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Let's Improve On This - Light/Heavy Sixer ATARI 2600 w/Built-In GAMES/ROM

unicorn ROM sixer built in game

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#1 atari2atari ONLINE  

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Posted Fri Apr 13, 2012 9:54 PM

I would like your help to make something awesome that we all can enjoy.


Here's the PURPOSE and POINT - -

The original ATARI 2600 had a spot on the motherboard where a built-in game was SUPPOSED to be installed:

DSCN1646.JPG

Basically, that was a spot where an integrated circuit ROM (Read Only Memory) with a game on it, just like what you find in a cartridge, could be built-in. Somewhere during production of the ATARI 2600, however, that AWESOME idea was scrapped, but the "solder pad" for the game remained behind. Then, in later revisions of the motherboard that location was removed entirely. It is only found in "Light" and "Heavy" six-switch ATARI 2600 units.

You can still install a game in that spot, though, and have it run automatically when no cartridge is loaded! There are some limits and challenges, however.



Over the years, others have documented there observations and efforts to make this work, but to date no one has written a CLEAR, SIMPLE tutorial on how it can be done so others can enjoy it. (I reference the work of others that I could find in more detail, below)

I'd like to change that, and I would propose that those who have something to CONTRIBUTE help write a definitive guide to make this work!



Here's my PROPOSAL - -

So far I've worked on this for a few weeks. I have gotten it to function - - to a point! I have major questions that are still unanswered, but I have done a lot of the first steps.


1) I would like to spell out below in tutorial format below what I have done so far.

2) And, I'd like to outline the MAJOR QUESTIONS that I think remain in order to get it to work properly



What I'd like YOU to do, if you are willing, is tackle any and all of the parts or answer questions that you feel you can CLEARLY and POSITIVELY contribute to!

What's that mean? It means that a "drive-by spray posting" of a few phrases with no illustrations or explanation, like "well, you just tie such and such to this and that, and there you go!" are not what we're looking for.

This is for "the record," so it needs to be easy to read, and people have to be able to follow the steps again.

Try reading some of these "BACKGROUND" threads below, and you will see what I mean . . .




###



BACKGROUND

There are several historic threads on the topic that provide some starting point for us.

The first is the work of AtariAge member "Longhorn Engineer" along with user "A.J. Franzman" - -

http://www.atariage....ost__p__1301781

Take a minute and scroll through it if you would like, but to summarize, there was a lot of back and forth on what was needed to be modified or added to the motherboard, some initial versions were tried with some success, and then it was abandoned. No real agreement on what you do or do not modify was reached.

Honestly, that was the best thread I have found so far after a fair bit of searching.



One other thread I found mentions the topic, along with a brief discussion of how to get larger games to work in that spot on the motherboard, but again no specifics are provided on how to DO it - -

http://www.atariage....ost__p__2113348



There is a quick mention of the "unicorn board" ATARI 2600 Jrs. in that last thread link I provided, where the "built-in" game made a brief manufacturing resurgence - -

http://www.atariage....ost__p__2113944

These are rare and not in every ATARI 2600 Jr. (don't start tearing yours apart!). Some of the "features" of this board work quite well, like accommodating larger built-in games. I have one of these boards and have gotten it to work with a built-in game (I chose PITFALL!), as I start to detail here - -

http://www.atariage....-compatibility/

And, other topics can be found about "unicorn boards" as well - -

http://www.atariage....2600jr-unicorn/

http://www.atariage....-unique-2600jr/


It's relevant because some of the "switching circuitry" that allows this to work in these late-model ATARI could be useful for this project.


[Do you have other links or references for this section? Contribute them if you do, please!]




###




QUESTIONS

Some of these questions won't make sense until you start reading the outline of the TUTORIAL below but it's good to get them out here first, I think.


There are just THREE main questions right now:



- How SPECIFICALLY (what is soldered where and why) is it possible to get games larger than 2k to work in the solder spot?

I have installed several 2k size games into the spot on a Light sixer, and had them work (COMBAT, etc.) but anything larger, like DONKEY KONG, will not work.

As mentioned above, "A.J. Franzman" alludes that it is possible (http://www.atariage....ost__p__2113348), but there are no details.

