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Finished and verified: SMD replacement boards for Horizon HRD+

As previously mentioned, I’ve been working on an old Horizon HRD+ RAMdisk card for the TI-99/4A.

Three of the chips (a 74LS138, a 74LS154, and a 74LS259) had other chips stacked and soldered on top of them, with leads bent out and wires connecting to various places on the board. One chip (the 154) is no longer available in .600 DIP form.

For obvious reasons, I decided to replace these chip stacks with plug-in boards. Each board is electrically equivalent to the chip stack that it replaced, with the exception of the 138 replacement board incorporating errata wiring later published by Horizon. All chips are SMD, and are currently available as HCT.

The Eagle schematic and board files are available here.

All three of these boards designs have been tested and are currently in use in my HRD+.

Enjoy. Hopefully this will help someone out there that’s trying to get one of these working, but is stymied by the stacked chips and/or can’t find a package-exact replacement for the 74LS154.

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