IIci onboard video + NEC LCD monitor

[Jamieson]

I'm using the onboard video on my IIci and it is connected to an NEC LCD19v monitor. I don't think the LCD monitor is "multisync". The adapter I'm using is one of those cheap things with 10 dip swiches. I've found a few dip switch settings that sort of make it work and the monitor reports a vertical refresh rate of 66Hz and 640x480. Picture is stable but washed out: pixels which should be black are medium-gray. Cranking down the brightness settings on the LCD don't help; it's something in the video signal itself. When I press the "auto adjust" button the LCD, there is a brief moment when the colors look good, but then it returns to the white washed out look. Tried changing the "gamma" setting in the monitor control panel and that improved things by a small amount. Still washed out.

Just a guess, but I think the IIci video is adding sync pulses to all three colors and it's confusing the monitor's auto adjust circuit and is not properly establishing the black levels. Any suggestions?

I'm going to try building the circuit that is shown in THIS VIDEO where the guy has the IIci internal video seeming working with an external monitor.

 

[Jamieson]

The IIci is definitely adding sync information to the R G B video signals and shifting the black/blank level up from 0V. This video setup is just strange.

The IIci video chip (UE9) looks at the state of the ID1,ID2,ID3 pins at power up. Depending on the state of those pins the video chip chooses the timing parameters, whether or not to drive the HSYNC and VSYNC outputs, etc.

"Portrait 640x870" in this mode the HSYNC and VSYNC is generated, and only the BLUE video is active. There is no sync pulse on the blue video signal in this mode. In this mode on my monitor the black levels look pretty good. Not bad, but some green and red would be nice.

In every other mode I've tested the video chip does not drive the HSYNC and VSYNC signals. There is the composite sync available on pin 3 and thankfully my LCD monitor can accept that and lock onto the video signal.

The problem is still that sync pulse is added to the video signal causing the signal to be shifted up significantly. My LCD monitor can't handle that shift. It's expecting the black level to be near 0V, and so I get that white washed out look.

The RAMDAC used on the IIci is the Bt478 (UA7). The datasheet shows what is going on. There is an input pin (5) on the RAMDAC called SYNC. When this pin is active the RAMDAC shifts the signal up and adds the sync pulse as shown. If the RAMDAC SYNC input is tied to ground, then the sync information is NOT added to the R G B video signals.

I think if UA7 pin 5 could be lifted and tied to ground, then the RAMDAC would then not add SYNC pulses to the video, and the video signal would be brought back down closer to "normal" of 0V during the blanking period. There would still be a small offset with the black level at 54mV, but that's better than the current situation with blank level shifted up to 0.286V and the black level all the way up at 0.340V.

Unfortunately UA7 RAMDAC is a PLCC package, so there's no quick way to lift a pin. A trace would need to be cut to try this out.

 

[kkritsilas]

From the schematic, you could always lift R48, and then use the R48 pad that goes to pin 5 can be connected to ground through a pull down resistor (suggest 47 to 75 ohms).

 

[Jamieson]

But UE9 pin 128 is an output, which would fight against that pulldown resistor. Plus the "comp sync" output on the video connector is useful for monitors (like mine).

Does anyone know if Apple ever released technical documentation on the IIci video system, specifically that "VGT8002-6216" ASIC? There seems to be a lot of confusion about the video modes and how they relate to those 3 ID pins on the connector, and I think a lot of that comes from bunch of different systems (LCs, other Mac II, etc.) all getting grouped together.

edit: found it Mac IIci Developer Notes

 

[Jamieson]

From the developer notes:

So it looks like there are two and only two supported video modes on the IIci internal video:

ID3, ID2, ID1 = 0 0 1, 640x870, HSYNC and VSYNC active, only BLUE video active and there is no sync pulse on blue. 68.9kHz and 75Hz. Intended for the monochrome portrait monitor.

ID3, ID2, ID1 = 1 1 0, 640x480, HSYNC and VSYNC disabled, Composite Sync enabled, R G B video active, 35.0kHz, 66.7 Hz. Sync pulse added to all video signals and all video signals shifted up significantly (blank level 0.286V, black level 0.340V). Intended for the 12" monochrome and 13" color monitors.

Modes 010 and 101 are "reserved". I tried these out but did not find anything useful looking at the HSYNC, VSYNC, and "comp sync" pins with the oscilloscope.

 

[Jamieson]

*learns how to access the super secret service menu in the LCD monitor*
*manually adjusts the VGA offset and gain controls until it looks good*

Bah, nevermind.