I can tell you that a 4k game DOES work in the "unicorn" - - PITFALL! is 4k, and I was able to get DONKEY KONG to work, too.



- What is the BEST way (again, with as much detail as possible with what gets modified or added and why!) to get maximum compatibility for the detecting and automatic switching of a cartridge being loaded?

I am pleased to say that by just straight installing a 2k ROM integrated circuit in the solder spot and doing NOTHING else, you actually get a LOT of compatibility. The console will automatically switch to the loaded cartridge in many situations. But not all.

I have not figured out a pattern. Here's an interesting list of games that DID boot and work with the built-in ROM in place (non-scientific, I was just grabbing carts from a box) - -

KEYSTONE KAPERS, DECATHALON, YARS' REVENGE, PRESSURE COOKER, COMBAT, FROGGER, WIZARD OF WOR

And, those that did not - -

GRAVITAR (you see the title screen, and then it rolls and glitches out), SPACE SHUTTLE, PITFALL II, and the HARMONY CARTRIDGE


The HARMONY is a known case, and is likely fixable, by the way - -

http://www.atariage....70#entry1943270

But, I have tried shorting those two pins that were mentioned, and still can't get it to work.


Once the ROM is pulled from the motherboard, ALL of the above games will work just fine, including the HARMONY, by the way.




- What is the cleanest and most direct way (once more, with DETAIL on exactly what to cut and solder!) to wire in a "kill switch" for when the above "compatibility mods" don't always work?

We need a way to absolutely and positively disable the on-board built-in ROM for times where you want to return the console to its native state.

I have looked and looked, and I just don't understand enough about disabling ATARI mask ROMs.

I think it will involve cutting some traces that go to the place where ROM is located on the motherboard, wiring some pins of the ROM to be able to be grounded (or electrified?), and putting a positive switch in there somewhere that gets flipped.

Any and all ideas on this one are welcome!





###




TUTORIAL

This will be short and sweet - - the steps thus far are simple!



Take apart your sixer and expose the motherboard.

I used this light sixer, with this serial number, and a manufacturing date of August 14, 1980 - -

DSCN1641.JPG

DSCN1642.JPG

DSCN1645.JPG

Ugh, this one was kind of gross before it got cleaned, and someone had kludged in a random non-ATARI cable - -

DSCN1643.JPG


Here is the motherboard and revision number - -

DSCN1648.JPG

DSCN1963.JPG

And, as shown before, the solder spot - -

DSCN1646.JPG


I simply cleaned the solder holes with soldering braid, and then soldered in an integrated circuit socket - -

DSCN1651.JPG

IMPORTANT - - note that the socket is BACKWARDS the way I installed it! The "key notch" that shows you which way to orient the integrated circuit when you socket it in is reversed!

I learned this because on the UNDERSIDE of the motherboard is a VERY small "1" etched into the traces that tells you where the first pin of the integrated circuit should go - -

DSCN1713.JPG

Look in the upper left hand corner, near my "1" that is drawn in marker (IGNORE my wire coming off the pin in this picture, it's something I was doing for testing)



Now, in order to install your ROM, I recommend you actually place it in ANOTHER socket, and you will be socketing THAT socket into the one that is soldered on the board. I do this so I can have some modularity with my ROMs, and swap and try different ones.

Here's how that looks - -

IMGP4141.JPG

I actually place a dot of solder at each of the four corners of the socketed ROM so that it doesn't come back out, too. If you are swapping these things in and out of the motherboard, it can happen, so this just secures it.




What ROM to use? Right now there is a 2k ROM size LIMIT! If we can figure it out, the maximum size is 4k. No bankswitching or fancy special chip ROMs will be possible.

EEPROMs are possible with the inclusion of an inverter [This is a section that needs to have more input on exactly WHERE and how an inverter would be placed to allow for an EEPROM]

How do you know? I used Kevin Horton's excellent list of cartridges and scanned down it for 2k ROMs - -

http://www.emulatron...tari8/sizes.txt (direct link)

Which was found here - -

http://emu-docs.org/...=All%20Consoles



NOTE: desoldering ROMs from cartridges is slow and laborious. Desolder EVERYTHING completely though, and go slow so that you don't hurt the pins on the ROM!



