2600 home brew compatibility on 7800?

[Albert]

1 minute ago, SearsRoebuck said:

I can happily report that LadyBug works on my Atari 7800 - this is what freaked me out, another poster saying this exact game didn't work for them - as a copy I had bought was on its way to my place! Excellent port btw.  Dark Chambers 2600 seems like a poor trade off; though I'd be curious if Secret Quest and Stargate don't work on these systems for any one who has them. 

The problems described here are the exception, not the norm, thankfully.  The vast majority of people are able to play homebrew games on their 7800s without any issues. 

 

 ..Al

[AtariLeaf]

20 hours ago, SearsRoebuck said:

I can happily report that LadyBug works on my Atari 7800 - this is what freaked me out, another poster saying this exact game didn't work for them - as a copy I had bought was on its way to my place! Excellent port btw.  Dark Chambers 2600 seems like a poor trade off; though I'd be curious if Secret Quest and Stargate don't work on these systems for any one who has them. 

 

As to why I don't just play 2600 games on a 2600 - a combination of both space and the fact that the 7800 does play about 99.9% of the 2600 games. My system is an AT85, and has an early 'purple box' (matching serial) so it is before the systems were modified any. 

Yep it's a revision issue as other 7800s I own play ladybug fine. I'll check and see if I can narrow down which one it is.

 

Edit. Heres a pic. Made in 1987?

Edited July 8, 2020 by AtariLeaf

[+-^CrossBow^-]

And this one doesn't play Ladybug is that correct? Again the blue stamp might indicate the year but to know when you usually have to check the chip date codes to get a better idea. It is possible that this one has the extra timing circuit as I'm not exactly sure when it was introduced but possible. Just have to open it up and see.

 

[+Mitch]

5 hours ago, AtariLeaf said:

Yep it's a revision issue as other 7800s I own play ladybug fine. I'll check and see if I can narrow down which one it is.

 

Edit. Heres a pic. Made in 1987?

First digit is year, second is month. So July of 1988.

 

Mitch

[juansolo]

For what it's worth, I get exactly the same symptoms with my PAL 7800 (Rev C mobo). Star Castle and Space Rocks don't work (just fires straight into 7800 Asteroids), but they do run ok from the Harmony Cart. I don't have a huge amount of other homebrew for the 2600 yet (working on it, waiting for the release of Tower of Rubble before I order my next batch), but Juno First, Draconian and Galagon are fine.

 

What does the C64 cap connect to? I'm fairly certain it's not the correct one in the PAL schematic, and I can't even find it on the NTSC one...

Edited July 10, 2020 by juansolo

[+-^CrossBow^-]

Correct, the only schemes I have on the NTSC 7800 are dated from '84 which, was before they added in the extra timing circuit. The cap sits between the 74x logic IC LS174 flip-flop and an LS02 quad 2-input NOR. C64 on the NTSC system connects Pins 6 and 13 of that 74LS02 together.

 

 

 

[juansolo]

Found it, it's C42 on a PAL machine. Snipped it with interesting results. Star Castle now works, Space Rocks still doesn't.

 

UPDATE: I wouldn't bother. Where I can get Star Castle to work, it's intermittent. Maybe 1 time out of 10 it'll start. Most of the time it fires up Asteroids. Not sure it's made any difference.

 

 

Edited July 10, 2020 by juansolo

[marauder666]

Here's a zip with the later NTSC model schematics.

 

7800n.zip

[Frank.Bullitt]

I have a REV C PAL 7800 board.

 

I've modified the board like above, and I've had interesting results.

 

Haunted House running on a Harmony Cart has been improved, the eyes are shaking around like before, but they're no longer able to walk through walls haha.

 

I've attached some before and after pictures where you can see the protagonist is now walking through the gap, rather than through the wall next to it.

 

The screen flicker issue still remains sadly, my screen goes blank and then comes back, almost like a refresh rate change on a 480i ps1 menu.

 

[RevEng]

44 minutes ago, Frank.Bullitt said:

Haunted House running on a Harmony Cart has been improved, the eyes are shaking around like before, but they're no longer able to walk through walls haha.

Haunted House has a programming error in the positioning routine that only manifests on 1 in 100 consoles. Try the following version in your Harmony, where I fixed the error.

 

[edit - the link to the HH fix is in my reply below]

 

[Frank.Bullitt]

11 minutes ago, RevEng said:

Haunted House has a programming error in the positioning routine that only manifests on 1 in 100 consoles. Try the following version in your Harmony, where I fixed the error.

Wow I didn't know that, are there any other games that will have issues on the same consoles?

