Questions about the N64 Jumper Pak

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Questions about the N64 Jumper Pak

Postby qwertymodo » Thu May 03, 2012 4:10 pm

I'm trying to trace out the jumper Pak's connections to attempt an on-board expansion pak transplant and removing the expansion slot entirely (mounting the resistors and caps on the board as well), but something just doesn't make sense to me when I'm looking at the jumper pak. There are 5 capacitors on the board, but 4 of them appear to be connected to ground on both sides. I took some pictures to show what I mean, and I marked the vias and traced the components onto the front-side image so I could see which components connected to which pins. All of the green vias are ground, so are the yellow (but I didn't realize that when I initially marked them), the red connect to the resistors, and the one blue via is the only connection to a cap that makes sense to me. Unless I'm missing something, the other 4 caps are just shorted across 2 regions of ground which are already connected by a trace, so they're not bridging the regions (you can follow the trace straight up from the yellow vias on the back of the board straight to several other ground vias). Can anybody tell me what's going on there? Also, does anybody know what value the caps are? They have no marking whatsoever. The resistor arrays are marked as 51ohm, so at least I know them.

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Re: Questions about the N64 Jumper Pak

Postby marshallh » Thu May 03, 2012 4:50 pm

I'm not sure what you're confused about-- your yellow marked copper fill is the Vtt supply island, which is decoupled via several caps. All the rambus data lines are pulled up to Vtt through the resistors. This is providing the other end of the line termination which is crucial at 500mhz

The one cap that doesn't fit is decoupling another voltage supply, probably 3.3 or 2*Vtt
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Re: Questions about the N64 Jumper Pak

Postby qwertymodo » Thu May 03, 2012 5:19 pm

What I'm confused about is that the Vtt island doesn't appear to be decoupled if you look at the front side, the traces continue straight up to the green-marked vias which connect to the ground plane on the back (the traces are kind of covered by the resistors I drew in on the front image). Also confusing me is the fact that the 4 capacitors are in parallel. Why not just use a single equivalent cap?
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Re: Questions about the N64 Jumper Pak

Postby marshallh » Thu May 03, 2012 10:07 pm

Look closely at the via pair that you marked yellow - Vtt.
It's broken and doesn't connect to ground, hard to catch unless you're careful.

The reason there are 4 caps is to provide the closest possible physical return path for current. Ideally you'd have a cap for each line that is being tied to Vtt. But this is "good enough".

http://www.murata.com/products/catalog/pdf/c39e.pdf
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Re: Questions about the N64 Jumper Pak

Postby qwertymodo » Fri May 04, 2012 11:26 am

Ah, thanks. I see the broken trace now and it makes a lot more sense. Thanks for the reading material, I'll look into it and if I have any more questions I'll come back and ask.
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Re: Questions about the N64 Jumper Pak

Postby qwertymodo » Fri May 04, 2012 1:00 pm

Ok, so now I'm looking at how to wire it all up on the mainboard and this is is what it's looking like. The white traces are apparently not connected in the jumper pak (I'm guessing they were used in the expansion pak, but I'm too lazy to pull mine out of my other '64 to check...) so I'm thinking I'll use 2 of them as additional Vtt lines by cutting the traces and connecting them at the vias. Also, I'm not sure on the redundant caps, but I just marked all of my options in this diagram, and I may omit some when I actually attempt it. Also, I'll be connecting all of the grounds using the green vias.

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