And, once you have your ROM and have socketed it, then you can place it on the board.

NOTE the proper direction and orientation of the "key notch" here, which is facing RIGHT! - -

DSCN1708.JPG

DO NOT press down all the way with your socketed ROM! Just enough to make contact is for testing is perfect! If you press it in all the way, it's going to be HARD to get it back out without possibly bending pins!

Once you are certain the project is done, you can "click" it positively in, however.


And, that's it.

Really.

Turn on the console with no cartridge in it, and COMBAT or whatever will boot. Put a game in the cartridge slot and it will likely boot - - depending on the game.


Now, there was a lot of talk in that thread referenced above about cutting ground traces, adding in a zener diode, and other "pull up circuits."

I tried all that. I also tried it all WITHOUT it. So far, it works the same EITHER WAY, compatibility bugs and all.

DSCN1711.JPG

(note my zener diode and the cut trace on the ground, just like "Longhorn Engineer" had)

DSCN1962.JPG

Now I have repaired the cut trace, and removed the zener diode - - and I have the same exact compatibility as I did before. This is how it is operating now.




I think that the "answers" to some of the compatibility questions and the kill switch idea will require cutting traces, but for now, this "works" without it.




###





SO, that's where I am at.

Still with me?

tl;dr ? (too long; didn't read?) ;)


I'm looking forward to any and all ideas that people have. I will happily cut and solder whatever, as long as there seems to be a decent reason behind it, so please feel free to make suggestions!


-atari2atari

#2 SoundGammon OFFLINE  

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Posted Fri Apr 13, 2012 10:28 PM

...I'm just going to toss this idea in to the wind, but have tried an eprom instead? Maybe a 2516 or a 2716 eproms for 2k and 2532 or 2732 eproms? Just a thought...

#3 Pioneer4x4 OFFLINE  

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Posted Sat Apr 14, 2012 6:53 AM

Very interesting work so far.
As I mentioned in another thread. I would mount an inverter between 2 sockets and wire it up by bending pins so they do not pass through. Then you could use it as an adapter for EPROMS. Is there any space limitations above it?

#4 atari2atari ONLINE  

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Posted Sat Apr 14, 2012 8:57 AM

Looks like I have lost my ability to edit the first post - - too bad!

I mean to include this "snip" of the 2600 schematics that shows the place where the ROM is also mentioned - -

Snip from 2600 Schematics.JPG




Now, on to your questions . . .


...I'm just going to toss this idea in to the wind, but have tried an eprom instead? Maybe a 2516 or a 2716 eproms for 2k and 2532 or 2732 eproms? Just a thought...


Great questions on EPROMs!

From what I know so far - -

- EPROMs will theoretically work, although I have not personally tried it

- The same 2k or 4k limit applies, with no bankswitching, etc.

- EPROMs have the same pin-out as an ATARI mask ROM, with the exception of . . .

- EPROMs are "active low", which means that a low voltage current to the right pin on the IC will enable the EPROM. ATARI mask ROMs are "active high", so a high current (+5 v, I think) to activate the chip. This means that there is a high current on the motherboard headed to the chip select pin (remember, they have the same pin-out) that needs to be inverted so that the EPROM can be used.




Very interesting work so far.
As I mentioned in another thread. I would mount an inverter between 2 sockets and wire it up by bending pins so they do not pass through. Then you could use it as an adapter for EPROMS. Is there any space limitations above it?


Would you mind showing us in pictures exactly where you would wire that inverter, and specifically what part number or type of inverter you would use? "A.J. Franzman" also mentioned that you could use an inverter, but never provided details - - http://www.atariage....ost__p__2113850

I would be great if you wanted to provide some concrete details, and then I could try it out!

Here's some handy pictures for your use - -

DSCN1968.JPG

DSCN1970 (2).JPG

:)

-a2a

Edited by atari2atari, Sat Apr 14, 2012 9:01 AM.


#5 atari2atari ONLINE  

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Posted Sat Apr 14, 2012 1:48 PM

Looks like I have lost my ability to edit the first post - - too bad!

