Getting Nibbles to run on a Tandy 1000A

Getting Qbasic Nibbles to run on an IBM Compatible PC, easy rite? Not when the computer is a Tandy 1000A with 128KiB of RAM!

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This project started out because my Tandy 1000A did not have enough conventional RAM to run Qbasic (among other bundles DOS apps from MS-DOS V5+). I like Qbasic Nibbles, it is one of the first games that I ever played on a computer. Sadly, I get the "There is insufficient conventional memory available" error message when I try to run qbasic. Thus began my quest for Nibbles.

The project quickly went from just adding an additional 512KiB of conventional RAM to also adding in 32MiB EMS, then a real time clock (RTC), and finally an IDE hard drive interface (only after I got this designed did I find the XT-IDE and Lo-Tech memory boards), and who knows what else I will try to squeeze in. As things got more and more complicated (and I was forced to wait longer to play Nibbles), I decided to push ahead and make a conventional RAM only expansion card that has 512KiB of onboard ram that will, along with the 128KiB onboard RAM, bring the Tandy 1000A up to it's 640KiB limit.

This is where I left off the other night, I am re-using some perf board from another project. I still need to remove the rest of the clock circuit, RAM, and ROM that is left over from a homebrew 6502 computer. the card edge connection was salvaged from a bad AT&T Merlin system phone card.

This is the very simple schematic for the expansion card. A quad 2-input AND gate, quad 2-input OR gate, dual 2-4 de-multiplexer, and 8 bit bus transceiver are used. 

Memory map showing the address bits that can be used for decoding the memory mapped to the card, as well as the on-board memory limits and the start of UMA.

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auclark1800 wrote 03/28/2016 at 23:05 point

I, too, have tried IC stacking:

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Benchoff wrote 09/26/2014 at 01:06 point
Have an update for this? There is a point where stacking RAM chips becomes excessive and awesome, and you have clearly passed it.

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Eric Wright wrote 09/26/2014 at 16:54 point
I sadly have not had time to work on this for the past while. Since my last update, I have been fixing things for the local e-cycler (1956 Dumont TV, a 1938 Hallicrafters SX-16, an IBM AT, etc) and have been doing many side jobs. These side jobs will soon be paying off though, as the proceeds are going towards a hackerspace/makerspace (we should have our lease next week!).

Once we have the lease for the space, I will be continuing work on this project (and my wife will be happy that I am not building things on the coffee table in the living room!)

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Adam Fabio wrote 08/04/2014 at 04:56 point
I remember stacking ram chips back in the day - but we only doubled them. 8 chips high is quite the tower! I can't wait to see how this project turns out! Good luck with the nibbles!

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Eric Wright wrote 08/04/2014 at 18:54 point
Thanks, if everything goes well, i should have time to get the prototype version of the card up and running tonight. In the past, I too have only stacked 2 ICs on top of each other, but not wanting to run wire to 384 different pins when most of them are the same connection made me decide to stack the two sets of 8 ICs.

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