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No single item may affect the reliable performance of the Apple Macintosh as much as its SCSI components. Near the top of the list (if not the top) are the external SCSI cables.

SCSI termination
A good rule of thumb is to terminate the first and last physical device in a SCSI chain. With some exceptions, this is a good rule. The most obvious exceptions are the Quadra 900 and 950. These two Macs require that NO internal device be terminated, and that the logic board be terminated by placing a terminator at the end of the internal ribbon cable. Middle-of-the-chain termination on the external SCSI bus should NOT be necessary if you use:

External terminators should be used on the last external device on the chain, due in part to the nature of Apple's hardware reset signal which travels on pin 40 of the 50-pin internal SCSI ribbon cable. Quality is critical when using active, pass-through termination. When using a scanner (or other high random noise devices like SCSI printers), only the Apple active, pass-through terminator should be used. Many scanners require the capacitance level of such a terminator to achieve the proper impedance level on the SCSI chain. The black Apple passive, pass- through terminator is excellent and works on most Macintosh models from the IIci through the Quadra 950. Models starting with the Quadra 800 require active terminators to ensure the most stable SCSI chain.

SCSI device order---it does matter!
Address Order
The Macintosh loads or addresses the SCSI devices in the following SCSI ID order: 0, then 6-5-4-3-2-1.

Physical Order
Physically, external devices should be connected to the Macintosh so that the most stable devices (Category 1) are last in the SCSI chain, and the least stable devices (Category 4) first (i.e. directly to the Mac). As an example, if you had a Macintosh IIci with an 80MB OEM internal drive, a 120MB external hard drive, a SyQuest drive, and a flat-bed scanner you would set the ID of the internal drive at 0, the external drive would be at 6, the SyQuest at 5, and the scanner at 4. Physically, the chain would start with the Macintosh, then the scanner, the next device the SyQuest, and so on with the last device being the external hard drive. Internal termination of external devices should normally be avoided because of its tendency to not handle the hardware reset signal and the constant SCSI polling in place on all Macintoshes since the SE.

Discussion of the idiosyncrasies of SCSI could go on forever. Let's summarize the proper load order for Macintosh SCSI devices:

This document is an edited and modified version of an article (mostly based on Apple technical guidelines) which originally appeared in MacInformedTM magazine . It is available on America Online, and subscriptions may be obtained by emailing keithTRON@aol.com.

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