I mean to include this "snip" of the 2600 schematics that shows the place where the ROM is also mentioned - -

Snip from 2600 Schematics.JPG




Now, on to your questions . . .



...I'm just going to toss this idea in to the wind, but have tried an eprom instead? Maybe a 2516 or a 2716 eproms for 2k and 2532 or 2732 eproms? Just a thought...


Great questions on EPROMs!

From what I know so far - -

- EPROMs will theoretically work, although I have not personally tried it

- The same 2k or 4k limit applies, with no bankswitching, etc.

- EPROMs have the same pin-out as an ATARI mask ROM, with the exception of . . .

- EPROMs are "active low", which means that a low voltage current to the right pin on the IC will enable the EPROM. ATARI mask ROMs are "active high", so a high current (+5 v, I think) to activate the chip. This means that there is a high current on the motherboard headed to the chip select pin (remember, they have the same pin-out) that needs to be inverted so that the EPROM can be used.




Very interesting work so far.
As I mentioned in another thread. I would mount an inverter between 2 sockets and wire it up by bending pins so they do not pass through. Then you could use it as an adapter for EPROMS. Is there any space limitations above it?


Would you mind showing us in pictures exactly where you would wire that inverter, and specifically what part number or type of inverter you would use? "A.J. Franzman" also mentioned that you could use an inverter, but never provided details - - http://www.atariage....ost__p__2113850

I would be great if you wanted to provide some concrete details, and then I could try it out!

Here's some handy pictures for your use - -

DSCN1968.JPG

DSCN1970 (2).JPG

:)

-a2a


Pioneer, the above question is still relevant, and if you can answer it, that's great, but perhaps this would assist your illustration or wiring diagram?

http://www.atariage....ost__p__2066625

-a2a

#6 Pioneer4x4 OFFLINE  

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Posted Sun Apr 15, 2012 5:36 AM

I will work up a diagram, but here is the visual idea of what I meant. Personally I would use wirewrap sockets with longer leads and cut to length, and insert them into holes, not solder.
http://ist.uwaterloo...rs/sockets.html
And it would be better to make it for a 28 pin 27256 or there about since 2732s are impossible to find.
basically, this is all from memory/quick checks, so do not assume it is correct.
  • bend out the leg on the upper socket for pins 18&20 and connect the line FROM the inverter pin 8 to the bent pin 18 on the upper.
  • put a wire TO the inverter pin 9 in maybe pin 20 on the lower.
  • Tap the GND and 5V from the pins on the top socket which will connect when inserted.


#7 e5frog OFFLINE  

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Posted Wed Feb 5, 2014 5:02 PM

Any more work done here?

So basically, if you insert a custom ROM in the socket then you won't be able to run some cartridges?


I was thinking you could add your own special startup-screen - that could be fun, how about the same type of image displayed as the Amiga, with a hand and a cartridge. ;-)

#8 atari2atari ONLINE  

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Posted Wed Feb 5, 2014 5:08 PM

Ugh, no, I'm stuck!

There's some way in which the ATARI can recognize when a cartridge with a ROM is inserted, and so NOT attempt to run the soldered-on ROM.

But I don't know what that "switching circuit" is or requires!

AJ Franzman mentions it here, but never details what it is, exactly - -

http://atariage.com/...tion/?p=1301954

The "switching circuit" is the "pullup" that he mentions.

I'd love to finish this project, I've got several boards now with soldered-on ROMS, it's lots of fun to just power it on with no cartridge and play PITFALL or DONKEY KONG, but I'd love to be able to insert any cartridge and have the ATARI "sense low" and ignore that soldered-on ROM!

Any help would be very welcome!

-a2a

#9 iesposta ONLINE  

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Posted Wed Feb 5, 2014 5:15 PM

I really wanted a built in Yars' Revenge, but that's 4K and the questions weren't answered or were not asserted clearly enough.
I don't wanna do it if it isn't still 100% compatible with everything else.

#10 atari2atari ONLINE  

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Posted Wed Feb 5, 2014 7:58 PM

I really wanted a built in Yars' Revenge, but that's 4K and the questions weren't answered or were not asserted clearly enough.
I don't wanna do it if it isn't still 100% compatible with everything else.

