Hyperfrog
Member
I have a couple of Hercules Graphics Cards of which I replaced the master clock crystal in order to generate a video signal that can be displayed using another type of monitor (SVGA or composite).
Those cards normally have a 16-MHz crystal. If you replace it with a 28.57-MHz one, you get a signal that is compatible with most multisync SVGA monitors, i.e. an horizontal refresh rate of about 33 KHz, and a vertical refresh rate of about 87 Hz. I used a 28.57-MHz crystal because I had one handy, but you could try a 29- or a 30-MHz one. So far, I got this mod to work with only one of the three cards I tested it on. Perhaps the other cards' oscillator circuits are not designed to work with the 28.57-MHz crystal I used. I'll have to use the scope to see what's going on. In addition, I could see some pixel corruption on the graphical display of the modified card, but only in certain areas of the screen; I suspect the video RAM chips are not fast enough for this clock rate. The text mode seems OK though. I wish I could get other cards to work and compare the results. Otherwise the display is crystal clear (pun intended) and very stable.
Another possible hack is to remove the 16-MHz crystal and route the OSC signal (pin B30) to the input of the oscillator circuit (that is, one of the pins normally connected to the 16-MHz crystal). This produces a video signal that some composite monitors can handle (HSYNC: 16.5 KHz, VSYNC: 46 Hz), though none of which I own can display a complete 720x348 screen. I tweaked one monitor so that it displays a picture that is almost perfect, but I miss a few columns of pixels on the left, and a few lines on the bottom. This time, there was no pixel corruption, but I didn't use the same card as for the SVGA hack. The nice thing about this mod is that you can display a full-screen picture (no border) and even enjoy artifact colours using an Hercules Graphics Card! However, the 46 Hz vertical refresh rate is quite annoying unless you use a monitor with a long-persistent phosphor, that is... a green monochrome monitor.
Cheers,
Christian
Those cards normally have a 16-MHz crystal. If you replace it with a 28.57-MHz one, you get a signal that is compatible with most multisync SVGA monitors, i.e. an horizontal refresh rate of about 33 KHz, and a vertical refresh rate of about 87 Hz. I used a 28.57-MHz crystal because I had one handy, but you could try a 29- or a 30-MHz one. So far, I got this mod to work with only one of the three cards I tested it on. Perhaps the other cards' oscillator circuits are not designed to work with the 28.57-MHz crystal I used. I'll have to use the scope to see what's going on. In addition, I could see some pixel corruption on the graphical display of the modified card, but only in certain areas of the screen; I suspect the video RAM chips are not fast enough for this clock rate. The text mode seems OK though. I wish I could get other cards to work and compare the results. Otherwise the display is crystal clear (pun intended) and very stable.
Another possible hack is to remove the 16-MHz crystal and route the OSC signal (pin B30) to the input of the oscillator circuit (that is, one of the pins normally connected to the 16-MHz crystal). This produces a video signal that some composite monitors can handle (HSYNC: 16.5 KHz, VSYNC: 46 Hz), though none of which I own can display a complete 720x348 screen. I tweaked one monitor so that it displays a picture that is almost perfect, but I miss a few columns of pixels on the left, and a few lines on the bottom. This time, there was no pixel corruption, but I didn't use the same card as for the SVGA hack. The nice thing about this mod is that you can display a full-screen picture (no border) and even enjoy artifact colours using an Hercules Graphics Card! However, the 46 Hz vertical refresh rate is quite annoying unless you use a monitor with a long-persistent phosphor, that is... a green monochrome monitor.
Cheers,
Christian
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