Getting Nibbles to run on a Tandy 1000A

Description

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.

Details

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.

Components

16 × M5M5257P SRAM Electronic Components / Misc. Electronic Components

1 × 74LS245 - 8bit Bus transcieverr Electronic Components / Misc. Electronic Components

1 × 74LS08 - quad 2-input AND gate Electronic Components / Misc. Electronic Components

1 × 74LS32 - quad 2-input OR gate Electronic Components / Misc. Electronic Components

1 × 74LS139 - dual 2-4 Demux Electronic Components / Misc. Electronic Components

Project Logs

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When life is too busy to prototype, design PCBs instead
Eric Wright • 08/11/2014 at 07:09 • 0 comments

I have not had a chance to finish up the prototype yet, I have spent the last week cleaning up an IBM XT clone and repairing an IBM AT (5170) for a local business. during my lunch breaks at work, I designed a simple PCB for a cleaner version of my conventional memory expansion card. a couple of PDFs of the board can be found below, but beware I still need to double check everything on them. On the PCB, I decided to rely on the SRAMs ability to go in to high impedance and omitted the 74LS245.
the SRAM chip used is an AS7C4096A-12JCN in an SOJ-400 package while the other three chips (74LS08, 74LS32, and 74LS139) are all SOL.
http://content.thetechnickel.com/misc/tandy-1000a/expansion/conventional-only/pcb-pdf/512KiB-RAM-Expansion-bottom.pdf
http://content.thetechnickel.com/misc/tandy-1000a/expansion/conventional-only/pcb-pdf/512KiB-RAM-Expansion-combined.pdf
http://content.thetechnickel.com/misc/tandy-1000a/expansion/conventional-only/pcb-pdf/512KiB-RAM-Expansion-silkscreen.pdf
http://content.thetechnickel.com/misc/tandy-1000a/expansion/conventional-only/pcb-pdf/512KiB-RAM-Expansion-top.pdf
quick note: use these PDFs at your own risk, they still need to be double checked for errors (well, tripple checked as I already double checked them).
it is turning in to a rats nest
Eric Wright • 08/05/2014 at 19:56 • 0 comments

I was able to get the two stacks of SRAM jumpered together for all address and control lines that are shared as well as connect the address lines (A0:17) for the SRAM to the card edge connector. Tonight I will connect the AND, ORs, demux, and bus transceiver up and finish wiring the SRAM stacks. This is going to be quite a mess. I really need to invest in some decent wire and stop using network cable. hehe
things are stacking up in a good way
Eric Wright • 08/02/2014 at 08:40 • 0 comments

I started working on the prototype tonight, armed with a general idea of what I wanted (and that included not wanting to do 24 connections on 16 different chips), I started tacking two sets of 8 SRAM ICs together. once both sets were all tacked together, I went thru and soldered the other common pins together. After getting the two sets of SRAM together, I decided that I should hammed out the memory decoder/demux so I hammered out a quick schematic based on the memory map I had created earlier

tomorrow at some point, I will scavenge around to locate the 4 7400 series ICs that I need and start to assemble the board.

View all 3 project logs

Discussions

auclark1800 wrote 03/28/2016 at 23:05

I, too, have tried IC stacking:

Are you sure? yes | no

Benchoff wrote 09/26/2014 at 01:06

Have an update for this? There is a point where stacking RAM chips becomes excessive and awesome, and you have clearly passed it.

Are you sure? yes | no

Eric Wright wrote 09/26/2014 at 16:54

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!)

Are you sure? yes | no

Adam Fabio wrote 08/04/2014 at 04:56

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!

Are you sure? yes | no

Eric Wright wrote 08/04/2014 at 18:54

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.

Are you sure? yes | no

Getting Nibbles to run on a Tandy 1000A

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When life is too busy to prototype, design PCBs instead

it is turning in to a rats nest

things are stacking up in a good way

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