>Your computer has two SCSI interfaces: an internal >=Fast<= SCSI chain and an >external standard SCSI chain
Note that these are TWO separate busses. >The internal SCSI chain supports the internal hard disk and CD-ROM drive. An >Apple-authorized dealer or service provider can connect one additional 3. >5-inch SCSI device to the internal SCSI interface. The SCSI device must be >no more than 1.625 inches (41.3 mm) high. "Full Height" is 3-1/8" high, therefore "half height" is 1-9/16" high, which is 1.5625", NOT 1.625". Most standard hard drives are 1" high, while some are less tall. >The internal SCSI chain supports >=Fast<= SCSI, which is capable of >transferring data at up to 10 MB per second (twice the rate of the external >standard SCSI chain). If you obtain a SCSI device that supports >=Fast<= SCSI, >you should receive faster performance if it is connected to the internal >SCSI chain. Devices 0 and 1 are reserved for hard drive and CD-ROM, respectively; Device 7 is reserved for the Mac itself. Devices 2, 3, 4, 5 and 6 are available. >The external SCSI chain supports up to seven external SCSI devices. No, only six, as device 7 is reserved for the Mac itself. >All devices on the same SCSI chain must have unique ID numbers, but devices >on different SCSI chains may use the same SCSI ID number. (For example, you >could have a CD-ROM drive with ID number 3 connected to the internal SCSI >chain and a tape drive with ID number 3 connected to the external SCSI >chain.) > >The drives that were installed in your computer at the factory, as well as >the computer itself, have already reserved certain SCSI ID numbers. Other ID >numbers are available for assignment to SCSI devices that are added after >you buy your computer, as described in the following table. You can change the SCSI IDs of the hard drive and the CD-ROM to anything else which is not conflicting. There are reasons why you would want the internal hard drive to be an ID other than 0. The device "roll call" starts at ID 6 and proceeds orderly down to ID 0. Therefore ID 0 is the LAST device in the "roll call". >SCSI chain SCSI ID number Device > >Internal* > >0 Factory-installed hard disk (terminated) 1 Available 2 Available 3 >Factory-installed CD->ROM drive 4 Available 5 Available 6 Available 7 Power >Macintosh computer (terminated) Correct. >External > >0 Available 1 Available 2 Available 3 Available 4 Available 5 Available 6 >Available 7 Power Macintosh computer (terminated) Correct. >* Although five SCSI ID numbers are available for assignment, only one other >SCSI device can be connected to the internal SCSI chain. Incorrect. All six may be used, and this was the case back in the Q950 and the 9600, and it is still the case in any Mac which includes a SCSI bus, internal or external. What is correct is only one other device is connected in any factory configuration. Usually this third device is either a Zip drive or a DDS-2 tape drive. >IMPORTANT The factory-installed internal hard disk and the Power Macintosh >computer are both terminated. If another SCSI device is attached to the >internal SCSI interface, it must not be terminated. If you attach a >terminated device to the internal SCSI interface, damage to the computerpis >main logic board can occur. Correct. There must be only two terminators on any SCSI bus because the drivers used are capable of "sinking" only 48 milliamperes. Each terminator consists of one 220 ohm pull-up and one 330 ohm pull-down resistor per SCSI signal. Each such terminator takes 24 milliamperes to "sink" its current. Most hosts are auto-terminating; PowerBooks may not be. An autoterminator counts as a terminator, but it may take less than 24 milliamperes to "sink" its current. All desktop Macs supply terminator power to the bus; few, if any PowerBook Macs do. -- PCI-PowerMacs is sponsored by <http://lowendmac.com/> and... Small Dog Electronics http://www.smalldog.com | Refurbished Drives | -- Sonnet & PowerLogix Upgrades - start at $169 | & CDRWs on Sale! | Support Low End Mac <http://lowendmac.com/lists/support.html> PCI-PowerMacs list info: <http://lowendmac.com/lists/pci-powermacs.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[email protected]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive:<http://www.mail-archive.com/pci-powermacs%40mail.maclaunch.com/> iPod Accessories for Less at 1-800-iPOD.COM Fast Delivery, Low Price, Good Deal www.1800ipod.com
