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Modify your Video Game Brain


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#1 batari OFFLINE  

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Posted Wed Jun 23, 2010 11:58 AM

I've been working in a Harmony-based cart dumper. I've been using a Video Game Brain to help. It wasn't working, so I decided to open up my Game Brain and I was horrified at what I found - the Game Brain switches power to carts and not chip selects! This is a horrible design, because when a cart is not selected, voltage is running through address and data lines while the power pin is floating. Chips are not designed to have power running through data and address pins when the chip itself is unpowered and it can result in damage to games, the system, or both. This explains why my Game Brain rendered my Kroko cart non-functional for a while. It started working again, but that experience was scary. I don't recommend anyone uses a Game Brain for playing games unless modified.

That said, it is possible to modify the Game Brain to work in a responsible way that will not damage any games. The Game Brain has DPDT switches. This means you can run GND or A12 from the 2600 to the appropriate pin on the cart port. To do it, you need nothing more than wire, solder, a soldering iron, and something to cut traces.

I haven't finished with the cart dumper yet. I will post it to the Harmony forum when something is going. For now, I have a proof of concept going - it operates kind of like the CopyCart.

Here are some pics of how the PCB was wired before the mod. NOTE: the fat traces are NOT grounds! This came as a surprise to me.

The yellow wire on the middle terminal of the switch was the +5v power connection. This wire was removed, as was the unsheathed wire through all of the middle terminals of all switches.
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IMG_3565.JPG

Here are some pics of the completed mod, with A12/GND switched with each DPDT switch:
IMG_3566.JPG
IMG_3574.JPG
IMG_3584.JPG
IMG_3585.JPG
IMG_3588.JPG
IMG_3589.JPG
IMG_3590.JPG

#2 Pioneer4x4 OFFLINE  

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Posted Thu Jun 24, 2010 12:07 PM

I think I understand this, I'll wait to I open mine up. Is there a handy link to a schematic or pinout of the 2600 cartridge port?
If I modify mine, I assume I can just modify the 2 close ports. What would be the best way to do that?

#3 A.J. Franzman OFFLINE  

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Posted Thu Jun 24, 2010 5:57 PM

The way the Video Game Brain was originally made is, as Fred wrote, a horrible design. If you're going to modify it at all, just go ahead and modify fix all of the ports.

#4 Video OFFLINE  

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Posted Fri Jun 25, 2010 3:29 PM

Cool I guess, but I always turned the system off before switching games (as I believe the instructions said to do anyways) so I've never had the problem. Of course, the little kids would always just hit the button, but half the time the game would crash and require a reset anyways so... :P

#5 batari OFFLINE  

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Posted Fri Jun 25, 2010 5:22 PM

Cool I guess, but I always turned the system off before switching games (as I believe the instructions said to do anyways) so I've never had the problem. Of course, the little kids would always just hit the button, but half the time the game would crash and require a reset anyways so... :P

The Game Brain can damage your games or system due to its electrical design, and operating it per instructions is still just as dangerous. If you never had a problem, you should consider yourself lucky.

#6 tetrode kink OFFLINE  

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Posted Sat Jul 3, 2010 1:41 AM


Cool I guess, but I always turned the system off before switching games (as I believe the instructions said to do anyways) so I've never had the problem. Of course, the little kids would always just hit the button, but half the time the game would crash and require a reset anyways so... :P

The Game Brain can damage your games or system due to its electrical design, and operating it per instructions is still just as dangerous. If you never had a problem, you should consider yourself lucky.

Just to clarify, what batari is saying is that the problem isn't merely switching games with the power on, the problem is that having the power on at all while there are any carts in the Game Brain that aren't selected is potentially harmful to such carts. Which means you shouldn't have any games other than the "selected" game inserted in the Game Brain while the power is on, which means the Game Brain is redundant. batari's mod eliminates the problem, making the Game Brain safe to use as originally intended.

-tet

#7 Buzbard OFFLINE  

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Posted Wed Aug 4, 2010 2:27 PM

Nice mod batari.:thumbsup: Thanks.

One question though,I notice that when a cart is not selected, you have its A12 line grounded, is it necessary to ground A12 or can it just be left disconnected?

#8 A.J. Franzman OFFLINE  

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Posted Wed Aug 4, 2010 9:43 PM

Best practice is to always put an IC's input pins into a specific state, and not leave them to "float".

Certain IC families will exhibit a known behavior if inputs are open (such as TTL, in which unconnected inputs assume the "high" state), and may be left open if this state gives the desired result. However, other IC families (such as CMOS) will exhibit erratic and unpredictable behavior on unconnected inputs, which may include latching, oscillation, chaos, or extreme electrical noise sensitivity. Even if all outputs related to that input are unused, the gate will draw far more power if it is in an oscillating or chaotic state, and it will produce electrical noise that may affect other gates in the same IC, or other parts of the circuit.

