Specifications

CHAPTER 1
Introduction
12 Floppy Disk Drive
Comparison of SCSI Arrangements 1
There are now three arrangements of SCSI cabling and termination used in
Macintosh computers.
The first arrangement is used on Macintosh computers that support only one internal
SCSI device (examples include the Macintosh LC II and the Macintosh Quadra 700).
Terminators built into the internal SCSI device terminate the internal end of the SCSI bus.
A separate terminator block at the last external device terminates the external end of the
SCSI bus.
The second arrangement is used on the Macintosh Quadra 900 and 950 computers. Those
machines have two SCSI driver ICs, one for the internal SCSI devices and one for the
external devices. (The software treats the two hardware buses as one virtual bus with a
single set of SCSI ID numbers.) The internal and external SCSI cables are both terminated
on the logic board. In addition, the internal cable is so long that it—like the external
cable—requires termination at both ends, so it has built-in SCSI terminators for the last
device. While this arrangement provides for higher transmission speeds because the two
segments of the bus are terminated separately, it is expensive because it has two driver
ICs and two sets of active terminators on the logic board.
The third arrangement is used in the Macintosh LC 520 computer as well as other
recently introduced models with more than one internal SCSI device (such as the
Macintosh IIvx and the Macintosh Centris 650). As described earlier, that arrangement
uses a single SCSI driver IC for both internal and external devices and provides
automatic termination on the logic board.
Floppy Disk Drive 1
The Macintosh LC 520 computer supports one internal high-density floppy disk drive
(Apple SuperDrive). The drive is connected to the logic board by a 20-pin connector.
Table 1-3 shows the pin assignments for the floppy disk connector.
The floppy disk drive uses a special connector adapter that allows it to be installed by
sliding it into a bay in the computer’s chassis. See Chapter 5, “Internal Storage Devices,”
for information about connector adapters.