 

In general I have discovered, both from reading and from experience, 2k is the max, no bankswitching.

 

If you just simply solder the masked ROM to the board, it's NOT 100% compatible with "everything else" (some carts will work, some won't).  Your game you attached directly to the board will always work with no other cartridge in the 2600, and some games inserted in the cart slot will boot, some won't.

 

I recommend soldering a socket to the board, soldering the masked ROM to ANOTHER socket, and then using THAT to test various masked ROMs stolen from cartridges.  This lets you "undo" anything that doesn't work, and makes the project modular.

 

I used this list to make a first-pass of potential ROMs to use: http://www.classic-g...bankswitch.html

 

Here's some example pictures - - 

 

DSCN1646.JPG

 

DSCN1651.JPG

 

Copy of DSCN1708.JPG

 

Copy of DSCN1709.JPG

(Last shot here, you can see the "stacked" sockets that make it modular.  That is me playing Combat, directly off the board)

 

 

** If anyone wants to help figure it out, the secret to this whole problem lies here, in these words - - 

 

http://atariage.com/...tion/?p=1301954

 

He even later says there's a way to make 4k games work, too - -

 

http://atariage.com/...nces/?p=2113850

 

 

Anyone know? :)  It'd be a great thing to figure out!  I'm just not a good enough electrical engineer to know WHERE to solder the resistors and make the circuit!  I need specifics, not theory!

 

-a2a



#11 e5frog OFFLINE  

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Posted Thu Feb 6, 2014 12:09 PM

It seems the suggestion was to separate a ground connection at the socket and then pull this connection high (with a resistor to +5V). When inserting a cartridge - that usually has these two grounds connected - it will pull down that signal to ground again, and disable the built in ROM I guess. It will require that the cartridge you're using have the two grounds connected in the cart. I guess not all of them does.

I haven't checked the details myself, I got curious when I saw this empty space in a light sixer I repaired, some kind of bios could have been inserted, I also do fancy videogames with built-in games.

Regarding the light sixer, it needed some button cleaning and also got the sound working:
http://atariage.com/...ic-cheap-patch/
Marvelous picture even through RF.


That Bomarc schematic snip looks odd, there's no VCC and the address-lines are offset where there should be enable signals.
EDIT: Ah... that's the cartridge (plug-in rom) socket, the "see sheet 4 for rom" is that where the internal rom is mentioned?


EDIT2:
There's an explanation here:
http://atariage.com/...s/#entry2113944

In the Jr with similar rom space:
"One of the CS pins is connected to cart port pin 24 and pulled high, while the other is connected to A12. When a cart is inserted it connects pin 24 to ground, so the internal rom is deselected. Eproms usually only have one active-low chip-select pin, so you need extra logic in that case (I posted a schematic of a clone console with built in games here).

Since a few carts do not have pin 24 and pin 12 connected togheter (harmony cart and a few activision games, for examaple), you should also add a switch between pin 24 of the cart port and ground so you can manually disable the internal rom, if needed. "


Is there a reason why the Harmony cart decided not to connect the two grounds?
Putting a switch in the socket instead would allow for all carts to work then I guess. If you want to use an eprom then you need to invert those chip select/enable signals as well.
micro.switch.jpg

Edited by e5frog, Thu Feb 6, 2014 12:39 PM.


#12 batari OFFLINE  

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Posted Thu Feb 6, 2014 6:24 PM

Since a few carts do not have pin 24 and pin 12 connected togheter (harmony cart and a few activision games, for examaple), you should also add a switch between pin 24 of the cart port and ground so you can manually disable the internal rom, if needed. "


Is there a reason why the Harmony cart decided not to connect the two grounds?

If the H6 schematics are correct, they have no provision for disabling the onboard ROM while a cart is in. If anything works at all, it's because the cart inserted overpowers the data bus of the onboard ROM. This causes currents in the chips that may be larger than they were designed to handle, which could cause damage to the onboard ROM, a cart, or both. I wouldn't do anything else with this mod until the disabling mechanism is in place.

 

To answer the question about Harmony, the two grounds were not connected because one of the ways a Melody board knows if it's in a console or a programmer is to detect the voltage applied to them. This doesn't affect all Harmony carts, actually: the two grounds were connected in newer boards. I'm not sure of the exact date when these started to ship but it's around August 2013 that this change was made.



