In the last log I posted, I had said that I would be ordering PCBs for my new SBC V2.1 design. However, after refining the schematic and PCB as much as possible, I decided that along with the SBC, I would build a modular, backplane-based design to allow for much more versatility, easy prototyping, and lower costs. Along with this, I decided I should finally give my project an official name. Z80 SBC is far too generic and could refer to any SBC design. Thus, the ZTO-80 was born.

The name ZTO-80 is the name of the general design of my Z80 system. It can be used to refer to either the SBC or the newly-made 40-pin bus I have designed for the system. The name itself comes from a combination of Z80, as this is the processor the system is built around, and my middle initials, T and O (yes, I have two middle names).

While the design of the ZTO-80 SBC has remained about the same since the last update, I decided that instead of having to order a completely new SBC everytime I made a change, I could break the main functions of the SBC into modules and order the new individual module when I changed something. As of now, there are 5 main parts of the ZTO-80 modular system: the backplane, CPU/Memory, Serial I/O, Parallel I/O, and CTC. Except for the backplane, all of the modules are on cards. Because the modules are seperate, unneeded features can be left out, reducing the cost of the system and adding space for custom modules.

The bus has been created to keep the most important signals from the Z80, such as the address and data bus, I/O and memory control lines, the clock, and interrupts, while leaving out non-essential signals, such as DRAM refresh and BUSREQ/BUSACK. This creates a bus that uses only 34 of the 40 available pin. Two of the remaining pins are used for TX/RX (only one module can use these signals at a time, modules should have jumpers instead of connecting directly to these lines) and the last 4 pins are left open for the user.

I will still be updating the SBC, and with this new design created I may also redesign the SBC to have much more efficient routing.

On my EasyEDA page, I have created a project that outlines the template for the design of a ZTO-80 module. It should be a very useful resource for creating cards of your own.

P.S. Grant Searle's page for his 7-chip Z80 computer was closed for some reason or another, and I couldn't find a new version, so I just replaced the link on the project page with an archived copy.