#9 Buzbard OFFLINE  

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Posted Thu Aug 5, 2010 8:08 AM

Ok, that's kinda what I was thinking, but thought I'd ask anyway.
Thanks A.J.

#10 Lendorien OFFLINE  

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Posted Fri Jan 7, 2011 12:46 AM

I'd like to do this mod to be able to use the game brain for dumping and to make it safe for use, but I do have a question because I'm not terribly technically proficient when it comes to electronics. I certainly see what's going on in the photos and can do the soldering required, but I do have a question.

At the top near the cable clamp, where does the yellow wire to the first capacitor come from? Is it from the same solder point that the black wire is soldered to? It's not clear in the photos.

#11 A.J. Franzman OFFLINE  

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Posted Fri Jan 7, 2011 1:35 AM

Going by the 4th photo in post #1, that appears to be correct. The black wire you mentioned seems to be connected to a power indicator LED in the lower left corner of that photo, and the white wire from the LED goes to a resistor, which then connects to a trace which connects to all of the VGB's cart ports' pin 12's. Since pin 12 of the cart port is a ground, for the LED to light, the black wire must be +5V. Since the yellow wires at the top of the photo tie into all of the capacitors' "+" leads, and those leads go to the cart ports' pin 23's (also +5V), it makes sense for the yellow wire to the leftmost capacitor and the black wire to the LED to originate from the same place.

Edited by A.J. Franzman, Fri Jan 7, 2011 1:37 AM.


#12 batari OFFLINE  

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Posted Fri Jan 7, 2011 1:46 AM

I'd like to do this mod to be able to use the game brain for dumping and to make it safe for use, but I do have a question because I'm not terribly technically proficient when it comes to electronics. I certainly see what's going on in the photos and can do the soldering required, but I do have a question.

At the top near the cable clamp, where does the yellow wire to the first capacitor come from? Is it from the same solder point that the black wire is soldered to? It's not clear in the photos.

Yes, it goes to the same place as the black wire (the black wire that connects to the power LED.) This is +5v, by the way.

EDIT: I posted before seeing AJ's reply, which is also correct.

Edited by batari, Fri Jan 7, 2011 1:48 AM.


#13 iesposta ONLINE  

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Posted Tue Jun 19, 2012 9:17 PM

I just did this modification project.
The Game Brain looks never used. I didn't use it before modding.
Switches fine on my Heavy Sixer, but Q*bert resets itself.
What do you think is causing that?
The capacitors?
Too thin gauge wire?

#14 alex_79 OFFLINE  

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Posted Wed Jun 19, 2013 1:47 AM

Just came across some detailed pictures of the "marjac romscanner" board at www.the-liberator.net (thanks to the link in this thread), and it apperas that it has the same flawed design. I marked the pins on the picture with different colors and you can see that the device switches power, while the A12 pins are in parallel between the cart ports.
Atari-2600-ROM-Scanner-by-MARJAC-1983-Rev-D-005.JPG
Atari-2600-ROM-Scanner-by-MARJAC-1983-Rev-D-006.JPG
Atari-2600-ROM-Scanner-by-MARJAC-1983-Rev-D-008_labelled.JPG

By looking at the board, the switches are SPDT, so it cannot be modified in the same way as the video game brain. Maybe using pulldown resistors on A12 for each cart port would work? I ask beacuse I'd like to buy one of these devices if I find one cheap enough, and I want it to work safely.
The leds would represent another issue in case of modification, so maybe it's not worth the effort.

#15 bcombee OFFLINE  

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Posted Sun Sep 29, 2013 4:37 PM

I was tracing the signals on an Imagic display kiosk game selector last night, and was surprised to see that it also seemed to be switching both power and ground instead of using A12 as a chip select.  That was based on some quick continuity testing while the unit was powered off, however. Has anyone traced out of these and made a schematic?  The construction of the board looked top notch, and while I saw multiple uses of 74LS244 buffer chips, there was another common chip with an obsolete part number that I can't remember.

 

The selector definitely had an issue with multiple chips driving at once, although usually interference was in the same column rather than across columns.

 

If I get a chance, I'll get some high resolution pictures of the front of the board, as the only ones I found online are of the back.  The design was neat, mostly all discrete logic.  It had a hefty 5V, 3A linear power supply to run the board, but I suspect 30 years haven't been kind to the capacitors.



#16 batari OFFLINE  

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Posted Sun Sep 29, 2013 5:19 PM

The fanout from A12 on the 6507 probably wouldn't support the number of slots on the Imagic kiosk, but switching off power is definitely not the answer. The correct use of buffer chips here should be to buffer A12 to the slots instead - at least I'd hope Imagic would be smart enough to do that.






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