When Apple launched the Apple IIGS in 1986, it contained a chip known as the Mega-II. This chip is described as “an Apple IIe on a chip.” This intriguing assertion made this engineer surprise: “Are you able to construct a standalone Apple IIe-compatible laptop across the Mega-II?” Within the newest element14 Presents venture video, I doc my multi-year journey to reply this query.

The 16-bit Apple IIGS featured near-perfect backward compatibility with the 8-bit II, II+, and IIe software program library. Many imagine the rationale for this compatibility was the Mega-II chip. Nevertheless, after analyzing the design of the IIGS, it turned clear to me it wasn’t accountable for software program backward compatibility in any respect! The Mega-II solely does two issues: handle IO gadgets and produce the Apple II video modes. The IIGS’s chipset and firmware deal with every thing else.

The video explains three different gadgets that use the Mega-II or variants: the Tiger Studying Pc (TLC), Video Overlay Card, and Apple IIe Enlargement Card. The enlargement card used a a lot later design known as Gemini. None of those gadgets seem to make use of the identical Mega-II within the IIGS to operate as an IIe. Though the TLC, which by no means left check advertising, comes very shut.

The element14 Presents video covers a short overview of the preliminary design known as Rev 1. It was a small assortment of PCBs that booted to the Applesoft immediate. However that was it! The remainder of the video walks by the rev2 and rev3 designs.

Rev 2 is a Mega-II {hardware} improvement platform. It incorporates a backplane design to permit for testing blocks of the Apple IIe circuit and features of the Mega-II. The backplane’s sign structure solved a major challenge from the primary revision. It eradicated 30 fly-wire connections by laying out the deal with and knowledge indicators so they might join on to the Digilent Digital Discovery! This one easy trick made debugging with the logic analyzer almost painless!

Total, Rev 2 supplied vital learnings concerning the Mega-II. Whereas I encountered a number of points, none have been essential roadblocks. For instance, the video briefly explains the function of one other IIGS ASIC known as Slotmaker. This chip supplied an Apple II-compatible slot AND the flexibility to speak to disk drives by one other IC. The Built-in Woz Machine (IWM) is a single-chip model of the Apple II Disk Card, designed by Steve Wozniak.

The Mega-II makes use of two Raspberry Pi RP2040 microcontrollers, one for enter and one for output. On the enter facet, an RP2040-based circuit offers an interface for contemporary USB keyboards. The microcontroller’s Programmable IO (PIO) modules for the GPIOs reply quick sufficient to the keyboard management indicators from the Mega-II. (Conventional microcontroller interrupts have been too sluggish.) On the output facet, an RP2040 captures the digital video bus from Mega-II and converts it to VGA output.

Rev 3 turned the main focus after testing and growing all of the {hardware} blocks. This model of the venture is a single-board model of Rev 2. The 4-layer printed circuit board measures roughly 165 by 175 millimeters. It consolidates a contemporary WDC 65c02, EEPROM, 128K of DRAM, the Mega-II, an IWM with 20-pin and 19-pin connectors, Slot Maker with Apple-II appropriate slot, and each Gameport connector varieties.

Mega IIe’s further interfaces embrace a VGA port, USB-Host for the Keyboard, a 3.5 earphone jack, and energy enter. The general package deal is simply barely bigger than the 5.25-inch disk drive used with the unique Apple IIe!

Sadly, the preliminary Rev 3 expertise was much less enjoyable than Rev 2. The issues began with the (first) PCB home manufacturing the boards incorrectly after which cascaded from there. The EEPROMs have been extremely unreliable, a board design drawback prevented them from working, and once I lastly obtained the board besides, I loaded the EEPROM with the mistaken binary. Nevertheless, persistence paid off, and ultimately, Rev 3 answered my query concerning the Mega-II!