Proper SYNC Output?

[Mask of Destiny]

I'm trying to get my Genesis to work with an Amiga (15KHz) monitor I have, but I'm having trouble getting a decent sync signal. Using the CSYNC pin on the A/V port gets me no picture, same thing using the /CSYNC in and out pins on the CXA1145. The only way I can get any picture at all is to use the composite video pin on the A/V connector as composite sync, but I can't get a stable sync on that.

As an aside, whats the obstacle to getting S-Video on a Genesis 1/2? Looking at the Genesis 1 schematics, the CXA1145 would appear to have Y and C outputs just like the CXA1645.

 

[lordofduct]

I just did a search and found this... for an Svid mod on a Genny model 1

As for the sync issue... That is weird, I thought Amiga's use Composite Sync for the signal! Their is no other sync signal that will come from the genny for you to use. Are you grabbing the right pin? (its the one just right of the dimple, clockwise of the dimple). And are you making sure to ground the signals? Because it really should work... dang it should work with an Amiga 9pin din plug cable from the store, if I recall correctly it has the same pinout.

 

[Mask of Destiny]

Well I have a Model 2 Genesis and an A4000 so neither have the standard 9pin DIN. Monitor definately uses Composite sync as the video cable only has a few conductors in it and one of them is composite sync.

I'm pretty darn sure it's the sync pin unless the pinout on gamesx.com is wrong. Ground is attached.

That S-Video mod looks a bit more involved than for the Genesis 3. I guess the Y and C output on the chip needs a little bit of conditioning before it's ready for display.

Funny that he says it can't be used on Genesis 2 models as mine has a CXA1145 (though it's the surface mount version of course).

EDIT:

I think I figured it out. The monitor wants /CSYNC (i.e. active low sync pulses), but the Genesis video port gives CSYNC (i.e. active high sync pulses). Both the VDP and the CXA1145 provide a /CSYNC signal; however, it looks like the signal needs some further manipulation before it's suitable for the monitor. If I short the /CSYNC IN to the AUDIO OUT pin and connect that to the monitor then I get a perfectly stable picture. For obvious reasons, that's an undesirable solution. Any ideas what circuitry I should put between /CSYNC and the monitor to get this to work?

 

[lordofduct]

Try taking the sync before the chip and don't short it...

what monitor do you have?

 

[Mask of Destiny]

Originally posted by lordofduct@Mon, 2005-08-01 @ 06:11 AM

Try taking the sync before the chip and don't short it...

what monitor do you have?

[post=137692]Quoted post[/post]​

The monitor is a Commodore 1070 (I think). /CSYNC IN is effectively the sync signal before the chip and as I said before, connecting to that alone gives me no picture.

I'm guessing there's some sort of DC bias that's screwing things up, but I don't know how to fix it.

 

[lordofduct]

OK I have an idea... try it if you like. I believe what is happening is when you ground the /csync on the audio out your just getting power for it. SO do this:

You'll need a 75ohm resistors 1% impedence...

short pin 10 (/csync) on pin 12 (vcc1) the line you bring out put the resistor on then feed this to the monitor /csync in.

I think it should work.

 

[Mask of Destiny]

Shorting VCC to a line connected to the VDP seems like a bad idea to me. Or do you mean something more like this:

pin 10 - resistor - pin 12

|

|

Monitor /CSYNC

Connecting to pin 11 would probably be a better idea in general (since it would isolate the VDP). I'll get some resistors and capacitors after work tonight and screw around some.

 

[cgfm2]

Originally posted by Mask of Destiny@Mon, 2005-08-01 @ 11:16 AM

Shorting VCC to a line connected to the VDP seems like a bad idea to me. Or do you mean something more like this:

pin 10 - resistor - pin 12

|

|

Monitor /CSYNC

Connecting to pin 11 would probably be a better idea in general (since it would isolate the VDP). I'll get some resistors and capacitors after work tonight and screw around some.

[post=137706]Quoted post[/post]​

I had unstable sync on my monitor when using the c-sync output too. As it turns out the video cable was too long. I fixed this by wiring two gates of a 74HC14 in series which gave a stable image (maybe a single gate of a 74LS244 buffer would work), though it took a second or two for the monitor picture to stabilize after power-on which I found too annoying to continue using.

Using a shorter cable between the two might help, or you'd want to boost the signal like I did. I'm guessing the Sony video conversion chip has limited drive capability on the c-sync output. Factor in that c-sync is also fed to the RF modulator so additionally driving a monitor may be too much of a strain, hence the seemingly weak output.

You can use the LM1881 sync splitter (www.mouser.com sells them) to extract the c-sync portion out of the composite video output. I have a few of these but have not tried them with the Genesis yet. At www.gamesx.com there are some articles about using the LM1881 if that helps.

If you find a good solution, it would be nice to hear about it.