I feel we need a simple 32k expansion that slaps onto the side of the 4a, or side of the speech synthesizer... So I've been trying to build one.
There are a number of new cartridges that require the 32k memory expansion to function. And with the Flash ROM 99 EA5 conversion craze, a console with 32k on the side can suddenly play a much larger library of classic and new TI software titles.
My intent is to build and offer finished product. I envision a vertical board about 3" high by 4" deep, chips to the right, edge connector to the left. And a jumper for external or internal power.
I've started with the circuit on TI TECH PAGES, under http://www.unige.ch/...ti99/mem32k.htm, scroll down to do-it-yourself-card.
This circuit maps logically to what Tursi describes in http://www.harmlessl.../TI 32k Mod.pdf from the best of my ability to tell.
Of course, I'm not in a PE-Box with this, so I have no RDBENA to worry about. So I've reduced the circuit, using the remaining 3 single line buffers in the 74LS125 to buffer MENEN, WE, and DBIN, instead of the 3rd 74LS244.
I believe I have all of the addressing correct, and can actually see the LED in the circuit activate when and only when I address memory in the expansion address ranges. However, I am having a real difficult time with the memory chip, and the data bus buffer 74LS245. Maybe some of you with more experience can spot what I'm doing wrong? It seems as though I'm stuck in high-impedance mode on the data bus buffer.
It is not shown in this schematic, but I've got 0.1uf caps across Vcc and Vss on all of the chips. And I'm driving the board off of an external 5v regulated supply.
I'm using a 62256 SRAM and not either the 55256 Theiry called for, or the CY7C199 from Tursi's circuit. They appear to be pinout compatible with the 55256. The address lines are numbered different in the CY7C199.
Here is my schematic:
SideCar32kSchematic.jpg 463.42KB 26 downloads
PDF if you want to look more closely:
32k-side-car-schematic.pdf 70.84KB 33 downloads
And a picture of all my jumper wires in use for fun:
IMG_20160703_171936.jpg 168.68KB 24 downloads
I now believe paying $30 for the 4k of ram in my mini memory cartridge was worth it, since it came with Easy Bug