Refurbishing a Horizon HRD+ RAMdisk Card

I recently came into possession of an ancient Horizon HRD+ RAMdisk card for the TI-99/4A PEB. Not the 2000, 3000, or 4000 — the original model, designed by Bud Mills and Ron Gries circa 1986.

It’s a variable-size RAM disk with battery-backed RAM. There’s a rudimentary charging circuit onboard, so that NiCad batteries can be used and charged while the PEB is on. This particular card hadn’t been turned on since 1989, based on the calendar taped to the front of the PEB that it was in … so, needless to say, the batteries were not only dead but had leaked into the chips below.

First order of business was to replace the corroded AAA battery holders at the top of the card and assess the damage. Lucky for me, the original owner decided to put gigantic ceramic 0.1uF decoupling capacitors directly beneath the batteries, and they blocked most of the battery goop from the chips below …

… which was a good thing, because this was a) fully-populated with 62256s (which are getting hard to find) and b) using a couple of 74LS154s soldered together to do the address decode, and .600 ‘154s aren’t made anymore as far as I can tell.

I pulled the top row of chips (including the ‘154s) and scrubbed everything with isopropyl alcohol anyway. Good thing that I did, too, as the top right (power) pin on the ‘154 stack came off as I pulled the chip. The stub was okay, but the bit that went into the socket had corroded away. Easy enough to fix with a bit of extra stiff wire left from an LED from another project.

For some reason, mid-80’s TI gear used a lot of stacked chips. There were wires flying from pins bent up on the stack, and some of those wires weren’t connected anymore. So I needed a schematic …

… and the go-to place for TI documentation (the Horizon subdirectory) didn’t have a manual for the plain HRD+.

A few days of searching led me to a post on AtariAge, where a person with the handle “schmitzi” posted a low-quality scanned version of the construction manual, including schematic (source here).

I fixed the wiring to match the schematic, powered it up, and it appeared to be okay. All memory tests passed, it could be formatted as a 384k RAM disk, and all was well …

… until I replaced the (ancient, power-wasteful, hot-to-the-touch) linear 7805 with a Minmax switching 7805. Now, suddenly, it didn’t work. I could see only a few volts across the power rails. That was not right.

It turns out that the 7805 ground connection isn’t really connected to ground. It has to go through a diode first (which isn’t usually a problem) and is fed back into the supply voltage via resistor R10 (which is a problem). The fix is to remove R10 and connect pin 2 of the 7805 directly to ground. Problem solved.

It also turns out that Bud Mills published errata for the HRD+ that fixed a data-corrupting reset-on-powerup bug, and added a switch to lobotomize the card in case the DSR somehow became corrupted. That document was found (in PDF form) in the TI-99/4A Yahoo! mailing list archives.

(Side rant: what is it with TI people and DSR-in-RAM? I’ve seen three projects now that put the DSR in battery-backed RAM, with predictable results. Did they all sleep through the engineering class where they were taught that critical code needs to be in non-volatile memory unless you have a damned good reason for not doing so?)

Digression aside … I have OCR’ed and cleaned up both the construction document and the errata and placed them here and here.

I have taken the liberty of correcting typos in the former; the latter didn’t appear to need correction.

As far as I can tell, there was only one source each for these documents on the net (schmitzi’s post on AtariAge for the construction manual — and that place is sort of a walled garden regarding web spiders), and I’ve therefore posted cleaned-up versions of both here with the intention that the various search engines can find them easier.

I hope these documents help the next poor fellow that finds one of these cards and decides to make it go.

About Chris Kobayashi

I'm a security systems engineer, specializing in UNIX, network, and physical security. I'm in Tokyo, and I'm mostly retired now. I'm well-versed in both electrical and software engineering, with a particular interest in old computers and game consoles. You can contact me here.
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