Hi forum, i have recently build a x68000 to vga adapter to use the sharp monitor with other vga machines/converters i have.
This is a monitor capable of doing tri-sync, 15khz / 24 / 31.... and for some reason when I plug anything vga the monitor stays at 15khz not switching to 31khz. Instead, when I use the x68k with the monitor it automatically switches to 31khz on boot time.
I wonder if anyone can give me a clue as to what may be happening.
ps: anybody knows what the Ys signal in the x68k video cable is for? http://www.gamesx.com/wiki/doku.php?id=av:japanese_rgb-15
Ys is a detect signal that tells the display there's incoming video. I'm not sure how it works off the top of my head, but I'm reasonably sure it's the reason my NEC CRT doesn't display anything with some devices.
EDIT: I found this page (http://www.hct.zaq.ne.jp/kyotani/onechipmsx/onechipmsx11.htm) which says:
Quote内部 / 外部RGB信号切り替え。 ハイレベルで入力した信号に切り替わる。
※インピーダンス75Ωなので要注意。 330Ω程度の抵抗で5V電源に接続する。
Which Google translates to:
QuoteInternal / external RGB signal switching. It switched to the signal input at a high level.
※ impedance 75Ω, so watch out. To connect to the 5V power supply with resistor of about 330Ω.
Small update, most definitely a problem with H sync as V sync seems to be detected. Also tried a different vga cable and redo the db15 connector, got the same result... the monitor wont switch to 31khz unless you connect it to the x68k.
Time to plug the oscilloscope...
Can you post here the diagram of your custom VGA adapter/cable?
I simply followed the pinout found here http://www.gamesx.com/wiki/doku.php?id=av:japanese_rgb-15
After plugin the scope I saw the sync output from the x68000 is inverted, both H and V, can anyone confirm? This is probably why the sharp monitor wont sync any of the vga devices I tried...
What I wonder then is why my LCD monitor takes the x68000 input through VGA without issues, perhaps modern monitors detect both inverted and non inverted sync inputs?
The moment I have a chance i will fit an inverter chip to my vga/x680k custom cable and see what happens.
Confirmed!! Placed an 74ls04 inverter on the vga H sync source and whoala, the monitor switched to 31khz.... I will build the final cable soon and test it properly... a pitty the cable will need a +5v input to feed the ttl inverter...
Good work.For powering the 74LS04 inverter you could add an external connector for a PSU on you cable like done here:
http://www.gamesx.com/wiki/doku.php?id=av:adding_5v_to_scart_lead