LCD Interface (PC Engine LT)
The LCD Interface deals with the digital pixel signals that go to the screen to be interpreted and displayed on screen.
Just like the PC Engine and PC Engine GT, the HU6260 is responsible for generating the RGB signals and sending them to the screen connector.
The LCD Interface ribbon is the one at the far back.
Flipped over on the back we can see the pinouts. From the signals I currently know I have labelled.
The pin 1 (245.9Mhz pulse wave) comes only when the LCD Ribbon is connected, so perhaps it is feedback to the backlight circuit, as it also appears exactly the same on the middle pin of the backlight connector even though the two pins are not the same.
To repeat that, when the LCD Ribbon is connected, this signal appears on both the LCD Ribbon pin 1, and backlight ribbon middle pin.
The HSYNC comes only when the LCD Ribbon is connected, so it is sent from the screen.
The HSYNC and VSYNC signals only appear when the LCD Ribbon is connected, otherwise they show a low 1V CSYNC noise.
The RGB signals are what you would expect, 1Vpp signals.
The CSYNC is 5Vpp signal.
NOTES
The RGB signals don't come straight from the HU6260 but seem to pass through some circuitry over by the MCU to the right, including passing through a 2.2k resistor divider setup, then to the LCD connector.
I have not reverse engineered this path yet, nor confirmed which order the RGB signals are in.
The contrast works by being supplied with the -23V, and then the adjustable 4.5V to 4.8V (which comes from the BRIGHT wheel, which controls the contrast.
