Typically when you’re replacing parts in an old computer it’s either for repairs or an upgrade. Upgrades like adding a more capable processor to an old computer are the most common, and can help bring an old computer a bit closer to the modern era. [Dr. Scott M. Baker] had a different idea, when he downgraded a Heathkit H8 from an 8080 to an 8008.
Despite the very similar numbers, the 8080 runs at four to nearly sixteen times the speed of its predecessor. In addition to this, the 8008 is far less capable on multiple fronts like address space, I/O ports, the stack and even interrupts. The 8008 does have one thing going for it though: the 8008 is widely known as the world’s first 8-bit microprocessor.
The custom 8008 CPU board for the Heathkit H8.
In the video after the break, [Scott] goes into great detail about the challenges presented in replacing the 8080 with the 8008, starting with the clock. The clock is two-phase, so that what would otherwise be a single oscillator now also has a clock divider and two NAND gates.
Boring clock stuff aside, he does some great hacking using the I/O ports including expanding the I/O port count from 32 to the full 256, bit-banging serial, implementing an interrupt controller and even memory mapping 64 KiB into 16 KiB of address space! With that and a few more special adapter circuits, we think [Scott] has done a great job of downgrading his H8 and the resulting CPU board looks fabulous.
Ask anyone to name a first-generation home computer from the 1970s, and they’ll probably mention the likes of the Altair 8800 and IMSAI 8080. But those iconic machines weren’t the only options available to hobbyists back in the day: Heathkit, famous for its extensive range of electronic devices sold in kit form, jumped on the microcomputer bandwagon with their H8. Though it always remained a bit of an obscure machine, several dedicated enthusiasts kept making H8-compatible hardware and software long after the computer itself went out of production. That tradition continues in 2023, with [Scott M. Baker] producing a brand-new DRAM board that’s compatible with any version of the H8.
Although the Heathkit H8 was designed around the Intel 8080 processor, it could also be equipped with a Z80. [Scott] had built an 8085 based CPU board as well, meaning that any other hardware he developed for the H8 had to support these three processors. For something as timing-critical as a memory board, this turned out to be way harder than he’d expected.
First off, he had already made things difficult for himself by choosing DRAM rather than the simpler SRAM. Whereas SRAM chips can be more or less directly hooked up to the CPU’s address and data buses, a DRAM setup needs refresh circuitry to ensure the data doesn’t leak out of the chips’ internal capacitors. [Scott] decided to use the classic D8203 DRAM controller to do that for him — a solution that was pretty common back in the day.
Getting the timing right for all signals between the CPU and the DRAM controller was not at all trivial, however. The main problem was with two signals, called /SACK and /XACK, which were used to pause memory access during refresh cycles. Depending on which CPU was on the other side, these signals apparently had to be combined with other signals, stored in a flip-flop or delayed by a cycle or two in order to align with the processor’s internal logic. None of this seemed to work reliably, so [Scott] looked elsewhere for inspiration.
Luckily, traces are easy to follow on a two-layer board.
He found this on eBay, where a few vintage H8-compatible DRAM boards were for sale. Although [Scott] didn’t manage to win the auction, the eventual buyer was kind enough to snap some high-resolution pictures of the board which enabled him to reverse-engineer the circuit. The board used the similar D8202 DRAM controller and came with logic that generated the proper signals to interface with the 8080 and 8085 CPUs. For the Z80, [Scott] dived into the documentation for Heathkit’s Z80 option and found a schematic with a few logic gates that would satisfy the Zilog chip as well.
[Scott] combined both of these solutions on a beautiful 1980s-style printed circuit board, with a bunch of 7400 series logic gates and even two GAL22V10 programmable logic devices. With full documentation and Gerber files available on the project’s GitHub page, Heathkit H8 owners can now get their own brand-new memory board — in kit form, as a Heathkit should be.
