The Macintosh IIsi was a compact 3-box desktop unit, effectively a cut-down Macintosh IIci in a smaller case, 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 colour and could drive a number of different external monitors, with a maximum screen resolution of 640 x 480 in 8-bit colour.
The IIsi used a 25 MHz 68030 processor that was down-clocked to 20 MHz to make it less competitive with the more-expensive IIci. This led to a popular hardware hack where the timing module was replaced with a faster one. The hack required soldering on the logic board so was not for the faint of heart. (Detailed instructions for the hack were available from the Berkeley Macintosh User Group.) The parts cost just a few dollars and the hack could be performed in about 30 minutes (if the user knew what they were doing). While the logic board was out of the case, it was an easy matter to fix the intermittent sound problems by taking a pencil eraser to the logic board contacts to the speaker.
It shipped with either a 40 or 80MB 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 a microphone/sound input socket.
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 1MB 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.
Because the IIsi had a single bank of four SIMM (RAM) slots, memory upgrades required four SIMMs of the same capacity. This meant that you had to buy a minimum of four SIMMs to increase your RAM and also that once you had increased the RAM, you could only increase it again by removing your initial upgrade. For a period RAM slot doublers were available that would allow you to install two SIMMs per slot. This made things somewhat more flexible, but even with these in place, the total RAM installed in each slot had to be the same. Unlike the LC, the IIsi was fully 32bit compliant, so was able to access all the RAM installed.
The IIsi's Processor Direct Slot was unique in that it required an adapter to be used with an expansion card. Adapters were available that would accept either NuBus cards or PDS cards. The PDS cards never really took off, however, so the NuBus adapters were the most popular. The Math Co-processor was installed on the adapter card. Apart from the Hardware hack described above, the best way to increase the performance of a IIsi was to install a NuBus video card.
The IIsi also suffered from sound difficulties --- over time, the speaker contacts would begin to fail, and sound would periodically drop out.
Sources and References
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The Macintosh IIsi Forevermac.com