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. 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.
This is a series of photographs showing how to assemble ZZ80MB.
Start off with bare pc board, component side & solder side.
I like to assemble a board with the lowest components first. This way, I can insert the component, flip it over, and use the weight of the board to keep the components flat against the pc board. So first group of components are through-hole 1/8W resistors.
Next is capacitors and a diode.
IC sockets, SIP resistor, and serial port header.
Two RC2014 expansion connectors are next
Last are PLCC socket, reset button, voltage regulator, 10uF tantalum cap, and power plug
CF adapter is quite tall, so it is assembled last. Here is ZZ80MB all assembled and populated.
I will cover programming and testing of ZZ80MB in next blog
Z280 has two modes of operations. 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. In ZZ80MB Z280 is operating in Z80-compatible mode.
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. One unusual feature of Z280 is its "serial bootstrap" function where it can be configured to boot without ROM through its serial port. With serial bootstrap, it receives 256 bytes of serial data and save them to memory location 0x0 to 0xFF. When the 256th byte has been received, it starts program execution from location 0x0. ZZ80MB utilizes the serial bootstrap function to load an EPROM programmer that can write the subsequent software into on-board EPROM.
Features of ZZ80MB are:
Z280 CPU configured to Z80-compatible mode running at 24MHz with bus speed of 12 MHz
1/2 megabyte of RAM
1/2 megabyte of EPROM
An internal UART at 115200 baud, odd parity, no handshake
Two modes of operation:
Serial bootstrap mode, loading file from the serial port,