Macintosh IIsi
Encyclopedia
Macintosh IIci
The Apple Macintosh IIci was an improvement on the Macintosh IIcx. Sharing the same compact case design with three expansion slots, the IIci improved upon the IIcx's 16 MHz Motorola 68030 CPU and 68882 FPU, replacing them with 25 MHz versions of these chips. The IIci came with either a 40 or...
in a smaller case (used for no other Macintosh model), made cheaper by the redesign of the motherboard and the deletion of all but one of the expansion card slots (a single Processor Direct Slot
Processor Direct Slot
Processor Direct Slot or PDS introduced by Apple Computer, in several of their Macintosh models, provided a limited measure of hardware expandibility, without going to the expense of providing full-fledged bus expansion slots.Typically, a machine would feature multiple bus expansions slots, if any...
). It was introduced as a low-cost alternative to the professional desktop models for home use, but offered more features and performance than the LC
Macintosh LC
The Macintosh LC was Apple Computer's product family of low-end consumer Macintosh personal computers in the early 1990s. The original Macintosh LC was released in 1990 and was the first affordable color-capable Macintosh. Due to its affordability and Apple II compatibility the LC was adopted...
series. It had color and could drive a number of different external monitors, with a maximum screen resolution of 640×480 in eight-bit color.
It shipped with either a 40-MB or 80-MB internal hard disk, and a 1.44-MB floppy disk drive. The MC 68882 FPU
Floating point unit
A floating-point unit is a part of a computer system specially designed to carry out operations on floating point numbers. Typical operations are addition, subtraction, multiplication, division, and square root...
was an optional extra, mounted on a special plug-in card. Ports included SCSI
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
, two serial ports, an ADB
Apple Desktop Bus
Apple Desktop Bus is an obsolete bit-serial computer bus connecting low-speed devices to computers. Used primarily on the Macintosh platform, ADB equipment is still available but not supported by most Apple hardware manufactured since 1999....
port, a floppy drive port, and 3.5mm stereo headphone sound output and microphone sound input sockets. The IIsi was the first Macintosh released with built-in sound-in capabilities. The Macintosh LC
Macintosh LC
The Macintosh LC was Apple Computer's product family of low-end consumer Macintosh personal computers in the early 1990s. The original Macintosh LC was released in 1990 and was the first affordable color-capable Macintosh. Due to its affordability and Apple II compatibility the LC was adopted...
, which was announced at the same time and also had a sound-in port, was released a number of months after the IIsi.
A bridge card was available for the IIsi to convert the Processor Direct slot to a standard internal NuBus
NuBus
NuBus is a 32-bit parallel computer bus, originally developed at MIT as a part of the NuMachine workstation project. The first complete implementation of the NuBus and the NuMachine was done by Western Digital for their NuMachine, and for the Lisp Machines Inc. LMI-Lambda. The NuBus was later...
card slot, compatible with the other II-series Macintoshes. The bridge card included a math co-processor to improve floating-point performance. The NuBus card was mounted horizontally above the motherboard.
To cut costs, the IIsi's video shared the main system memory, which also had the effect of slowing down video considerably, especially as the IIsi had 1 MB of slow RAM soldered to the motherboard. David Pogue
David Pogue
David Welch Pogue is an American technology writer, technology columnist and commentator. He is a personal technology columnist for the New York Times, an Emmy-winning tech correspondent for CBS News Sunday Morning, weekly tech correspondent for CNBC, and a columnist for Scientific American...
's book Macworld Macintosh Secrets observed that one could speed up video considerably if one set the disk cache size large enough to force the computer to draw video RAM from faster RAM installed in the SIMM
SIMM
A SIMM, or single in-line memory module, is a type of memory module containing random access memory used in computers from the early 1980s to the late 1990s. It differs from a dual in-line memory module , the most predominant form of memory module today, in that the contacts on a SIMM are redundant...
banks.
The IIsi also suffers from sound difficulties: over time, the speaker contacts can fail, causing the sound to periodically drop out. This problem was caused by the very modular construction of the computer, where the mono loudspeaker is on a daughterboard under the main logic board, with springy contacts. Speaker vibrations led to fretting
Fretting
Fretting refers to wear and sometimes corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration...
of the touching surfaces. The problem could be solved by removing the logic board and using a pencil eraser to clean the contacts of the daughterboard holding the loudspeaker. As the si is the only Macintosh to use this case design, these issues were never corrected in a subsequent model.
Because of its heritage as a cut-down IIci, it was a simple modification to substitute a new clock
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...
crystal
Crystal oscillator
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency...
to increase the system's clock rate
Clock rate
The clock rate typically refers to the frequency that a CPU is running at.For example, a crystal oscillator frequency reference typically is synonymous with a fixed sinusoidal waveform, a clock rate is that frequency reference translated by electronic circuitry into a corresponding square wave...
to 25 MHz.
External links
- Macintosh IIsi technical specification (or here) at apple.com
- Macintosh IIsi teardown at ifixit.com