The IIsi is a fairly compact desktop Macintosh which many believe Apple purposely limited to 20MHz as to not directly compete with the IIci model. The IIsi was released in October 1990.
The IIsi runs on a Motorola 68030 CPU at 20MHz. Generally shipped with either a 40MB or 80MB internal 50-pin SCSI hard disk.
The IIsi uses a single PDS for expansion and requires an adapter to install a Nubus card. The PDS to Nubus adapter will typically also include an FPU. See below for wiki information on IIsi expansion cards:
The IIsi has 1MB RAM on the logic board and 4 slots for 30-pin RAM. The maximum RAM at the time was 17MB (4 sticks of 4MB plus onboard) though later RAM sticks up to 16MB apparently can be used, giving the system a maximum of 65MB.
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. You should be able to accomplish this by setting the disk cache in the Memory control panel to 768K. In another edition of the book, it does mention that you should not exceed 512K cache if you're using below System 7.5 due to a glitch which would actually hurt performance.
If you're connecting the IIsi to a VGA screen instead of an Apple CRT, the VGA monitor must be sync on green compatible. This is generally not a requirement on other old Macs.
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