Is any input device for Famiom/Famiclon that uses /IRQ line?

Discuss hardware-related topics, such as development cartridges, CopyNES, PowerPak, EPROMs, or whatever.

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krzysiobal
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Is any input device for Famiom/Famiclon that uses /IRQ line?

Post by krzysiobal »

Cause I analyzed all devices described on wiki and generally for every line of 15-pin port (except /IRQ) there is device that uses it.
lidnariq
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Re: Is any input device for Famiom/Famiclon that uses /IRQ l

Post by lidnariq »

Have you also checked out the other peripherals that Nocash mentions that the wiki doesn't? (edit: He doesn't think any of them use the /IRQ line either)
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Re: Is any input device for Famiom/Famiclon that uses /IRQ l

Post by zzo38 »

I don't know of any, although I have once suggested a use for it in imitating mapper 99 (I have seen the schematic for RF Famicom, and it is just wired directly to the cartridge /IRQ line), but only if the game does not use IRQ. Other uses are also possible, including actually using IRQ.
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krzysiobal
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Re: Is any input device for Famiom/Famiclon that uses /IRQ line?

Post by krzysiobal »

I thought for my entire life that any devices connected to the /IRQ line should be open collector, just like wiki says and those discrete/pirate mappers that were connecting /IRQ to 7474, OR/AND gates or PALs were just an unofficial exception to the rule.

But after seeing that the official decapped MMC3's /IRQ output is also not open collector I started wondering that any input device that uses it, would not work with such mappers.

Nintendo M82, which is basically a box for many cartridges to be plugged at once and then played back, does not wire /IRQ to any of the cartridge slots, resulting in interrupt games not to be working at all. They knew about the non-open collector nature of IRQ just to connect all of them in parallel.

In contrary, all famiclones with multiple cartridge connectors that I remember so far, had the /IRQ pins shorted between every connector.

What's the behaviour of /IRQ in powerpak or everdrive?
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Re: Is any input device for Famiom/Famiclon that uses /IRQ line?

Post by nocash »

Interesting that MMC IRQs aren't open collector, that's making it a bit more mystifying how Nintendo had intended to interact with cart and controller IRQs (if that had ever really considered such things).
krzysiobal wrote: Sat Jul 03, 2021 5:16 amNintendo M82, which is basically a box for many cartridges to be plugged at once and then played back, does not wire /IRQ to any of the cartridge slots, resulting in interrupt games not to be working at all. They knew about the non-open collector nature of IRQ just to connect all of them in parallel.
Possible. But if they knew that then they could have added diodes to all IRQ pins to make them open collector. The bigger problem would be any deselected carts holding the IRQ pin low, that would require more logic. And the M82 thing seems to have been released in 1985, I don't know if any games did even use IRQs back then (MMC3 was released only three years later, in 1988).
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aquasnake
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Re: Is any input device for Famiom/Famiclon that uses /IRQ line?

Post by aquasnake »

Code: Select all

assign irq = (irq_scanline_mmc3_out | irq_scanline_mmc5_out | irq_cpu_out) ? 1'b0 : 1'bZ;

assign irq = ~(irq_scanline_mmc3_out | irq_scanline_mmc5_out | irq_cpu_out);

The IRQ output generated by the above two logics can run normally on my development cart, and I tested it for about half an hour.

The former IRQ output pin structure is an od-gate, the latter is a push-pull output.

So I infer that there should be a pull-up resistor at the IRQ input of the host.

If IRQ input has multiple front-end drivers, it should be designed as OD gates for all.
But if you need to work with other designed push-pull output at the same time, you can also modify the circuit and use diode isolation at the output end to ensure that only low-level is activated
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