I am working on guessing connections of one particular cartridge.
This cartridge has two write registers (74161, 7474), one read register (74244), and RAM memory.
When those elements need to be enabled is shown in table belowI've reverse engineered 95% of the connections so far, though the last remaining 5% makes me crazy as the chip which is responsible for decoding those addresses has unreadable markings. I guess this must be 74138 or 74156
Second one is much probable as RAM need to be enabled for both reads and writes and 74138 does not allow that, 74156 has open collector outputs and there is resistor ladder nearby so wire-AND could be done.
Anyway, to decode all that stuff, M2, RnW, /ROMSEL, A14, A12, A12, A11 need to be taken into account which is too much for just single 74138/156.
What is left nearby is half of 7474 unused, I am trying to guess the connections and I am wondering if that would work? (7474's pins 4 and 2 are for sure shorted).
According to 7474's datasheet, if /PRE and /CLR are both low, Q and /Q both remains high.
This cartridge has two write registers (74161, 7474), one read register (74244), and RAM memory.
When those elements need to be enabled is shown in table below
Code:
11111 0000000000 M2 RnW /RMS 43210 9876543210 -- --- ---- ----- ---------- 1 * 1 10100 xxxxxxxxxx $5000-53ff RAM (read/write) 1 0 1 10110 xxxxxxxxxx $5800-5bff 74161 (write) 1 1 1 10110 xxxxxxxxxx $5800-5bff 74244 (read) 1 0 1 10111 xxxxxxxxxx $5C00-5fff 7474 (write) Second one is much probable as RAM need to be enabled for both reads and writes and 74138 does not allow that, 74156 has open collector outputs and there is resistor ladder nearby so wire-AND could be done.
Anyway, to decode all that stuff, M2, RnW, /ROMSEL, A14, A12, A12, A11 need to be taken into account which is too much for just single 74138/156.
What is left nearby is half of 7474 unused, I am trying to guess the connections and I am wondering if that would work? (7474's pins 4 and 2 are for sure shorted).
According to 7474's datasheet, if /PRE and /CLR are both low, Q and /Q both remains high.
Statistics: Posted by krzysiobal — Sun Jun 16, 2024 4:55 pm — Replies 0 — Views 78