 

I've tried that binary file and there's no more eye shaking, now it works perfectly with no issues being caused by the compatibility circuit. Shame it's NTSC so the colours are wrong on my PAL console.

 

I'm going to try Space Rocks and Star Castle on the harmony tomorrow if I can find the binaries, hopefully this simple mod fixes the issues if the games are run via a Harmony Cart.

[RevEng]

2 minutes ago, Frank.Bullitt said:

Wow I didn't know that, are there any other games that will have issues on the same consoles?

 

I've tried that binary file and there's no more eye shaking, now it works perfectly with no issues being caused by the compatibility circuit. Shame it's NTSC so the colours are wrong on my PAL console.

My NTSC and PAL Haunted House fixes were posted here originally, so you can grab it from there.

 

Dodge Em has a similar bug that messes up the sound. It's a pretty common programming error, so there's probably more examples I'm not aware of.

[+DrVenkman]

34 minutes ago, RevEng said:

It's a pretty common programming error, so there's probably more examples I'm not aware of.

Given what I’ve uncovered about 2600 titles that don’t play nicely with my RetroTink, with hardly any effort on my part except trying a handful of games, I’m guessing you’re right. The screwy ways some games manipulate scanlines just don’t agree with line-doublers outputting HDMI.  

[RevEng]

31 minutes ago, DrVenkman said:

Given what I’ve uncovered about 2600 titles that don’t play nicely with my RetroTink, with hardly any effort on my part except trying a handful of games, I’m guessing you’re right. The screwy ways some games manipulate scanlines just don’t agree with line-doublers outputting HDMI.  

Well, that's an entirely different sort of error - messing up the sync requirements - which is unfortunately also very easy to do.

 

The sort of error in Haunted House comes about when a programmer means to type something like "LDA #1" (load A with the value of 1)  but instead types "LDA 1". (load A with the contents of memory location $01.)

 

On the 2600, most of the bits at $01 are undriven/floating for reads, but typically if you read from that location you'll get a value of the last byte in the opcode. (e.g. a "1" in the case of "LDA 1") since it was the last legit value on the bus, and hasn't yet decayed. This makes "lda 1" look and feel like "lda #1" most times, except on that rare 1 in 100 console, where the value decays quicker than usual. This issue also holds for other lower-register locations too, so it's a problem for values other than my example of "1".

 

[Frank.Bullitt]

I've got Star Castle to run with no issues from my Harmony now that the C42 has been lifted.

 

Lifting C42 definitely improves compatibility, at least on my 7800. I'd recommend the OP gives it a go.

[juansolo]

Star Castle and Space Rocks run both from the Harmony cart with C42 in place, but don't run as the stand alone carts. Space Rocks doesn't work either way on mine, Star Castle is intermittent afterwards, I'll have to put the cap back to see if it was intermittent before I snipped it. Both are fine in my VCS.

Edited July 12, 2020 by juansolo

[juansolo]

Yep, we tried some other bits and bobs. Pulling C42 on the PAL 7800 doesn't work to get Star Castle or Space Rocks working.

[+wongojack]

On 6/28/2020 at 4:40 PM, Jase said:

That’s interesting! Would be cool if there is a mod...

Hey Jase, did you ever pop C64 C42 and see if it got Space Rocks, Star Castle, and Scramble working?

 

 

*Edited reference per PAL schematic

[+-^CrossBow^-]

1 hour ago, wongojack said:

Hey Jase, did you ever pop C64 and see if it got Space Rocks, Star Castle, and Scramble working?

I believe Jase has PAL versions so they use a different capacitor marked on the board for that and the details of disabling it are noted above from some others who have tested it. I don't know about space rocks or Star Castle as I actually don't own those in physical cart form, but I do have a Scramble and it works fine on my 7800 though I never used it for testing others and stuck with the known issue games like the Activision titles as an example.

 

[Jase]

On 7/22/2020 at 7:54 PM, wongojack said:

Hey Jase, did you ever pop C64 C42 and see if it got Space Rocks, Star Castle, and Scramble working?

 

 

*Edited reference per PAL schematic

No I didn’t, I’ve just been running the carts on my jr until a tried and tested fix is found.. if it ever does! Seems we both have the same version PAL 7800’s, the issues are the same?

[+batari]

For PAL 7800s with Asteroids, I think the problem is with the PAL BIOS falsely detecting that no cart is installed, and so it boots Asteroids. The Harmony cart passes this detection, but it sounds like some Melody-based carts do not?

 

A fix for this issue (if other measures fail) is to replace the BIOS chip in the 7800 with one without the built-in Asteroids (such as with the DevOS BIOS, which is a nice mod anyway), or use a Harmony cart.