape holes for the extra fuse holder and for the DIP switch. Modifying the back panel is the hardest part of constructing a SCSI hard disk and will take you the most time. It took me about 3 hours to do.

3). Replace the power switch, the power supply, the fan, the 110 volt AC socket and fuse for the 110 volt AC input power. Attach the extra fuse holder to the back panel. Make sure that both of the metal tabs point up so that you can more easily solder wires to them.

4). Remove the controller board from its protective bag. You will see two resistor packs that are mounted in sockets. They are labelled RP3 and RP4 on the board (I have labelled the same in fig. 1). They are usually red or yellow and have ten pins all in a straight line. Carefully remove both of these resistor packs using a pair of pliers. Be careful not to damage the controller board. You can throw both of the resistor packs away.

5). Mount the controller board on the frame of the disk drive. There are threaded holes on the frame of all 5 1/4 inch diameter drives for this purpose. (If your disk drive does not have mounting holes for the controller board, drill four holes in the base of the box and mount your controller board directly on the box.) The board should be mounted so that the component side faces away from the disk drive. This will give you room to mount connectors at J4 and J5 (fig. 1). In addition, the red LED on the controller board should be at the same end as the red LED on the disk drive. This places the 20 pin and 34 pin connectors on both the controller board and the disk drive next to each other.

Use the 6 x 32 x 1/2 inch screws to mount the controller board to the disk drive. The 1/4 inch standoffs (those little metal cylinders) will be put between the controller board and the disk drive and will provide an air space for cooling. Put a nylon washer on either side of the controller board to insulate it from the screw head and the standoff. This prevents the controller board from shorting out.

6). Remove the mounting rails from the box if you haven't already done so. The mounting rails are the two pieces of sheet metal that are attached to the base of the box. They are designed to be attached to your disk drive, and will hold it in place. Attach the disk drive/controller board assembly to the mounting rails. There are screw holes in the frame of your disk drive for this purpose. When you mount the disk drive, the controller board should be facing the base of the box. There should be at least a 3/4 inch clearance between the controller board and the base of the box so that you can plug in the connectors at J4 and J5 (fig. 1). You may have to drill new holes in the mounting rails to provide this clearance.

7). Make a 6 inch piece of 34 wire ribbon cable from the 50 wire ribbon cable that you bought. Attach one 34 pin edge connector to each end. Make sure that pin #1 of the first connector is connected to pin #1 of the second connector. Attach one end to the 34 pin edge on the controller board at J2 and the other to the 34 pin edge of the disk drive. Make sure that the two connectors are correctly oriented with respect to the boards. Each male edge connector has a notch between pin #4 and pin #6. Use these notches to help you attach the connectors correctly.

8). Make a 6 inch piece of 20 wire ribbon cable. Attach one of the 20 pin female connectors to one end and the 20 pin edge connector to the other end. Check that pin #1 of the edge connector is connected to pin #1 of the female connector. Attach the 20 pin edge connector to the disk drive. Again make sure that you have it in the right orientation so that pin #1 is at the end which is notched. Join the 20 pin female connecter to the 20 pin male connecter on the controller board which is marked J0 (see fig. 1). The male connector is marked in white lettering on the controller board itself.

9). Now take a look at the remaining 20 pin male connector on the disk drive. It should have one jumper on it connecting a pair of pins which are next to each other. Make sure that it connects the pair of pins that are nearest the 20 pin edge connector. This sets the drive's address for the controller. The controller can only communicate with the disk drive when the disk drive is labelled with the lowest possible number.

10). Make a 2 foot long 6 wire ribbon cable. Connect one end of this cable to the last remaining 20 pin female connector. The six wires should be connected to the sockets numbered 1 - 6 on the female connecter. Separate the six wires for 1/2 inch at the other end and strip 1/4 inch of insulation from each one. Solder wires 1 and 2 to the first switch on the DIP switch. Solder wires 3 and 4 to the second switch and the last two wires to the last switch. Now check that closing switch number one connects pins #1 and #2 on the female 20 pin connecter. Close switch number two and check that it connects pins #3 and #4. The third switch should connect pins #5 and #6.

11). Epoxy the DIP switch into the hole that you cut for it in the back panel. Connect the 20 pin female connector to the controller board at the 16 pin male connector labeled J5 (see fig. 1). Make sure that the socket is installed with pin #1 of the female connector attached to pin A of the male connector (labelled in white lettering on the board itself). You will notice that the male connector on the board has only 16 pins and that your female connector has 20 pins. This is okay, the extra 4 pins simply aren't used.

