Dr. Scott M. Baker's Vintage Heathkit H8 Gets a Modern Add-On, Powered by a Raspberry Pi
The "Pinculum" includes transfer, serial console, and VGA terminal capabilities driven by a Raspberry Pi Zero and Parallax Propeller.
Dr. Scott M. Baker has got a vintage Heathkit H8 kit-form microcomputer talking to a Raspberry Pi for file-transfer tasks, and more, blending the old and the new to work around ongoing component shortages.
"The Society of 8-bit Heathkit Computer (SEBHC) has a very useful project that uses a Vinculum VDIP1 module to transfer files from USB thumb drive to the H8 computer," Baker explains. "Transferring files by traditional means can be difficult and problematic due to the primary floppy drive on the H8 computer being the hard-sectored H17 disk unit. Gotek [floppy drive emulators], with HXC firmware, sometimes but not always work. The Vinculum-based USB thumb drive support designed by Glenn Roberts, Norberto Collado, and others is an excellent alternative. Except… When I went to build mine, everyone was out of VDIP1 modules, and I do mean everyone."
Component shortages are a recurring theme in recent years, owing to major supply chain issues — and, like many makers, rather than waiting for stock to appear Baker opted to design an alternative using something he already had on hand: a Raspberry Pi single-board computer, with a handy and easy-to-access general-purpose input/output (GPIO) header right there on the board just waiting to be carefully connected to the kit-form Heathkit H8 first released back in 1977.
"The basic idea is relatively straightforward," Baker explains. "Two 74HCT574 latches hold the incoming and outgoing data. When the H8 writes a byte, it goes into the first latch, and the [Raspberry] Pi can later read it by enabling the latch's output. When the Pi writes a byte, it goes into the second latch, and the H8 can later read it by enabling the latch's output. In and of itself, this implements a simple 1-byte bidirectional FIFO [First-In First-Out buffer]. Resistor dividers are used to protect the Pi from the 5V logic."
While Baker's initial design was enough to talk to the Heathkit H8, he decided to take the concept still further — adding serial connectivity to allow the Raspberry Pi provide a terminal service, then adding a a Parallax Propeller-based terminal. The result is the Pinculum: a custom board which hosts a Raspberry Pi Zero for communication with the H8 plus the Propeller-powered VGA terminal with USB and PS/2 keyboard support. When Vinculum's VDIP1s come back in stock, there's even a socket on the board in which one can be inserted as an alternative to the Raspberry Pi.
More details on the build are available on Baker's website, with source code available on GitHub under an unspecified open source license.
