Introduction

I'm mainly interested in 68K family of processors. I worked with 68K processors during the early phase of my professional career for embedded applications, but I am unfamiliar with how 68K were used in desktop applications. I like to explore the various uses of 68K in the general desktop usage which means interface with disk, keyboard, ethernet, USB, video and operating systems.

During the recession of 2007/2008 I had many opportunities to collect 68K components, memories, and peripherals at bargain basement prices mostly on eBay and some on local electronic auctions. I purchased them as memorabilia and to tinkering with during my retirement. I'm retired now and this is a record of my tinkering.

The first step is put together a tinkering set of tools, then the pathfinder projects to exercise these tools, and finally explore the uses of 68K in the general desktop environments.

I stumbled across the RC2014 community in early 2018. I like the simple backplane and large collection of I/O modules. The single inline connector can also plug into a solderless breadboard for prototyping experiments.

Z280RC is a SBC Z280 designed to plug into the RC2014 and interface to its many I/O modules.

ProtoRC is a 100mm x 100mm prototype board. It centered around a Altera EPM7128 CPLD which interface RC2014 signals on one side and 0.1“ grid array experiment area on the other side.

T68KRC is Tiny68K that plugs into the RC2014 backplane and interface to its many I/O modules.

I was shocked to find out how cheaply pc boards can be fabricated, especially in the 100mm x 100mm format. This capability encourages modular construction in the size of 100mm x 100mm format and the 80-20 prototyping approach of 80% pc board and 20% hand wiring. Unfortunately, most of the board shops are in China, so there are greater cost in shipping and turn-around time. I can amortize the cost and better use the waiting time by working on multiple projects at the same time. The “Tinyxxx” naming convention denotes the 100mm x 100mm format (Tiny) followed by the name of processors, e.g. Tiny030 is the MC68030 pathfinder in 100mm x 100mm format. For ease of construction, I prefer through-hole components over surface-mount components and I'll stay with the 5V components for now.

The 68302 is one of the early 68000-based MPU of the 683xx family. The availability of System Integration Module and abundance of I/O devices make it easy to interface thus it is the first path finder project.

Tiny302 Prototype

Tiny302 pcb

Tiny302 software

Tiny302 construction log

MC68020 is arguably the most perfectly crafted 32-bit CPU. The dynamic bus sizing makes it easy to interface with 8-bit wide memories and peripheral; low power consumption means external heat sink is not necessary; abundance of registers and orthogonal instruction set make writing assembly level language easy; vector base register allows relocation of the exception tables; coprocessor architecture for expanded features notably the floating point and memory management; and it is widely used so parts & tools are readily (cheaply) available. I want to build Tiny020 in the “retro” style which means using through-hole TTL glue logics.

Tiny020 prototype

Tiny020 software

Tiny020 pc board

Tiny020 construction log

MC68030 is a mc68020 with integrated memory management unit plus data cache. I want to do Tiny030 for 3 reasons: I stumbled across a box of new, never opened, wrongly-labelled 68030 tucked away in the garage. I believe it was a part of large crate I successfully bidded on in a local auction and it may well be the most valuable item in that lot sitting there unopened for 10 years! I also want to use the large pile of 4/8/16 megabyte SIMM72 memory modules, and I want to check out Altera's tools by using the EPM7128 CPLD as the glue logic. Since I'm already successful with Tiny020, I'll use the 80-20 pcb-handwiring approach.

Tiny030 schematic

Altera 7128 CPLD schematic

Tiny030 pc board

Tiny030 pc boardbuild log

MC68040 is effectively the end of line for the 68K family. There is 68060, but it was not widely used and part is hard-to-find and expensive. I may work on a 68060 later, but not as a path finder project. The first concern with 68040 is power management. It is a power hog and heat sink design is essential–not ideal in the prototype development environment. 68040 does not have dynamic bus sizing, so all executable memory MUST be 32-bit wide. An external dynamic bus sizer can be used, but there is no space for it in the Tiny format. I'm better off using 4x 8-bit flash for boot ROM and SIMM72 for RAM.

Tiny040 schematic

Explore the concept of $10 68K initially proposed here: https://www.retrobrewcomputers.org/forum/index.php?t=msg&th=152&goto=2253&#msg_2253

Tiny68K design

Tiny68K Software

Tiny68K pc board

Tiny68K construction log

Prototype Z280 board in 100mm X 100mm format

Soldering SMT components

Repurpose the ADC SPX-MPU board–MC68302 single board computer

CP/M 68K on the MPU302, a repurposed ADC SPX-MPU board