ZEUS is a custom modular standalone 8-bit computer kit for boys 40+ and computer science students. It was designed to be robust, handy in assembling, usage, storage and good for Z80 assembly language learning and code developing in a bootstrapping way.

I always wanted to have a computer good for lowest possible level of programming (machine code/assembly). My wish was to enjoy the magic of gradual bringing to life a piece of a hardware, that I can easily comprehend and where I can control everything with use of simple yet effective machine language subroutines, reinventing my own ways towards building operating system, assembler/disassembler and maybe even some homebrew high level language. And this small machine had to have an ability to exchange data with a modern PC, but also be completely autonomous from it. My first 8-bit design was based on 3 solderless breadboards. And the experience of hand wiring hundreds of connections made me dream of a common bus modular PCB design. And this kit is the final result of that effort and it can be further expanded with some additional modules, that now are easy to develop and test with this kit.

ZEUS specs:

• Z80 CPU 10 MHz
• EEPROM 32 kB
• SRAM 128/512kB
• TFT2.8" 320x240 (itead9341)
• UART-USB 115200 baud
• PS/2 port for keyboard

ZEUS memory map:

• highmem($8000-$FFFF) SRAM
• lowmem($0000-$7FFF) SRAM banks C,D,E (128k) or banks 0-E (512k)
• lowmem($0000-$7FFF) EEPROM bank F

Current modules decode only HIGH nibble of IO port, low nibble is used for device ports selection. Thus one can address up to 16 IO devices with up to 16 ports each.

ZEUS IO map:

• port 0x0X (SIO) UART, PS/2
• port 0x1X (PIO) TFT, SD
• port 0xFX (flip-flop) memory banking and other

During design development I decided to use SMD capacitors and resistors with footprints large enough for soldering iron. It made ZEUS modules compact and neat while still simple enough to solder. I have also designed a simple and compact shield which turns Arduino MEGA into EEPROM programmer, that is necessary for bootstrap coding. There is latest version of Arduino code that writes ZEUS Monitor (current version of my ZEUS coding efforts) to EEPROM with use of the shield.

ZEUS hobby computer comprises:

• ZEUS6 backboard
• ZEUS CPU module
• ZEUS MEM128k(512k) module
• ZEUS SIO module
• ZEUS TFT2.8 module

Apart from 40-wire Z80 CPU bus ZEUS has auxiliary bus for System Register Q0-Q7 (partially used for memory bank switching), interrupt daisy-chaining (see Z80 manual) AX0-AX7 and UART signals Rx/Tx. ZEUS is powered through USB-B port which also used for communication with PC in case USB-UART bridge IC is installed in backboard. Otherwise one can use FTDI header on ZEUS SIO module for communication with external world. ZEUS TFT module has GPO line "LED" which drives orange LED and also can be used as CS for TFT driver IC (itead9341). TFT Arduino shield is also equipped with microSD card slot which can be used for data storage. It's better to use microSD card up to 2GB as such cards can process smaller chunks of data at a time. MicroSD card is not included in the kit. ZEUS SIO module is compatible with any Z80 family SIO (0,1 or 2). There is WR-WRE jumper on ZEUS MEM128k/512k module that provides (if shorted) opportunity to write data to EEPROM in situ. But AT28C256-15PU is too slow for 10MHz system clock, so if you don't want to corrupt EEPROM data you should switch system clock to 7.3728MHz (by removing 10MHz generator from ZEUS CPU module and shorting CLK-UCLK jumper on ZEUS SIO module) before writing data to EEPROM.

• ZEUS schematics
• ZEUS Monitor tips
• ZEUS kit assembly manual

Components tested: TFT shield, EEPROM, PIO, SIO, clock generators.