12). Cut a 2 foot piece of 50 wire ribbon cable. Attach the 50 pin female connector to one end. Connect one of the SCSI connectors to the other end. Again make sure that pin #1 from one connector is connected to pin #1 of the second. You will find that the SCSI connector is labelled with tiny numbers. These numbers are on the outside which is the side with the latches. The SCSI connector and the 50 pin female connector use different numbering schemes. (Only pin #1 on the SCSI connector will be connected to the same pin number on the 50 pin female connector. Pin #2 of the 50 pin female connector will be connected to pin #26 on the SCSI connector, etc.)

Measure 3 inches up the cable from where you installed the first SCSI connector and mount the second one here. Again make sure that pin #1 of this connector is connected with pin #1 of the other two connectors.

13). You must now wire the SCSI connectors on the back panel to provide termination power (+5 volts). Measure up 4 inches from this second SCSI connector and mark it with a magic marker. Now count over from the red wire (wire #1) and find the wire #26. Use a razor blade to cut between wire #26 and #27 from this point back to the second SCSI connector. Make sure that you don't cut through to the wire #26 or #27. You are only cutting the insulation between them. You should now have a 3 inch slit in the ribbon cable. Repeat this operation to separate wire #25 and #26. Now cut the wire #26 as far away from the SCSI connector as you can, about 4 inches. Strip 1/4 inch of insulation from it. Cut a 6 inch piece of wire and strip 1/4 inch of insulation from each end. Solder this piece of wire to the separated wire from the 50 wire ribbon cable and cover the joint with a length of heat shrink tubing. Connect the other end of the 6 inch wire to one of the metal tabs on the extra fuse holder (the one that you installed).

14). Now you must connect the fuse holder to the +5 volt line from your power supply. Identify which of the wires coming from your power supply is the +5 volt line. It will be pin #4 on one of the white plastic connectors that plug into the disk drive and the controller board. Mark it with a piece of tape. Cut a 9 inch piece of wire and strip 1/4 inch of insulation from each end. Solder one end of this wire to the unused metal tab of the fuse holder. Solder the other end of the 9 inch wire to the +5 volt line. The easiest way to do this is to cut the +5 volt line in the middle and strip the insulation from each end. Then solder the two ends of the +5 volt line together with the 9 inch wire from the fuse holder. Be sure to put a piece of heat shrink tubing on one of the wires before you solder them so that you can cover the joint when you are done.

15). Install a 500 milliamp 1 1/4 inch fuse into the fuse holder that you added to the back panel.

16). Connect the two SCSI connectors to the back panel. Use toothed washers between the back panel and the connectors to ensure that the connectors are properly grounded.

17). Plug the 50 pin female connector into the controller board at J4 (see fig. 1). You should check the connection to make sure that pin #1 of the male connector is attached to pin #1 of the female. The pin numbers for the male connector are marked in white lettering on the board.

18). Attach both power connectors, one to the disk drive and one to the controller board (see fig. 1). These connectors are made of white plastic and have four pins arranged in a line. The male connector has a ridge on it to ensure that it is properly inserted.

19). Cut a one foot piece of wire and attach the female AMP connector to one end. Solder the spade connector to the other end. Attach the AMP connector to the ground tab on the disk drive frame. The ground tab is located next to the 20 pin and 34 pin connectors on the disk drive. Attach the spade connector to the chassis ground where the power supply is grounded.

20). Screw the rails into the base of the box.

21). Replace the cover to the box.

22). Make sure that both your Mac and your hard disk are turned off. Connect the Mac to one of the SCSI connectors on the back of your hard disk. Connect a terminator to the other one.

23). Cross your fingers. [MacTutor is not responsible for anything that happens at this point! -Ed]

24). Turn on your hard disk (the Mac should still be turned off). You should hear the disk drive spin up. While it is starting up the red light on the controler card should be on. It should go off when the disk reaches the correct speed. If it starts blinking, or the drive does not start spinning you have not connected the drive to the controller board correctly.

25). Now turn on your Mac. You should see the LED on the controller board blinking at 1 second intervals. The Mac will not boot correctly because the hard disk is not formatted. The blinking LED on the controller card tells you that the controller card is correctly connected to the Mac. If it does not blink, you have either incorrectly wired the Mac, or you have incorrect termination. You may have forgotten to remove the terminator packs from the controller board or don't have a terminator attached to one of the SCSI connectors on your hard disk).