#13 e5frog OFFLINE  

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Posted Sat Feb 8, 2014 11:07 AM

I wouldn't recommend anyone adding an internal rom chip without doing the needed changes with pull-up resistor and cut the ground connection and possibly add a switch as well for full compatibility. Instead of cutting the trace one could just as easily lift the enable signal of the internal rom-chip and add a switch to turn it on or off.

Nice to know what it was for, it could have been fun adding your own specialized rom in there.

#14 e5frog OFFLINE  

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Posted Sat Feb 8, 2014 4:10 PM

Found this picture when googling
http://www.atariage....rtridge_PAL.gif

Shows pin 24 as /EXROM (external rom) hooked to GND, seems it was meant to indicate the presence of external rom, but as it was in reality hooked to GND internally there's no reason for the carts to do this. Explains the idea about disconnecting the internal rom though.
If a cartridge masked rom works with no modification, maybe the space was meant for some resellers own eprom, imagine it saying Sears in rainbow colors when starting and then asking you to turn off and insert a game cartridge, maybe it would have some simple pong-like game built in...

#15 atari2atari ONLINE  

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Posted Sat Feb 8, 2014 10:12 PM

Thanks for all the interest in this thread, fellows!  It's a topic I've been thinking about for a while, so I appreciate the renewed conversation.  It's sometimes hard to make progress just thinking in your own head!

 

Lots of good comments and suggestions here.  Among them (thanks, e5frog!) is the one that I tentatively decided on back when I was initially working on it last year - - a simple hard switch.  Just a good old physical ON/OFF for the onboard ROM.

 

...

In the Jr with similar rom space:
"One of the CS pins is connected to cart port pin 24 and pulled high, while the other is connected to A12. When a cart is inserted it connects pin 24 to ground, so the internal rom is deselected. Eproms usually only have one active-low chip-select pin, so you need extra logic in that case (I posted a schematic of a clone console with built in games here).

Since a few carts do not have pin 24 and pin 12 connected togheter (harmony cart and a few activision games, for examaple), you should also add a switch between pin 24 of the cart port and ground so you can manually disable the internal rom, if needed. "

 

...

Putting a switch in the socket instead would allow for all carts to work then I guess. If you want to use an eprom then you need to invert those chip select/enable signals as well.
micro.switch.jpg

 

 
I got the switch idea from reading about another mod, the "Flashback 2" modification for a cartridge port.  Very similar idea - - built in games, and the desire to override those when you insert a cartridge.
 
Your comment, efrog5, made me go back to look for a build-log I had seen of almost that exact same switch you show in your post.  I remembered someone adding it into the cartridge port to turn of the internal games.
 
And, as luck would have it, I didn't find that page, and instead came upon a discussion on a website I had previously missed regarding an "automatic selection circuit."  I think that circuit would work here in this situation!  He uses either a CD4016 or CD4066 CMOS transmission gate to build the circuit.
 
 
(and, here's a PDF of that page, just in case some day it gets taken down Attached File  Flashback 2 Cart Mod - Autoswitch.pdf   771.17KB   15 downloads )

 

 

HOWEVER, even that author states that "some cartridges still didn't work." :(

 

 

So, in the end, I am starting to lean towards the best solution just being a physical switch.  Not as classy, but very effective.

 

I agree with you both here, it's not safe for the board or cartridges until some sort of circuit or switch is built - - 

 

... This causes currents in the chips that may be larger than they were designed to handle, which could cause damage to the onboard ROM, a cart, or both. I wouldn't do anything else with this mod until the disabling mechanism is in place...

 

I wouldn't recommend anyone adding an internal rom chip without doing the needed changes with pull-up resistor and cut the ground connection and possibly add a switch as well for full compatibility. Instead of cutting the trace one could just as easily lift the enable signal of the internal rom-chip and add a switch to turn it on or off...

 

(Hey, thanks for jumping into the conversation, batari, by the way - - your perspective and engineering mind are very appreciated!)

