|This is the DAD-1. DAD stands for Digital Analog Digital, a moniker that applies because this is a D/A card with an integral LM311 comparator that allows for A/D conversion either by writing a ramp, or a series of outputs that are derived by successive approximation, against one comparator input while the input signal is applied to the other.|
You repeatedly checked D7 on read to see if the comparator output had flipped. Took 8 writes and 8 reads to definitively measure the analog input using successive approximation. Using a ramp, it could take 255 writes and 255 reads... a very good reason not to use a ramp. D7 was handy to use because it let you use the 6800 or 6809 signed branch directly after the read. About as efficient as you could get.
So you could use the card as either an A/D, or a D/A. Lots of uses for such things.
It's a single-sided PCB, hand-laid out using tape on my first light table (which was about two feet square, IIRC.)
I designed the card for the SWTPC SS-30 bus some time in the mid to late 1970's. I think this was the first product of my own that I designed and sold in quantity, but the memories... they've faded. I'm old, lol.
Those Molex connectors... man those were annoying. Some really odd spacing, which made the PCB artwork and subsequent drilling of the board just that much more annoying.
I think I still have one of these somewhere. Maybe.
This image is a scan of an old B&W photograph.
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