The IIsi shares some features with the SE/30, the LC series, and the Mac II series. Like the SE/30, it has a 68030 PDS for expansion. Like the LC, it has no built-in NuBus slot, is quite short, and has a curved front. But with an adapter, the PDS can be converted to a NuBus slot, making it a legitimate member of the Mac II family.
The IIsi was designed as a less expensive, less expandable alternative to the Mac IIci. Cost saving measures included eliminating NuBus expansion slots, soldering 1 MB of RAM to the motherboard, and using a slower CPU (20 MHz vs. 25 MHz). Although the IIsi was marketed as a 20 MHz computer, users quickly discovered it used parts rated at 25 MHz. (Apple had intended it as a 25 MHz computer, but chose to scale back the speed to avoid cutting into IIci sales.) Chipping the IIsi to 25 MHz -- or even 28 MHz - is not unusual.
Like the IIci, the IIsi uses onboard RAM for video, which slows the computer. One way to speed things up is to add either a PDS or NuBus video card (see our NuBus Video Card Guide for more information). Another is to set aside the first 1 MB of RAM, since that is the bank shared for video and program space.
Along with the LC, the IIsi was one of the first Macs with audio input.
The IIsi is noted for a sound problem where the internal speaker may fail to sound. This is caused by poor contact between the speaker wire and the plug on the motherboard. This can usually be fixed by cleaning and coating the contacts on the motherboard with electrical cleaner and lubricant.
There is a ROM SIMM slot on the Mac IIsi which may be filled with a IIsi ROM, although this is rare, since the IIsi generally has ROMs on the motherboard. If you have a IIsi with this ROM, the computer will not function without it. There are mixed reports from the field concerning compatibility with the IIsi ROM and the SE/30; installing a IIsi ROM may make an SE/30 32-bit clean. From Low-end Mac