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SNES USART v3 
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Joined: Fri 10 Apr 2009, 15:00:08

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Post SNES USART v3
SNES USART v3 is finished.

New to this version: I've dropped the SPDT switch, because it was too fickle and would easily come off the board.
In its place, I now have a DIP SPST switch. Since it had two switches, I had to make use of the other. So I wired it up to an optional LED power indicator.
I thought about turning on the LED only when the USART is active, but I've never had much luck inverting digital logic. Plus blinking is annoying.
Technically the Teensy has an LED on it too, so this is a bit silly, but my LED is brighter so there's that.

Forced myself to learn about pulldown resistors on the fly. Apparently the SPST in open state was allowing some residual (<0.02V) voltage, that was preventing the SPDT IC from flipping to logic-lo. Went with 1Kohm even though much less is actually needed, so that you can use the same resistor as the one on the Teensy.

This time I managed to make the entire board with absolutely zero wires crossing over the top of another. It's really not a big deal if they overlap, I just kind of enjoy optimizing the paths.

http://imgur.com/v5Jjb,9BADK,sPKGv

Image

USARTv2 (left), USART v3 (right)

Image

USART v2

Image

USART v3

Sun 02 Sep 2012, 20:32:17

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Post Re: SNES USART v3
I am selling USART v2 and v3 boards if anyone is interested.
USART v2: $150
USART v3: $175

I timed myself, it takes me about five hours to make these from scratch. Another five hours to solder up the passthru cables. Another two for ordering parts, getting them from the post office, shipping, and testing the board. Parts are $27 (Teensy) + $16 (DB9 breakout boards) + $2 (wiring) + $1 (3x resistors) + $2 (LED) + $2 (switch) + $4 (ISL84544s) + $15 (breadboard) + shipping (~$15, multiple orders from three different places.) Then the game extension cable is $20 for 3-4 (I have a high failure rate on these) and seven 24WG crimp terminals and wire and hot glue, and another $15 for shielded DB9 hoods, grommets, heat shrink tubing, solder, soldering iron tip, etc. So raw parts are $120. Shipping to you is $10. So that's about ~$3-5 an hour for my labor :P

If no one wants any, no worries. Working for $3 an hour is no fun, but the experience helps. In fact, I'd rather not sell to someone unless they intend to use this. It's primarily for SNES developers. Homebrew and/or emulation testing. You will need a flash cart to run the code on, and Linux or OS X to run the serial program on.

I will sell at-cost (so $120) to anyone who has helped with bsnes development, but I will welcome >$120 if anyone in that category wants to pay more =)

Sun 02 Sep 2012, 20:38:29
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Post Re: SNES USART v3
For pulldown, 10k ohm is kind of the standard. Whenever you see 10k ohm attached to GND in a schematic, it is almost always a pulldown resistor.

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Sun 02 Sep 2012, 22:08:26

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Post Re: SNES USART v3
Well I can easily switch it with a 10Kohm one, but the 1K is working just fine. In fact, I tried it with a spare 390ohm resistor (I was messing with an LED to test things first and had it lying around) and that worked great too.

It was only leaking ~0.02V. For some crazy reason that was enough to make the ISL go logic hi.

EDIT: hmm, it seems that a lower ohm resistors drains more current. I don't quite understand that, meh. So does anyone else concur that I should be using a 10K ohm resistor instead? That will make the board require three different resistor types >_<

Sun 02 Sep 2012, 22:21:13
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Post Re: SNES USART v3
byuu wrote:
EDIT: hmm, it seems that a lower ohm resistors drains more current. I don't quite understand that, meh. So does anyone else concur that I should be using a 10K ohm resistor instead? That will make the board require three different resistor types >_<


No problem. You can have them tie 10 1 Ohm resistors together in serial :D But somehow I'm imagining that might actually cost more.

The main point is the schematic. Whenever anyone sees 10k Ohm pulled to ground, they know exactly what you are doing and can substitute accordingly.

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Mon 03 Sep 2012, 01:55:11
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Post Re: SNES USART v3
byuu, why not assemble on perf board? lasts a lot longer than bread board. in fact, why not design a pperfcb in eagle and etch your own boards

perf board: http://www.youtube.com/watch?v=3N3Apzmy ... ata_player
etching: http://www.youtube.com/watch?v=tWnfnt2r ... ata_player

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Mon 03 Sep 2012, 08:55:49

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Post Re: SNES USART v3
byuu wrote:
Well I can easily switch it with a 10Kohm one, but the 1K is working just fine. In fact, I tried it with a spare 390ohm resistor (I was messing with an LED to test things first and had it lying around) and that worked great too.

It was only leaking ~0.02V. For some crazy reason that was enough to make the ISL go logic hi.

EDIT: hmm, it seems that a lower ohm resistors drains more current. I don't quite understand that, meh. So does anyone else concur that I should be using a 10K ohm resistor instead? That will make the board require three different resistor types >_<


Well, as far as draining more current with lower ohms, pretty sure that's just Ohm's law in effect. In this case, since V=IR, and your voltage is fixed, then increasing resistance must decrease current and vice-versa to maintain that. Put another way, I=V/R, and there, trivially, as R increases, I decreases.

