The Macintosh IIsi was a compact three-box desktop unit, effectively a cut-down Macintosh IIci 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). 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 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 was an optional extra, mounted on a special plug-in card. Ports included SCSI, two serial ports, an ADB 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, 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 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'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 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 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 IIsi 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 crystal to increase the system's clock rate to 25 MHz.
Read more about Macintosh IIsi: Trivia