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Daughterboard replacement for Horizon HRD+ chip stacks

TI-99/4A Earlier, in my article about refurbishing an old Bud Mills Horizon HRD+, I asked why TI gear (both official and homebrew) tended to stack similar chips on top of each other, straightening pins out, and using flying wire to connect to other stacked chips.

The question was mostly rhetorical because it was pre-electronic-CAD ... so it saves real estate on the board, saves time laying out interconnects on the board, and so forth, because a human was doing the layout. It doesn't make much sense now, especially that nearly every 74xx chip ever made is available in a form factor that's 25% the size and consumes 10% of the power of the original chip.

The Horizon HRD+ has three chip stacks -- one stack of three 74LS138s, one stack of two 74LS259s, and one stack of (obsolete in PDIP) 74LS154s.

I have designed replacements for all three that fit into the original socket, with small solder pads at the edge of the board to handle the flying wires that were originally attached to the chips.

The '138 replacement has been tested and seems okay. It incorporates the "hide" function as described in the HRD+ errata, so you'll need a 4.7k resistor and a small two-position switch to superglue up by the positive end of the batteries, with the switch actuator facing towards the rear of the PEB. Connect the middle switch contact to "HIDE" on the PCB, and either of the other two switch contacts to ground (but not both!), and you're good to go.

You may have clearance problems between the daughterboard and the surrounding socketed chips. Add an extra 16-pin socket between the HRD+ and the board, and again you're good to go.

A zipfile containing the Eagle .sch and .brd files for the '138 replacement are here.

The '259 replacement board will be forthcoming. I accidentally ordered N-suffix (DIP) parts rather than D-suffix (SMD) from Mouser, so it'll be a few weeks before I can have that ready and tested. After that, the '154 ...

... and maybe, just maybe, a daughterboard that uses two 512kx8 SRAM chips to bring the HRD+ up to one megabyte. No promises, but the hardware can certainly handle it. This would entail pulling all of the 62256 SRAM chips, plugging into one socket, removing a resistor on the board underside and running a couple of CS/address lines to the '154 daughterboard.

Better than stack-and-solder, anyway. I hope that someone else finds the Eagle files useful when refurbishing the Bud Mills Horizon HRD+.

(Repetition of the full product name for search engines :-)

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