Now that we are near completion, I want to try the external cartridge bay that Zozo sugested. But I need some schematics.
2x 74ALS138
1x 74LS32
First 74LAS138:
/E0 and /E1: /MREQ
E2: /RFSH
A0: A19
A1: A20
A2: A21
The 8 outputs are valid for 8x512K memory areas:
/Q0: 00H-1FH
/Q1: 20H-3FH
/Q2: 40H-5FH
/Q3: 60H-7FH
/Q4: 80H-1FH
/Q5: A0H-BFH
/Q6: C0H-DFH
/Q7: E0H-FFH
With the second 74ALS138 divide one 512K are for a 64K areas:
/E0 and /E1: /Qx from first 74ALS138
E2: +5V
A0: A16
A1: A17
A2: A18
If using /Q0: 00H-1FH from first 138, then the outputs of the second:
/Q0: 00H-03H
/Q1: 04H-07H
/Q2: 08H-0BH
/Q3: 0CH-0FH
/Q4: 10H-13H
/Q5: 14H-17H
/Q6: 18H-1BH
/Q7: 1CH-1FH
The selected output will be the new /CART signal. Using Q0 from the first, and Q2 from the second, then the new external cartridge will be at 08H-0BH, next to the original.
One gate from the 74LS32 make the /CART&RD signal.
Also need a 7805 (or compatible) or a +5V from bus card.