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Author Topic: AV Famicom JIO Chip Replacement Idea  (Read 2762 times)
Great Hierophant
Disk System
Posts: 199

« on: December 02, 2017, 03:42:09 PM »

In the AV Famicom, the Australian PAL NES Top Loader and the rare revised US NES Top Loaders, Nintendo replaced three chips, a 74LS139 and two 74HC368s, with a single 32-pin chip designated BU3266 or BU3270. Nintendo may also have incorporated a resistor pack inside the chip to pull up the input lines. In my opinion, at least for the AV Famicom, the chip should be replaced for one or more of the following reasons :

1. Easier to reimplement the Famicom Microphone functionality
2. Restore Famicom Expansion Audio mixing levels to old-style Famicom
3. Restore the missing NES functionality of Zapper support for Controller Port One
4. Replace a dead JIO chip

How can the chip be replaced? I think the best solution would be to use a PCB with small (SOIC) versions of the three chips in a flat-pack form factor. This PCB would be placed over the holes where the JIO used to be and attached via a pair of interposer-style sockets supporting the shrink/skinny DIP pitch of the JIO chip. That pitch should be .070" or 1.78mm. These sockets would seem to fit the bill : https://www.mill-max.com/assets/pdfs/127.pdf (Use two of the 16-pin sockets to get 32 pins). With the added height of the interposers, you should be able to fit a board on top which may be slightly wider and longer than the JIO chip and give you more room to work.

The 32-pin chip has several redundant or not-connected pins : http://nesdev.com/nes_pio_pinout.txt It looks like the holes where pins 2, 12, 15, 31 & 32 went can be repurposed for inverter input and output.

Now let us look at how the inverters were used on the Famicom and AV Famicom

Famicom   AV Famicom
4016 D0   4016 D0
4016 D1   4016 D1
4016 D2   
4017 D0   4017 D0
4017 D1   4017 D1
4017 D2   4017 D2
4017 D3   4017 D3
4017 D4   4017 D4
Aud Out   Aud Out
PA13   PA13

As you can see, the Famicom used all twelve inverters provided by the 74HC386s for various signals. The AV Famicom only ended up using nine inverters. It has been established that M2 is sufficiently strong that it does not require buffering. My NES controllers with their long cables work just fine in my AV Famicom. Each 74HC368 is a hex buffer/inverter (3-state) with six inverters. We can use the three extra inverters provided by the 74HC386 to handle microphone and zapper inputs.

The AV Famicom's Controller Ports do not connect the D3 and D4 pins. I restored Controller Port 2 Zapper functionality by connecting those pins to the corresponding $4017 D3 and D4 on the Famicom Expansion Port. However, we can instead use one of those pins for the microphone input and connect a Controller Port 2 Zapper to the Expansion Port via a physical adapter. One inverter will be used to handle the microphone input at $4016 D2.

Two inverters will be used to implement $4016 D3 and D4 on Controller Port 1. This is where things may get a little aesthetically unpleasing. We only have five unused pins, but we need six to implement all three inverters. While some wiring will need to be added, I would like to have done so entirely on the underside of the board to keep things looking as neat as possible. However, one wire will need to go topside from the board across to some of the bare metal of the exposed pin for D4 on Controller Port 1. It may be possible to design the board to extend to the Controller Port pin and require only a little bit of solder rather than a wire stretched like a clothesline.

It has been discovered by Ace that the reason why the Famicom's internal audio goes to the cartridge so quietly is because of the audio amp constructed with the JIO inverter. He got Famicom audio levels to match his old, pre-GPM Famicoms by recreating the amp found in those Famicoms and bypassing the JIO chip. I thought the quieter audio levels were due to the higher CPU revisions (G & H) found in the later consoles, but they were not. We do not know what kind of resistance is being applied inside the JIO chip. GPM Famicoms were first introduced in 1988, so games developed on the earlier models' boards would likely expect the mixing levels of those boards. That includes most FDS games and probably the majority of the cartridge games. The new board revisions probably took a while to percolate and it was mostly the oldest Japanese licensees that dabbled in expansion audio.

One issue with board design would be to make the board in such a way that it would not conflict with the NESRGB board, which is a very popular mod for AV Famicoms. The NESRGB is a large board that sits on top of the PPU and covers more surface area than just the PPU. The Hi-Def NES's PPU interposer is much smaller and therefore not an issue. The board's pin holes may have to be shifted toward the front of the console somewhat to give enough clearance to the NESRGB. Another issue is that the mode contemplates wires running from the data bus to the JIO pins, and the nearest convenient place to obtain some of these signals is across the PPU. This could add noise to the AV Famicom's renowned jailbar-free video signal.

Any thoughts, comments or suggestions would be appreciated.

Check out my retro gaming and computing blog : http://nerdlypleasures.blogspot.com/
Posts: 13

« Reply #1 on: December 07, 2017, 01:53:31 AM »

I have nothing technical to add but Im glad you keep trying to make this a reality.  Id love to play around with microphone games but I have no interest in investing in a ln original famicom or twin just for this purpose.  I really hope this becomes a reality and someone creates a board that can even allow the internal and external audio levels to be tweaked on the fly.
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« Reply #2 on: December 07, 2017, 10:19:09 AM »

Yes if this can defeat the weaknesses of the AV Famicom it may be a good mod.

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