Statt Emulator: Bastler baut Apple II Plus komplett in Hardware nach
In an impressive display of creativity and technical skill, Simon Boak, a savvy hobbyist from Scotland, has constructed an Apple II Plus from modern components. Rather than relying on software emulation, he opted for a genuine hardware recreation. His project, the SB Mini II, closely adheres to the original circuit principles conceived by Steve Wozniak while integrating certain modern changes.
The Foundation of the SB Mini II
In his blog, Unimplemented Trap, Boak reveals that the original Apple II/II Plus was built from standard parts, many of which are still available today. The 6502 processor and various logic chips used in the original design can be found on the market, making it feasible for Boak to replicate the classic system. Apple even published complete schematics in the manual for the Apple II, allowing enthusiasts like Boak to understand how the machine operated. This openness was crucial to his project.
At the heart of the SB Mini II is a standard 65C02 processor, clocked at approximately 1.024 MHz. This frequency is achieved by using a 4.096 MHz quartz oscillator, which is divided by four. This is remarkably close to the original 1.023 MHz frequency used in the 60 Hz models.
Memory Configurations
Unlike the original Apple II, which used DRAM, the SB Mini II utilizes 48 KBytes of SRAM, accomplished through one and a half 32-KByte SRAM chips, leaving 16 KBytes unused. This transition from DRAM to SRAM simplifies the circuitry significantly, eliminating the need for the original’s refresh logic. Boak notes that DRAM was primarily a cost-saving measure in the past; today, SRAM is cost-effective and widely available.
Originally, Boak had considered incorporating a Language Card logic that would allow for a full 64 KBytes of memory. However, due to the complexity involved, he decided against it. Instead, he has included a replica of a Saturn 128-KByte card in Slot 0. The ROM is supplied as a 12-KByte image contained within a 32-KByte EEPROM, with the option to switch between different sections using jumpers.
Design Limitations
Boak openly addresses some of the compromises made during his reconstruction. Notably, the slots do not accommodate DMA signals, USER-1 signals, or a 7 MHz clock. Unlike the original model, there is also no cassette connection.
The keyboard and peripheral integration demonstrates a balance between authenticity and modern technology. A Raspberry Pi Pico manages a USB keyboard interface while producing electrical signals that mimic the original Apple II’s ASCII keyboard. Remarkably, level shifters are unnecessary, as the Pico operates solely on logic within the board, using 3.3 Volts that are compatible with the CMOS logic.
Video Output and Design Innovations
Boak has chosen to eliminate onboard graphics entirely. Video output is provided exclusively through an Apple II VGA card installed in the expansion slot. In the original Apple II, a significant portion of the circuitry was dedicated to generating a composite video signal. Boak’s approach favors the VGA card, resulting in enhanced image clarity.
The SB Mini II is also notable for being Boak’s first four-layer PCB design. Power is supplied through 12 volts, converted into 5 volts via a Pololu regulator.
Custom 3D Printed Enclosure
The enclosure of the SB Mini II consists of several 3D-printed components that are glued together and subsequently painted. The design closely resembles the Apple ProFile hard disk enclosure but includes ventilation slots and the rear structure of the original Apple II. The cover can be easily removed without tools to provide access to the interior.
In conjunction with the computer, Boak has also created the Studio II Monitor, which is designed to house an 8-inch LCD with a resolution of 1024 × 768 pixels. This customizable housing features aesthetics reminiscent of early 2000s Apple displays.
Conclusion
The SB Mini II stands as a testament to the enduring legacy of the Apple II and the passion of enthusiasts who cherish this pivotal system in home computing history. Boak’s project not only celebrates the classic design but also serves as an inspiring example of how modern technology and classic architecture can come together to revive history. For details and schematics related to the project, visit GitHub.
