Plasmode's ZZ80MB Packs a Zilog Z280, Offers 'Self Programming' Bootstrap Over Serial
A less-successful 16-bit successor to the Zilog Z80, the Z280 powers a clever "self-bootstrap" mode that gets beginners running quickly.
Pseudonymous retired hardware engineer Plasmode has released a vintage-style single-board computer built around Zilog's unusual 16-bit Z280 chip — and, unlike his earlier designs, the ZZ80MB can be bootstrapped with no need for a dedicated EPROM blower.
"This my 4th Z280 design. In this design, I want to make Z280 more accessible to general hobbyists so all components are through-hole parts and glue logic are common TTL logic," Plasmode writes of the ZZ80MB. "Another hobbyist-friendly feature is no external EPROM programmer is needed to program the on-board EPROM."
"ZZ80MB has a 'self programming' feature that can serially bootstrap even with a blank EPROM and load the necessary files to program its blank EPROM. Afterward it can boot up normally with the freshly programmed EPROM. The PC board is in the inexpensive 102mm X 102mm form factor. All parts for this board are readily available for about $35."
While many Z80-based single-board computers exist on the market, those targeting Zilog's lesser-known Z280 are rarer beasts. Released in 1987, the Z280 was designed as a 16-bit enhancement to the eight-bit Z80 — and is itself based on the earlier Z800, which never made it to mass production. Fans of the chip praise it for the inclusion of a range of relatively advanced features, including a built-in memory management unit (MMU) with memory protection, instruction pipelining, and both a high-performance 16-bit Z-BUS interface and a backwards-compatible eight-bit interface.
"Z280 has two modes of operations," Plasmode explains. "In its native mode, it operates with 16-bit data bus and 24-bit of address. Z280 can also operate in Z80-compatible mode where it has a 8-bit data bus with 24-bit address, but only 64K of memory is accessible at any given time. While Z280 was not a commercial success (too slow, too late), it contains many modern CPU features such as MMU, DMA, cache, and on-chip peripherals."
The ZZ80MB design makes use of few of this features, mind, preferring to use the chip in Z80-compatible mode at a clock speed of 24MHz with a 12MHz bus. The board includes 512kB RAM, 512kB EPROM, a 115,200bps internal UART, a 44-pin IDE interface for a Compact Flash drive, two expansion connectors compatible with boards designed for the popular RC2014 range of modular computers, and comes ready to run CP/M 2.2 and 3.
Design files, schematics, and a bill of materials are available on Plasmode's Hackaday.io page.