Heathkit H8 Computer Enthusiast Upgrades Memory with a Custom DRAM Board Implementation

In 1977, Heathkit introduced the H8 computer kit. Initially, it shipped with an Intel 8080 CPU and had a 4K RAM board as an option. Early computers like the H8 typically used static RAM (SRAM). While these chips were more expensive than dynamic RAM (DRAM), they required no support circuitry. Even though DRAM chips are ubiquitous today, retro computer enthusiasts typically use SRAM because it is so cheap now. Well, except for one. Dr. Scott M. Baker designed the "H8 Hellboard," a 64K DRAM board for the H8 that works with multiple CPUs.

The main system bus was a significant aspect of computer kits from the late 1970s. At the time, despite some design issues, the S100 bus had gained popularity. Heathkit introduced a 50-pin "Benton Harbor Bus" with the H8. This new design resolved several problems with S100. A side-effect of these standardized buses is that these computer kits can support different CPUs. The H8 is such an example.

Originally, the H8 shipped with an Intel 8080-CPU. However, other CPU cards exist. For example, there was a Z80 upgrade available. And Baker designed a board with an 8085 CPU. When creating the DRAM board, Baker aimed to support all three CPUs.

The fundamental difference between SRAM and DRAM is that DRAM requires constant refreshing. When you write data to SRAM, the contents stay valid for as long as the device has power. Data in DRAM, however, decays over time. So each memory location needs a refresh cycle where the current state is read and then written back. Different computer designs handle DRAM refresh in various ways. For example, Baker's approach uses a dedicated DRAM controller.

Getting the timing correct for DRAM refresh is tricky. And as Baker learned, even when using a DRAM controller, it continues to be tricky! In the H8 Hellboard's write-up, you can follow the learning journey. Baker made several assumptions about how a D8203 controller worked. Even though Baked based some of those assumptions on the D8203's datasheet, the design still had bugs! A final solution for an operational DRAM board came from distributed teamwork.

On eBay, several D8203 DRAM boards showed up for sale. Baker contacted one of the buyers and, with the help of others, virtually reverse-engineered how these vintage boards worked. The learnings from these functional D8203-baed designs helped. For example, the older board used a relatively simple RC delay circuit to create a delay with the write-enable signal.

The result is a 64K DRAM board that works with the 8080, Z80, and 8085. (Or at least it should!) With the Gerber files Baker provides, a GAL22V10D programmable device, and a handful of other chips, you can build an H8 Hellboard for your Heathkit H8.