 

 

My plan now is to try that CMOS transmission gate circuit, just for fun, and then also that hard switch from cartridge pin 24 on the cartridge slot to ground (as discussed here, which you also quoted - - http://atariage.com/...es/?p=2113944).

 

Regarding the hard switch, as I understand it, couldn't you also disable a standard masked ROM by taking pin A12 and grounding it, or "pulling it low" - - that's the correct terminology, yes?   Is that also a valid method?  Is it a better method than a switch to ground cartridge pin 24?

 

Thanks again for the conversation, I appreciate it . . . !

 

:D



#16 iesposta ONLINE  

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Posted Sat Feb 8, 2014 10:27 PM

Relating to the Atari Flashback 2 that "some cartridges still don't work" That has to do with the fact that one of the signals was not brought out of the 'Atari on a chip' so some games with certain bank switching fail.
>Not due to a switch. <
For instance the Flashback has Millipede, but the real cart will not play on the Flashback 2.

It is probably common knowledge, but hardware newbies like me are still not sure what selects a chip and what disables it. I rely on following steps others write to build mods.

#17 e5frog OFFLINE  

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Posted Sun Feb 9, 2014 12:29 AM

That Flashback 2 mod seems to rely on the same type of switching, getting pin 24 grounded by the cart. As this doesn't work all the time perhaps some other method would be better. Maybe combined with a current sense on the +5V pin in the cartridge connector. But I think a physical switch would be the easiest hack, cart in => internal rom disabled and vice versa.

An optical switch in the cart chute could work if you don't want a electromechanical.

#18 e5frog OFFLINE  

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Posted Mon Feb 10, 2014 1:15 PM

Regarding the hard switch, as I understand it, couldn't you also disable a standard masked ROM by taking pin A12 and grounding it, or "pulling it low" - - that's the correct terminology, yes?   Is that also a valid method?  Is it a better method than a switch to ground cartridge pin 24?


We can call it A12 but the function in that masked rom is probably Chip Enable or Output Enable, sometimes just called CS/CS1/CS2.

The cartridge connector in the 2600 is connected to GND already, assumed to be a design change when deciding not to include an internal ROM. By breaking this ground connection to pin 24 of the cartridge connector (that I assume the pin position "A12" of the non present internal ROM is hooked to) you'll enable the detection of a cartridge by expecting pin 24 to get grounded when a cartridge is inserted. Cartridges should/must all have the two "ground connections" bridged.

If you want to be sure all cartridges work it's a better method to have a toggle switch to the internal ROM. That means you also don't need to permanently modify the motherboard. If using a socket you can bend the enable pin away from the board and solder a wire to it then connect it to either +5V or GND with a switch. However, you will be able to accidentally activate the internal ROM when inserting a cartridge - which is not good so the optimal solution would be a switch that is activated by the cartridge or a switch that will be deactivated when powering off. Optical switch, relayed button, mechanical switch, clever electronic switch (for example sense current on cartridge 5V).
To prevent accidentally having the internal rom active you could have a relay or electronic circuit that release the active signal when powering off and it's not enabled unless a button or switch is pressed after powering on again.

If using a dipswitch it's necessary to have a resistor or you'd just be able to set one condition unless there's an internal pull-up or pull-down.

If you put the end with the "Two-state output" in the picture below to the internal ROM it will stay low until switch is closed (turned on), if you swap ground and voltage (that's the arrow up) you'll have a default condition of high. I would use 10k Ohm instead of just 1k as showed in this example picture.
design_14.JPG
If using a ON/ON switch you'll put the internal rom signal hooked to the middle common pin and then +5V on one of the outer pins and GND on the other one, resistor is recommended to prevent shorting the console if switch breaks.


Personally I think if you're going to have an internal ROM at all you want it to be automatically activated whenever no cart is plugged in.

In theory you could have several internal ROMs and toggle the enable signals to choose chip, for example via a randomizer circuit. ;-)

Edited by e5frog, Mon Feb 10, 2014 1:50 PM.


#19 e5frog OFFLINE  

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Posted Tue Feb 11, 2014 11:43 AM

Image saved and then uploaded to postimage, moderator can feel free to copy this link and replace in the previous post. (and erase this post)
design_14.jpg





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