![]() ![]() The board used the similar D8202 DRAM controller and came with logic that generated the proper signals to interface with the 80 CPUs. Although 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. ![]() He found this on eBay, where a few vintage H8-compatible DRAM boards were for sale. Luckily, traces are easy to follow on a two-layer board. None of this seemed to work reliably, so looked elsewhere for inspiration. 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. The main problem was with two signals, called /SACK and /XACK, which were used to pause memory access during refresh cycles. Getting the timing right for all signals between the CPU and the DRAM controller was not at all trivial, however. decided to use the classic D8203 DRAM controller to do that for him - a solution that was pretty common back in the day. 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. For something as timing-critical as a memory board, this turned out to be way harder than he’d expected.įirst off, he had already made things difficult for himself by choosing DRAM rather than the simpler SRAM. 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. That tradition continues in 2023, with producing a brand-new DRAM board that’s compatible with any version of the H8.Īlthough the Heathkit H8 was designed around the Intel 8080 processor, it could also be equipped with a Z80. ![]() 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. 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. 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. ![]()
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