TOPIC -----------------------------------------------------------
What are the major differences between the Macintosh Plus and the Macintosh II?
DISCUSSION ------------------------------------------------------
Several notable differences exist between Macintosh Plus and Macintosh II
functions. Some of these items are listed below:
External Drive Port
There is no external drive port on the Macintosh II. An Apple HD20, 400K
external drive, or 800K external drive cannot be connected to the Macintosh II.
Battery Location
The battery is no longer located within the power supply. Instead, it is
soldered onto the logic board, and can only be replaced by a level 1
technician.
Serial Port Pinouts
While both the Macintosh Plus and Macintosh II use mini-DIN-8 connectors for
serial ports, one signal has been changed on the Macintosh II and the Macintosh
SE. Pin 7, which is not used on the Macintosh Plus, is now a general purpose
input pin. It is connected to the serial communication chip's data carrier
detect line on the Macintosh II. Pin 7 on the modem port may also be used as a
SYNC line, since it is connected to the clock line from the VIA.
SCSI Terminator Power
The Macintosh Plus does not provide terminator power on its SCSI port. The
Macintosh II has added this signal to pin 25 of the external SCSI connector.
Keyboard and Mice Connections
Keyboards, mice, and other input devices on the Macintosh II are now connected
through an Apple Desktop Bus port. The DB-9 connector used for this purpose on
the Macintosh Plus is no longer available.
SCSI Interleaving Schemes
The ROMs of the Macintosh Plus and Macintosh II have different SCSI disk
interleaving schemes. Although disks can be used interchangeably, they will
work most efficiently on the system on which they were initialized. The
Macintosh Plus uses a 3-to-1 interleaving scheme, while the Macintosh II uses
1-to-1 interleaving.
Therefore, a SCSI disk initialized on a Macintosh Plus and moved to a
Macintosh II will not transfer data as quickly as a Macintosh II SCSI drive,
since the Macintosh II will be forced to wait for every third block of data,
rather than using them in sequence.
If a Macintosh II SCSI drive is taken to a Macintosh Plus, the speed
difference will be even more apparent. The Macintosh II drive will have blocks
allocated in sequence on each track. But the Macintosh Plus will not be able to
accept the data that quickly, so the drive must make almost a full rotation
before the second block again passes under the head, and the information can be
read from the drive.
Power On/Off Switch
The Macintosh II has no hard power on/off switch. Instead, the Power On key on
the Apple Desktop Bus keyboard initiates a request for power-on to the
Macintosh II. This key is not a power-off key; power-off is provided through
software via the Shut Down command in the Finder.