Power Macintosh 7500, 8500, 9500 Series: SCSI-2 Compliance 9/95
Article Created: 5 July 1995 Article Reviewed/Updated: 18 September 1995
TOPIC
The following is a list of common questions and answers about SCSI (Small Computer Systems Interface) buses in the Power Macintosh 7500, 8500, 9500 series computers.
DISCUSSION
Question: How many SCSI buses do the Power Macintosh 7500, 8500, 9500 series of computers have?
Answer: There are two SCSI buses on the Power Macintosh 7500, 8500, 9500 series of computers. There is an internal bus and an external bus.
Question: What are the speeds of the SCSI buses?
Answer: The internal bus has a maximum transfer rate of 10 MB/sec. The external bus has a maximum transfer rate of 5 MB/sec.
Question: How many SCSI devices can I attach to this computer internally?
Answer: There is a connector for the external bus inside the computer on the logic board, however, it is generally easier to simply hook internal devices to the internal bus.
There are 4 drive bays inside the Power Macintosh 7500, 8500, 9500 series but one is used by the floppy drive. If your computer has a hard drive and CD-ROM drive you will have room for one more internal SCSI device for a maximum of 3 devices on the internal SCSI chain due to physical constraints.
Question: How many SCSI devices can I attach to this computer externally?
Answer: The external bus can have up to 7 SCSI devices attached to it. Make sure that you do not exceed the maximum cable length (6 meters or 20 feet) including the cables inside the devices. Additionally all SCSI devices must be powered on when your Macintosh is on.
Question: Does one SCSI bus have precedence over the other?
Answer: Because you can have two SCSI devices at the same SCSI address (one on each bus) the internal bus (all addresses) is set up to take precedence over the external bus. This is rarely an issue, for startup drives the setting in the Startup Disk control panel will be used.
Question: Is the external SCSI port SCSI-2 or Fast SCSI-2?
Answer: The external SCSI channel on the Power Macintosh 7500, 8500, 9500 series computers is SCSI-2 compliant but does not support Fast SCSI-2. The external SCSI channel supports the SCSI-2 standard at 5 MB per second. The internal SCSI channel supports the Fast SCSI-2 standard at 10 MB per second for a hard disk array. Both internal and external buses are 8-bit only, not wide.
Question: Is our MESH Fast SCSI controller fully compliant with the SCSI-2 'Fast' specification as defined in the SCSI-2 spec?
Answer: Our Fast mode is completely SCSI-2 compliant.
Question: Since the Power Macintosh 7500, 8500, and 9500 are SCSI-2 compliant will we see better SCSI performance?
Answer: By itself, SCSI-2 does not mean added performance, although, generally speaking SCSI-2 products are newer and tend to be faster just because they take advantage of the latest software and hardware advances. Optional SCSI-2 modes like 'FAST' and 'WIDE' offer improved SCSI performance.
Question: Did we implement SCSI disconnect/reconnect?
Answer: Yes, we implemented disconnect/reconnect per the SCSI-2 standard.
Question: Will third-party manufacturers be able to optimize their equipment to take advantage of Fast SCSI-2?
Answer: Apple can not comment on the compatibility of third-party products. Please check with the manufacturer of your third-party products for compatibility with the Fast SCSI-2 standard on the internal SCSI channel of the Power Macintosh 9500 computer.
Question: Do we implement the new Windows '95 "Plug and Play" features related to hard drives?
Answer: No, Apple did not implement the new Windows '95 "Plug and Play" features because it is not in the SCSI-2 standard.
Question: If I remove the internal Apple hard drive, is the internal SCSI bus terminated or not?
Answer: The Apple internal drive supplies termination to the internal SCSI bus when it ships from the factory. If the drive is removed and another drive is installed, then that drive should be terminated. If no drive is installed on the internal SCSI bus, then the ribbon cable should be removed from the logic board to prevent problems with termination.
Article Change History: 18 Sep 1995 - Added information on Power Macintosh 7500 and 8500. 09 Aug 1995 - Reformatted to make more readable. 03 Aug 1995 - Added information about internal termination.
Support Information Services Copyright 1995, Apple Computer, Inc.
Keywords: supt,kppc,kpci
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