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Yurkie

2 cartridge slots

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I was wondering if it would be as simple as adding a switch to have 2 cartridges connected to the Atari and flip a switch to toggle between them. 

 

My idea is to have one slot internal and have the plus cart or harmony cart insert in it, and a regular single game cartridge in the original external cartridge slot.

 

If I was reading the slot pin out correctly there is a 5v and a ground pin. Would a SPDT or a DPDT switch accomplish this?

 

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Switching off power to a cartridge while still having it connected to the address and data bus is BAD. Don't do that!
The correct way to is to switch the "chip select" pin (which on the VCS is connected to CPU address A12).

CPU A12 should be connected only to the "selected" cartridge, while the corresponding pin on the "deselected" one must be tied to GND. So you need a DPDT switch.
See here:

https://atariage.com/forums/topic/164987-modify-your-video-game-brain/

 

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Switch the VCC is not the better option as the second memory can be ghost fed by internal protection diodes.

Better use the A12 (chip select) pin.  The selected cart will receive the chip select on its pin A12 while the other will receive a GND (on its A12 pin) and stay unactive. 

 

image.png.f6a6569b04e72ab5bd75f8d96bd401ac.png

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How did the vintage 2600 cartridge selectors/changers (like the Video Game Brain and others) do it?  Those seemed to work.

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11 minutes ago, doubledown said:

How did the vintage 2600 cartridge selectors/changers (like the Video Game Brain and others) do it?  Those seemed to work.

 

Click on the link above.

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When I added a game rom to the board in my heavy, I drilled a hole in the aluminum shielding so that a micro switch sticking its arm through the hole disabled the on-board rom when a cartridge was inserted.

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Cooler idea, build a cartridge slot into a cartridge,  like a Sonic 3 knuckles cart.  Put a switch on that and no need to have two cart slots.  Or like a 2600 copy cart, cart...

 

Just a thought.

 

 

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On 4/16/2021 at 3:35 PM, Danjovic said:

Better use the A12 (chip select) pin.  The selected cart will receive the chip select on its pin A12 while the other will receive a GND (on its A12 pin) and stay unactive. 

That depends on what you mean by inactive.

If it only means it does not respond to placing new data on the bus with a change of address but that any existing data is still being driven on the data pins that may not be sufficient as unless the active cartridge outputs can overpower those of the inactive one it will create contention on the data bus where one cartridge is trying to drive the line high and other is trying to drive it low potentially resulting in corrupted data. However if the cartridge data pins are tri-state and so go high impedance when the CS pin is pulled to ground then all should be OK. 

 

If the cartridge data pins do not go high impedance then a better method would be to place a buffer IC with tri-state outputs like a 74HC244 between both cartridge ports data pins and the data bus and use the switch to enable the outputs of the relevant buffer as that completely isolates the outputs of the "deselected" cartridge from the data bus, thereby completely eliminating a contention issues on the bus. All cartridge input pins can be connected in parallel.    

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I guess all cartridges have tri-state data pins, else there would be bus contention every time the VCS accesses TIA, RAM and I/O registers, which are mapped to the low 4k of the memory area (A12 low).

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7 hours ago, Stephen Moss said:

That depends on what you mean by inactive

Tristate or Standby depending on either Output Enable or Chip Enable signal is used by the cartridge. 

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