The revised Plus Too will use a real 68000 CPU, and will add a microcontroller for the floppy disk SD card interface. The next version of Plus Too will use a custom-designed circuit board instead of the Altera DE1 kit.
The disk is read-only, and there is no keyboard, sound, SCSI, serial ports, or real-time clock. It exhibits some stability problems, and implements only the absolute minimum of hardware necessary to boot the computer. This implementation is capable of booting from a single System 6.0.8 floppy disk image stored in ROM, and running programs from the disk. The 68000 CPU is a soft-CPU module within the FPGA. The current implementation of Plus Too is built using a commercial Altera DE1 FPGA development board. The current implementation performs the encoding offline, using a custom-made utility program, and the encoded result is stored in ROM. This encoding is planned to be performed on the fly by a microcontroller. To correctly model a Macintosh floppy disk, the disk image must be encoded into a series of virtual tracks and sectors, with the proper low-level layout, header, footer, checksum, and GCR disk byte format. Plus Too uses an FPGA to model the Mac’s IWM (floppy controller), SCC (serial ports), VIA (general purpose I/O), mouse, and keyboard hardware. The toolbox ROM data was copied from my real Mac Plus (nicknamed “Lonely”) for use with Plus Too. The original Macs contained a Motorolla 68000 CPU and about half a dozen major support chips, with all of the Mac-specific behaviors and OS routines defined in a 128K toolbox ROM. The current implementation has 512K of RAM. The CPU runs at 8.125 MHz, slightly faster than a real Mac. Floppy disk images are planned to be loaded from an SD card, and the current implementation stores a single disk image in ROM. A standard PS/2 mouse and keyboard are used for I/O. The Mac’s 512 x 342 screen is pixel doubled to fit the VGA width, which leaves some extra space below the Mac screen region to display hardware debugging information. It is similar in concept to the Minimig Amiga recreation and the Replica 1 recreation of the Apple I.
The project’s goal is to recreate these classic computers using modern electronic components, producing a stand-alone machine able to run period Macintosh software from the mid-1980’s. Plus Too is a working hardware replica of the Macintosh Plus and Macintosh 512Ke computers. Tux2000 on First Look at the RP2040 – Raspberry Pi Microcontroller.Gorf on First Look at the RP2040 – Raspberry Pi Microcontroller.Steve on Yellowstone Universal Disk Controller for Apple II.Steve on Ron Nicholson, Early Mac and Amiga Engineer.Yellowstone Universal Disk Controller for Apple II Floppy Emu Disk Emulator for vintage Apple II, Macintosh, and Lisa