where the /ROMR signal is originated?

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nippur72
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where the /ROMR signal is originated?

Post by nippur72 » Tue Mar 22, 2022 10:55 am

I am not able to locate where the "ROMR" signal that goes in the 2K BIOS ROM is originated (datasheet, Circuit 1.pdf).

Any clue ?
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Scouter3d
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Re: where the /ROMR signal is originated?

Post by Scouter3d » Tue Mar 22, 2022 6:14 pm

Hi,

/ROMR (or ROMreturn as i like to call it) is thru an inserted cartridge normaly connected to /ROM0 (the chipselect signal for the uppermost 16k (BIOS region))

in the CSL cartridge and the MegaSDCart we use /ROM0 to use an external BIOS
the parallel interface takes /ROM0 uses a part of it and returns the rest to /ROMR

Cheers, TOM:0)
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nippur72
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Re: where the /ROMR signal is originated?

Post by nippur72 » Wed Mar 23, 2022 7:29 am

ok thanks, so if I've understood correctly /ROMR is used by an external cartridge when it wants to use the BIOS ROM; it does that by sending back the /ROM0 signal. If instead the cart wants to use its own ROM then /ROMR is just kept high for all the time.

This also implies that when you turn on a CreatiVision without a cart, it just executes random code because the BIOS ROM is never selected, right?

Apart from CSL and MegaSDCart, are there any cartridges that do not use the BIOS ROM ? I guess not.
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Scouter3d
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Re: where the /ROMR signal is originated?

Post by Scouter3d » Wed Mar 23, 2022 7:45 am

Hi,

Yes, you are correct :0)

1) All gamecartridges use the internal BIOS (/ROM0 is simply connected to /ROMR on the game PCB)
2) CSL and MegaSDCart use /ROM0 for themself (to replace the internal BIOS) and give nothing back to /ROMR (or pull /ROMR high)
3) the parallel Interface divides the 16k /ROM0 romselect into 8x 2k ROM areas - uses $E800-$F000 for itself - combines the remaining 7x 2k into 1 signal and gives this signal back as /ROMR

Yes, If you turn on a Creativision without cartridge inserted you only get a blackscreen and nothing is working... - correct again :0)

Cheers, TOM:0)
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Scouter3d
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Re: where the /ROMR signal is originated?

Post by Scouter3d » Wed Mar 23, 2022 7:49 am

PS: The (never released) Colecovision Adapter for the Creativision would have also disabled the internal ROM (and the CPU) on a Creativision MKII...
AFAIK there is only such an adapter for the Salora Manager (using its external port) and no adapter for the Creativision... (Madrigal is working on connecting the Manager-version to the CV)
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