ADDING DEVICES TO CMD HD SYSTEMS
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Adding on addition SCSI drives or devices to a CMD HD system is generally a very simple process. There are some guidelines and general information which, if understood before starting the process of adding a device, can help to assure success.

The SCSI Bus
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The standards for SCSI devices seem somewhat shrouded in mystique, especially to the Commodore user. SCSI (Small Computer Standard Interface) is a standard which was written to insure compatibility between a wide variety of devices manufactured by different companies for connection to small computer systems. These devices include floppy disk drives, hard drives, tape backup systems, removable cartridge storage systems, CD ROM drives, WORM drives, Scanners, and more.

An SCSI system consists of two parts - a host adaptor (controller) which is in charge of managing the SCSI bus, and one or more SCSI devices such as those mentioned above.

Each device attached to the SCSI bus has its own individual ID (device number), including the host adaptor. A device may be further broken down into a number of individual units all addressed under the same device number, each having their own unit number. This is similar to the way the CMD has its own device number on the Commodore serial bus, yet contains a number of different partitions - each individually addressable. Similar, but not the same. Under the SCSI standard, up to eight device numbers may used - SCSI device numbers 0 through 7. Each device may contain up to eight units, logical unit (LU) numbers 0 through 7.

The CMD HD SCSI Device Assignments
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On the CMD HD system, the host adaptor has been designated as SCSI device number 7, logical unit number 0. The built-in SCSI hard drive device in a CMD HD hard drive has been designated as SCSI device number 0, logical unit number 0. SCSI systems do not allow having more than one device with the same SCSI device number, even if the logical unit number is different. Therefore, only six 'slots' remain available in the CMD HD SCSI system for other devices, SCSI device numbers 1 through 6. 
Most any SCSI device that you may buy will provide jumpers or a switch of some type to allow you to change the SCSI device number assignment for that device. On the many bare hard drive units we have seen, jumpers have always been located either on the circuit board of the imbedded SCSI controller, or on an additional IEC connector on the rear of the drive assembly. Usually when jumpers are used, there are three jumpers in all assosciated with the SCSI device number. The values of the jumpers are normally arranged in a binary fashion, with one jumper having a value of 0 or 1, the next having a value of 0 or 2, and the final jumper having a value of 0 or 4. By adding or removing 'shorting blocks' to these jumpers, the device number may be programmed. The following chart shows how most drives are programmed for SCSI device numbers:

SCSI ID  Jumper 1  Jumper 2  Jumper 3
-------  --------  --------  --------
   0       off       off       off
   1        on       off       off
   2       off        on       off
   3        on        on       off
   4       off       off        on
   5        on       off        on
   6       off        on        on
   7        on        on        on

Most SCSI drives use this scheme, though we have seen at least one
manufacturer who has reversed the scheme (jumper attached gives value of 0 for that jumper). The best advice to follow when setting SCSI device numbers on a particular drive is to make sure that you get the manufacturers information about that drive - you'll often need it anyway to locate these and other features on the mechanism.

If you purchase an add-on drive which comes supplied in a case with its own power supply, there will often be a simple switch located on the back panel to allow setting of the SCSI ID. 
Parity Setting
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While the CMD HD does not use the parity bit on hard drives, it is recommended that the parity mode on your drive mechanism be enabled. Some drives appear not to operate properly on the CMD HD system when parity is not enabled. Again, parity is usually controlled by a jumper located somewhere on the drive mechanism - see the manufacturers data on your mechanism to find out how to enable the parity on your drive.

Connections
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The CMD HD comes supplied with an Apple Macintosh(r) compatible DB-25 (female) SCSI connector. If you buy a bare drive mechanism, it should come with a 50 pin IEC connector on the rear of the mechanism. This is the internal standard for SCSI interconnection. Two external standards exist - the Apple Macintosh DB-25 (used by CMD as well as many Apple and Amiga(r) hard drive manufacturers), and a 50 pin Centronics (used by some Apple manufacturers as well as by many manufacturers whose products are intended for IBM(r) and mini- or mainframe computer systems).

When going from a bare drive mechanism to a CMD HD, you will need a special cable which adapts the 50 pin IEC connector on the drive mechanism to the DB-25 connector on the CMD HD. While we do not recommend making this cable yourself, if you are qualified it is possible to do so. This cable is available from custom cable makers such as Cables to Go in Ohio, and as a stock item (part number RCA614) from The Grapevine Group, Suffern, NY. The Grapevine Group may be contacted for information at 1-914-357-2525, or toll-free orders only at 1-800-292-7445.

When connecting an external drive mechanism (one supplied with its own case and power supply) to the CMD HD, the cable will depend on the connector supplied on the external drive's case. If it is a DB-25 (female) Macintosh compatible connector, then a shielded 25 pin parallel cable will do the trick (this should have a male DB-25 connector on each end). This cable can be obtained from almost any computer supply store or mailCentronics 50 male). This is available from most any Apple dealer, Apple mail order houses such as MacWAREHOUSE (part number ACC 0270) phone number 1-800-255-6227, and from The Grapevine Group (part number AP256). 
The Terminator
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Most SCSI mechanisms will have terminating resistors mounted on the embedded controller board. These fulfill a requirement that the SCSI bus be terminated with a certain resistance value at each end of the bus. The mechanism mounted in the CMD HD has terminating resistors mounted on it. When you add another drive onto the SCSI port on the HD, it too should have terminating resistors mounted. Refer to the manufacturers data for the drive to determine if that drive does or does not contain terminating resistors. If it does not, you should try to obtain these from the supplier from whom you purchased the drive. You can also purchase terminating connectors which attach to the external drive connectors from most computer supply stores and mail order houses.

If you are adding a third (or higher) device to the SCSI bus, then only those devices physically located at the ends of the bus need to have terminating resistors. All devices in-between should have the terminating resistors removed. 
Power Supply And Case Considerations
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If you buy an external hard drive from a reputable dealer, it will come with a proper power supply and case. If, however, you opt to 'do-it-yourself', you're in for a real treat...often buying a hard drive mechanism, case, power supply, and connecting cables will add up to over what you would have spent for an external drive and cable.

You'll have to consider just how much power will be needed by your drive mechanism - again, check the manufacturers specs, and don't forget that it is always wise to over-rate the power supply by at least 10%. There are almost always two voltages involved here - +5 volts DC for the electronics, and +12 volts DC for the motor. The requirements you'll need to meet will be the operating current (usually in milliamps - ma) on the +5 vdc and the starting surge current (in milliamps or amps) on the +12 vdc. The +5 vdc current drain is usually fair