TITLE
    Macintosh Quadra 700, 900 & 950: SCSI Termination Issues 1/93
Article ID:
Created:
Modified: 		9399
12/20/91
1/26/93
TOPIC




    Article Change History
    ----------------------
    01/26/93 - UPDATED
    * To include Quadra 950.
    08/21/92 - UPDATED
    * Clarified how to get internal SCSI terminator for Quadra 700



    This article addresses SCSI termination issues with the Macintosh Quadra
    700, 900 and 950 computers.

DISCUSSION


    What is SCSI?

    SCSI (Small Computer System Interface) is a standard by which one connects
    a device to the CPU. This device can be an external hard drive, CD ROM,
    Printer, Scanner, etc. The SCSI bus needs to be terminated at both ends
    to eliminate signal reflections.

    How is the Quadra 700 Terminated?

    With an internal hard disk drive, there are termination resistors on the
    drive itself. This would terminate the internal end of the bus.

    If the internal hard drive is removed, a internal SCSI terminator must be
    installed (Apple Part Number 630-0408). This part was originally designed
    for the Macintosh Classic computer and a service provider can locate this part
    listing in the Macintosh Classic section of the Service Source CD, Technical
    Procedures or Macintosh Service Guide book. Apple US Service Administration
    can help a service provider order this part by overriding the standard
    minimum order.

    Floppy only system: Apple will install a plug-in terminator (Apple Part
    Number 630-0408) into the internal SCSI connector on the motherboard;
    thereby terminating one end of the bus as an internal drive would do. As
    external devices are added the last device should have an external
    terminator installed (Gray Terminator, Apple Part Number 658-8032). This
    way the bus is terminated at both ends.

    How is the Quadra 900 and Quadra 950 Terminated?

    The Quadra 900/950 has two SCSI buses, Internal and External.

    The Internal Bus is terminated on both ends (one termination on the logic
    board and the other at the other end of the internal SCSI cable). THIS
    MEANS THAT ANY INTERNAL DEVICE SHOULD NOT HAVE TERMINATION RESISTORS.
    THEREFORE, MAKE SURE THE TERMINATION RESISTORS HAVE BEEN REMOVED BEFORE
    INSTALLING A DEVICE INSIDE THE QUADRA 900/950!

    Note: All prior Macintoshs required the internal hard disk drives to have
    termination resisters on them. Hence, most hard disk drives' vendors ship
    their internal disk drives with termination resistors installed. With the
    Quadra 900/950, the terminators need to be removed since the internal bus is
    already terminated at both ends.

    External SCSI is terminated on the logic board. As external devices are
    added the last device should have an external terminator installed (Gray
    Terminator- Apple Part Number 658-8032).

    Though the Quadra 900/950 has two independent SCSI buses, system software
    views the two as a single logical bus. Like with other Macs, the CPU ID
    is 7. This allows for a total of 7 SCSI devices (IDs 0-6) to be added
    internally and externally. Users must be careful not to use the same ID
    on more than one device (internal or external).



Document Information
Product Area: Computers
Category: Macintosh Quadra & Centris
Sub Category: General Topics

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