Making Genesis Jamma Adapter, Having video issues

[Tighe]

I am in the process of making a JAMMA adapter for my Genesis, but I am having the following video issue:

http://www.youtube.com/watch?v=CKv2kMTUU60

This Web page says that you don't need any sync splitter:

http://pc2jamma.mameworld.info/misc_arc1.html

What do you think I am doing wrong?

[MKL]

What pin of the A/V port are you tapping sync from? Are you sure you're actually using sync (pin 7) and not composite video (pin 3)?

I think the Megadrive/Genesis outputs a positive composite sync, i.e. the sync line is pulled high. If your monitor only accepts a negative composite sync (typical of most arcade boards), then that could be the problem (assuming you're not using composite video instead of sync by mistake). Try inverting the sync state with an inverter IC (74LS04) and see if it solves the problem.

[NFG]

Nothing uses positive sync, as far as consoles are concerned.  The only device I've come across in my long and entirely non-conclusive modding history with positive sync was my Robotron PCB.  All consoles use negative sync.

[Tighe]

Nothing uses positive sync, as far as consoles are concerned.  The only device I've come across in my long and entirely non-conclusive modding history with positive sync was my Robotron PCB.  All consoles use negative sync.

Thanks, so what do you think is the issue?

[MKL]

Nothing uses positive sync, as far as consoles are concerned.  The only device I've come across in my long and entirely non-conclusive modding history with positive sync was my Robotron PCB.  All consoles use negative sync.

As I said, I don't have a Megadrive at the moment, so I can only rely on pics I took when I had one. Here is shown the path of the sync line from the VDP to the A/V port:



If I'm correct in identifying that 5v spot (it's connected to the 5v leg of the crystal), the sync line is held high through a (2.2K?) resistor. I'm sure that if you probe the sync line it will be high, unlike on a Neo Geo (just an example) where it's low. The Megadrive/Genesis is known for displaying sync issues when used with some RGB converters which are solved if the sync line is made low. I wouldn't exclude the problem here can be solved the same way (the OP has yet to confirm he's tapping sync from the correct point).

[Tighe]

The Megadrive/Genesis is known for displaying sync issues when used with some RGB converters which are solved if the sync line is made low. I wouldn't exclude the problem here can be solved the same way (the OP has yet to confirm he's tapping sync from the correct point).

Yes I am using the sync line, as picutured here:

http://www.gamesx.com/avpinouts/genesisav.htm
The image is of the cable side.


So how do I make the sync line low?

[RGB32E]

Ahhh!!!!   Just in time... while connecting my Genesis 2 to my FC-14 (RGB->Component converter) I noticed some issues that turned out to be sync related.  Check out the gamesx wiki and the pieces of the puzzle (sync separator + hex buffer == nice genesis sync)!

http://gamesx.com/wiki/doku.php?id=av:lm1881n

Read and enjoy!!!    

[Tighe]

Ahhh!!!!   Just in time... while connecting my Genesis 2 to my FC-14 (RGB->Component converter) I noticed some issues that turned out to be sync related.  Check out the gamesx wiki and the pieces of the puzzle (sync separator + hex buffer == nice genesis sync)!

http://gamesx.com/wiki/doku.php?id=av:lm1881n

Read and enjoy!!!   

But my Genesis is a Genesis 1 with a separate sync...  Am I wrong?

[viletim]

Tighe,

This is a problem with all Megadrive and Genesis consoles. The C-Sync line is unbuffered and isn't really suitable for driving the sync input on an external monitor.  The signal has enough "fan out" to drive the input of its own video encoder and that's about it.

The fix is to use a CMOS (4000 or 74hc series) logic chip to buffer the signal. A CD4050 should do the job, connect a couple of gates in parallel for extra drive and ground the inputs to all the rest.

Alternatively, the LM1881 trick works just as well here as anywhere else.

[MKL]

Ahhh!!!!   Just in time... while connecting my Genesis 2 to my FC-14 (RGB->Component converter) I noticed some issues that turned out to be sync related.  Check out the gamesx wiki and the pieces of the puzzle (sync separator + hex buffer == nice genesis sync)!

http://gamesx.com/wiki/doku.php?id=av:lm1881n

Read and enjoy!!!    

That's an inverting buffer and does what I said needs to be done: inverting the logic state of sync from high to low.

[RGB32E]

That's an inverting buffer and does what I said needs to be done: inverting the logic state of sync from high to low.

Well... both the LM1881 and CD4049 are available in PDIP packaging, so order up and get to work! 

[MKL]

That's an inverting buffer and does what I said needs to be done: inverting the logic state of sync from high to low.

Well... both the LM1881 and CD4049 are available in PDIP packaging, so order up and get to work! 

Not sure why the LM1881 would be needed. The Genesis already outputs sync, you don't have to strip it from composite video. And if he wanted to do it really cheap he could just open the console, remove the resistor I pointed out above from 5V and ground it.

[RGB32E]

Not sure why the LM1881 would be needed. The Genesis already outputs sync, you don't have to strip it from composite video. And if he wanted to do it really cheap he could just open the console, remove the resistor I pointed out above from 5V and ground it.

Ahh... found your post that sums up the genesis 1 sync... Since the Genesis 1 uses a CXA1145P, it actually has CSync output on pin 11.
http://nfggames.com/forum2/index.php?topic=3387.msg25055#msg25055


However, things are different with the Genesis 2, they use encoders that do not have sync output pins.  I guess I'll start another thread in the near future, as I was having a different issue... and feeding a LM1881 composite video or composite sync gives different results....

[Tighe]

Not sure why the LM1881 would be needed. The Genesis already outputs sync, you don't have to strip it from composite video. And if he wanted to do it really cheap he could just open the console, remove the resistor I pointed out above from 5V and ground it.

OK i read your previous thread about this, but I did not see where anyone tried this and said it worked.  I prefer you method since it does not require having to buy any chips.  I don't get enough information from your image to do this mod.  Can you give me a little more details please?  Thanks!

[MKL]

Not sure why the LM1881 would be needed. The Genesis already outputs sync, you don't have to strip it from composite video. And if he wanted to do it really cheap he could just open the console, remove the resistor I pointed out above from 5V and ground it.

OK i read your previous thread about this, but I did not see where anyone tried this and said it worked.  I prefer you method since it does not require having to buy any chips.  I don't get enough information from your image to do this mod.  Can you give me a little more details please?  Thanks!

Ignore that thread. While it's true that the Sony encoder RGB and sync outputs lack caps and resistors (actually the encoder doesn't even have a sync output!), the sync issue is due to the high level of the sync line. So you have to change the level from high to low somehow. As I suggested there's more than one way to do that: if you don't want to use a chip, you can do that little resistor mod on the Genesis PCB. If you can't find the resistor on your system (the board layout will be different from my pic as that's a Megadrive), post pics of the PCB (good enough so traces can be followed) and I'll show you.

[Tighe]

...if you don't want to use a chip, you can do that little resistor mod on the Genesis PCB. If you can't find the resistor on your system (the board layout will be different from my pic as that's a Megadrive), post pics of the PCB (good enough so traces can be followed) and I'll show you.

Thanks MKL!

I will post some tonight!  Off to work!