I'm in the process of redesigning a RAM expansion board for an old Apple IIe series computer to help me better understand how electronics work. Some of you may remember the Apple IIe from back in the 80's. I've found it's a great platform to learn on. Anyway, the RAM expansion board uses 1 Meg x 1 bit DRAM's and I want to modify the board to use 1 Meg x 4 bit DRAM's. Just a little homebrew project to further my knowledge of DRAM IC's and the circuits that drive them.
Here's the datasheet for the 1 Meg x 1 bit DRAM's:
Here's the datasheet for the 1 Meg x 4 bit DRAM's:
A friend of mine has been helping me a bit along the way too. After we studied the two data sheets we noticed that the 4 bit DRAM has an Output Enable pin which the 1 bit DRAM does not. It would seen to me that if I invert the Write signal with a 74LS05 then I could use it to drive the Output Enable pin. So I wired the 74LS05 in to the circuit . I then wired up the rest of the signals accordingly. I took IO1 and connected it to where the first 1 bit DRAM I/O would be, IO2 and connected it to where the second 1 bit DRAM I/O would be, and so on. On my board the 1 bit DRAM's have pin 1 and 17 wired together. I then have the four remaining sockets in the bank of eight DRAM's filled with 1 bit DRAM IC's.
Next I insert my board in to the computer, run my RAM test and... it fails. The program tests each DRAM separately, so the 1 bit DRAM's pass the test but the 4 bit DRAM fails. I've even tried several different brands of 4 bit DRAM's too. I can't seem to see a reason why it would fail, at least not according to the data sheets.
Does anyone have any experience with what I'm doing or maybe can see something I'm missing from the data sheets? As you can probably guess I'm a little stumped and have been trying all sort of things for the past couple weeks. Any help would or guidance would be appreciated.