Plasmode's Diagnostic Overlay Is Designed to Make Debugging MOS 6502 and Other Retro Chips Easier

Pseudonymous retired hardware engineer Plasmode has released a compact board designed to piggyback onto a MOS 6502 or compatible DIP-packaged eight-bit CPU, as a means of making debugging easier while building breadboard microcomputers — connecting the chip to a complex programmable logic device (CPLD) and RAM.

"[This] diagnostic overlay is an add-on board that helps the debugging of 6502 breadboard hardware," Plasmode explains. "The basic idea is a board that stacks over the 6502 to provides user interface. This board has 2 rows of pins 0.8" wide with pin functions [to] match 6502. It plugs over [the] 6502 [to] monitor/control the activities of the microprocessor with several modes of operations. This board contains two components, 128k RAM and CPLD, and two headers, serial port and JTAG port."

Like all great projects, the core concept is simple: when building a breadboard microcomputer from a MOS 6502 or modern equivalent, there's a chunk of time which needs to be sunk into figuring out whether things are working during the prototyping process. This overlay board simplifies that, offering a way to test a 6502 with only power, ground, reset, and clock pins wired up — talking to a debugging machine over a serial connection.

"The diagnostic overlay board is derived from the CRC65 design but without the compact flash interface and using [an] external 6502," Plasmode explains, referring to an earlier design of single-board computer he designed in 2021. "It has a 128k RAM and a 64-macrocell CPLD (EPM7064S). The CPLD has a small (64 bytes) bootstrap ROM, a serial port, and address decoding logic. [The] diagnostic overlay + 6502 can bootstrap over the serial port and load/run application[s] downloaded over the serial port. [The] CPLD can be re-programmed to provide other modes of operation such as instruction tracing and simple bootstrap/testing."

While the current diagnostic board is designed specifically for the MOS 6502 and modern equivalents, Plasmode says the core concept is generic — and it only needs the wiring changing to be compatible with other chips. "Different flavors of diagnostic overlay board can work with other DIP retro processors such as Z80, 6809, 8085, 68008," he explains, "[and] will have different routed add-on board[s]."

More information on the project is available on Plasmode's Hackaday.io page; while the maker plans to create a PCB variant once the concept is fully proven, the current design is based on prototyping board with hand wiring — and a schematic is available for those who would like to try building the same.