ZTO-80 SBC Features:
- 6MHz Z80 processor
- 32K of ROM, from $0000 to $7FFF
- 32K of RAM, from $8000 to $FFFF
- Z80 SIO (DART) controlling a full RS-232 serial port (57600 baud)
- Z80 PIO controlling two channels, one connected through a darlington transistor array
- Z80 CTC with four software-programmable timer/counter circuits
- Modified NASCOM BASIC (Microsoft BASIC) interpreter
- Bus expansion header
- Open-source hardware!
Ideas for the future:
- CP/M capability
- I'll have to research CP/M further before trying to implement it
- An additional serial channel
- Completed for V2.1
- An external keyboard/video board to allow standalone functionality
- VGA and PS/2?
- A memory expansion board
- An improved expansion bus
- Completed for V2.1
- Backplane-based (separate design and boards)
- Better PCB layout
- In progress for V2.1 and future versions
- Combining the RAM/ROM and I/O decoding chips
- Possible, but would likely change memory map and prevent more on-board I/O
- A version that supports 74HC chips
- V2.1 fully supports CMOS chips
- Maybe power supply regulation
- Would be on backplane
- Linear regulator has min. input voltage of about 7V, so USB power (5V) couldn't be used without some kind of bypass
The heart of the ZTO-80 SBC is the Z80 CPU, first designed by ZiLOG in 1975.
My Z80 SBC is designed around hardware that would have been available during the 1980s, with only a few exceptions. Despite this, however, all of the hardware to create the SBC is still manufactured and readily available, including the entire Z80 family and the 74xx family. Every part can be found on Digi-Key, except for the Z80 PIO and the SIO/0 (DART), which can both be found on Mouser. They can be expensive, so I have found that eBay is a good option to find chips for lower prices. Once I create a good BOM I'll add it to the project files.
Important hardware info:
The term "ZTO-80" can refer to either the SBC or the modular bus design. Usually, to clarify which I am talking about, I'll say either "ZTO-80 SBC" or "ZTO-80 Bus".
Note that the power input for the SBC is 5V only and is not regulated on the board. Anything higher than 5.5V will damage or destroy some ICs! I choose to use a cable such as this one so that I can use a USB port or AC adapter as a power source.
The Z80 DART is a version of the Z80 SIO with all synchronous functions dropped. It is hardly manufactured (although still active) and a CMOS version was never created. For this reason, it may be desirable to substitute the DART for a Z80 SIO/0. This chip is still produced in its CMOS variant and is nearly pin compatible with the DART. The difference between the chips is that the SIO/0 has pins 11 and 29 as SYNCA and SYNCB for synchronous operation, while the DART uses them for RIA and RIB (ring indicators for modem control). These pins are grounded on the SBC, so either the DART or SIO/0 will work just fine.
I have taken the time to create and layout schematics and PCBs for this project, and they can be viewed in the project gallery and interacted with at my EasyEDA page here. The boards have SMD passives, but they are large enough to easily solder even without magnification. I currently don't sell PCBs, but you can have them ordered through JLCPCB or any other fabrication service. An order of 5 of the SBCs (excluding shipping) costs $8 from JLCPCB. I will likely start a Tindie page to sell boards and modules soon.