Mon 03 Sep 2012, 09:22:37

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Post Re: SNES USART v3
SuperMikeMan wrote:
byuu wrote:
EDIT: hmm, it seems that a lower ohm resistors drains more current. I don't quite understand that, meh. So does anyone else concur that I should be using a 10K ohm resistor instead? That will make the board require three different resistor types >_<


Well, as far as draining more current with lower ohms, pretty sure that's just Ohm's law in effect. In this case, since V=IR, and your voltage is fixed, then increasing resistance must decrease current and vice-versa to maintain that. Put another way, I=V/R, and there, trivially, as R increases, I decreases.

To give a conceptual analogy, imagine you have just a line to ground, in other words a short, which would provide 0 ohms of resistance. That'll drain current as fast as possible. On the other extreme you can have no connection at all, effectively an infinity-ohm resistor. No current will go through. Everything in between these two extremes will lower current drain proportionally to how much resistance you have.

Mon 03 Sep 2012, 22:42:34

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Post Re: SNES USART v3
vxbinaca wrote:
byuu, why not assemble on perf board? lasts a lot longer than bread board. in fact, why not design a pperfcb in eagle and etch your own boards

perf board: http://www.youtube.com/watch?v=3N3Apzmy ... ata_player
etching: http://www.youtube.com/watch?v=tWnfnt2r ... ata_player

I am betting that cost is a primary factor.

Mon 03 Sep 2012, 22:46:25

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Post Re: SNES USART v3
I hate soldering. I'm not good at all with it. Perfboard would be an ugly, wired mess on the bottom. A custom PCB wouldn't be modifiable (I'm already at v3 here, although to be fair I'm pretty much done with it now), and would cost a lot of money to make. Given I've "sold" exactly one of these, and that it only really cuts out the breadboard cost ($15 of $120), I don't see much point.

The breadboard is plenty stable for what I am using it for. I intentionally cut all the wires to their minimum lengths to fit in the holes (which is exactly why it takes so long to make ... tons of repeated attempts to get the exact right length), and paid a whole lot for those DB9 breakout boards that are strapped in. There is no flex, no give, no accidentally knocking anything out.

Aside: replaced the pulldown resistor with a 10Kohm one. Still works great.

Mon 03 Sep 2012, 22:56:33
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Post Re: SNES USART v3
HJRodrigo wrote:
vxbinaca wrote:
byuu, why not assemble on perf board? lasts a lot longer than bread board. in fact, why not design a pperfcb in eagle and etch your own boards

perf board: http://www.youtube.com/watch?v=3N3Apzmy ... ata_player
etching: http://www.youtube.com/watch?v=tWnfnt2r ... ata_player

I am betting that cost is a primary factor.


Not at all intended as a slight against byuu, but I seriously doubt he wants to deal with this kind of bullshit:

Image

Granted this was a far more complex project than his USART, the point still stands. Not to mention the fact that to even get that much wire on there, you have to use an extremely thin gauge. I've already had one break loose in spite of the hot glue, and I don't think I will ever figure out which without simply tearing off every wire and rebuilding the board.

Luckily, I have some people who believe in the project and either have the skills needed to make a real fabbed PCB, or at least access to people with those skills. I'll be retooling that YMZ284 board design to accept MIDI in and output line-level Mono as part of the Hardchord project.

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Wed 05 Sep 2012, 12:04:17

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Post Re: SNES USART v3
> I seriously doubt he wants to deal with this kind of bullshit:

Yeah. I do wish I could find some veroboard like that with the little silver blobs though. The stuff at Radioshack here is just orange board that you have to manually snap (and it never breaks well.)

But any time I try soldering onto things with contacts like that, I am always concerned about not using enough solder and use too much :P

> as part of the Hardchord project.

You really did come up with the best name for a project ever there.

Wed 05 Sep 2012, 19:22:33
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Post Re: SNES USART v3
byuu wrote:
The stuff at Radioshack here is just orange board that you have to manually snap (and it never breaks well.)


Tin snips work great for cutting PCB.

byuu wrote:
But any time I try soldering onto things with contacts like that, I am always concerned about not using enough solder and use too much :P


Practice will fix that. Why not buy a TV-B-Gone kit and try that. Those are awesome novice projects.

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Wed 05 Sep 2012, 19:29:21
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Post Re: SNES USART v3
byuu wrote:
Yeah. I do wish I could find some veroboard like that with the little silver blobs though. The stuff at Radioshack here is just orange board that you have to manually snap (and it never breaks well.)


It's not veroboard :( It's a full on PCB. Can't even cut it unless you use a damn hacksaw. I hate it, but there's not many other options for pre-drilled boards that have holes on BOTH sides.

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Sat 08 Sep 2012, 01:14:05
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Post Re: SNES USART v3
I remember watching a few videos of some guy on youtube making his own electronics, and he would print his own circuits using a laser printer, get the ink onto some substrate material (don't remember what), then use some chemicals to replace the traces of ink with what I assume was copper. It actually looked pretty simple, though I guess you'd need to prepare the chemical baths yourself. Considering you already have a design where no wires cross, maybe making your own PCBs is an option worth looking into in the medium to long term?

Sat 08 Sep 2012, 01:21:44
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