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Author Topic: External Cartridge Bay Interface (Read 43 times)

Online gflorez

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External Cartridge Bay Interface
« on: 2019.March.17. 17:53:45 »
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.

Ok thanks. I will study it and more questions will follow, for sure....

Similar decoding is made inside DAVE:



I understand that /ROM is no more necessary, with your modified EXOS a ROM can exist on any 16kb boundary(Segment).

/CART&/RD  at B3 on the Cartridge Bay is decoded outside DAVE:

23096-0
--------------------------------

Ok, I will make a prototype of an External Cartridge Bay with all the 4MB addressing range options selected by jumpers. It could also be useful for testing a lot of other projects, to put 1 to 4 16KB RAM/ROM segments exactly and easily on the position where we want. A 4 socket 16KB cartridge PCB would be useful for it.

I only see a drawback, the code of the SD-Reader has to be re-written "segment-position-aware" to work externally. By now it only works at 04-07 segments because the RAM window is accessed at 07. The same with the MIDI interface.
« Last Edit: 2019.March.17. 18:29:28 by gflorez »