Monday, November 5, 2018

64K 4164 DRAM Decoding Integration

I integrated the 4164 DRAM decoder wiring into the logic board this weekend. It was very difficult to troubleshoot, and on retrospect, I think an adapter board would have been easier and less problematic. The main hangup was that the A14 signal to pin 10 of the J1 socket is connected on both sides of the logic board. One side in from expansion slot 7, and the other side goes out to H1 socket which creates the (A14&A15) signal. I had routed the (A14&A15) back to the J1 socket to decode the upper 16K of RAM. After disconnecting the A14 line from pin 10 and re-connecting pin 10 to a new video decoder signal (HIRES&PAGE2), the video decoder signal was getting sent through to H1 to give (HIRES&PAGE2&A15), which is wrong and caused computer to not be able to access the ROM.

Anyway, I fixed the problems and 74ls153 now sits in the J1 socket like it was always supposed to be there. The signals to decode the upper 16K of memory in the 4164 DRAMs are now available. I need to build circuit with registers to set to enable the upper bank and bank switch the 4K that overlaps the I/O area of the Apple II+. It will basically be 1/2 of an Apple 16K RAM/Language card.

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