Plasma TVs -what do the various boards do?

[SuperSkippy]

If you google How does a plasma TV work, it's not all that instructive. Is there a web page anywhere that explains what (in Panasonic-speak) the SC, SU, SD, SS, C boards actually do?

I feel like I understand how an LCD panel works, but I'm not so sure about plasmas.

[tom66]

Re: Plasma TVs -what do the various boards do?

SC = Scan and Sustain driver. Responsible for applying the Row Select voltage (-Vy) and the SetUp/SetDn waves. Also responsible for one side of the sustaining.

SU = Scan Multiplexer, top side. Responsible for selecting between the Row Select voltage and the SC board's output for each line on the panel. Each chip is a 30~128 bit shift register (typically 64 or 90 bits). Data is clocked in at the top or bottom, and shifted up the panel. A "1" may be clocked in initially, to select the first row, this is then shifted along the panel to select each successive row. Then "0" is clocked in to all channels, possibly using the RESET line, so they receive the common sustain signal.

SD = Same as SU but for bottom side.

SS = Common Sustain Driver. Responsible for applying the Ve voltage and also for applying the other side of the sustaining.

C = Responsible for feeding the signal from the control board to each of the panel data drivers, which load the actual image on the plasma panel.

[twbranch]

Re: Plasma TVs -what do the various boards do?

Good question and even better answer.

[SuperSkippy]

Re: Plasma TVs -what do the various boards do?

I started this thread yonks ago, and realised I never really did anything with it.

I kind of get your answer.

Basically what I know so far is that the panel is made up of rows and columns. To light a pixel, what happens is a high voltage is applied between the wires that make up a row of that pixel, and the wire that makes up the column. Then a spark goes between the two wires that excites the gas between them which phosphoresces, and what we see is the light that comes from this phosphorescence. I don't get what Row Select voltage (-Vy) and the SetUp/SetDn waves are, or what sustain is.

Any help?

[tom66]

Re: Plasma TVs -what do the various boards do?

What you describe is a DC plasma panel. Modern plasma panels are not DC because they have even worse efficiency than AC panels (500W+ for a 21" VGA display.) They are also a lot more expensive to make as you must use much higher voltages to drive the panel. So AC panels are used in basically every modern plasma TV.

A plasma panel is a memory device, in a way very similar to LCDs. The panel is scanned one row at a time. The appropriate 1-bit image is programmed into each line. Va and -Vy are used to do this. Vscn, the other voltage, typically only a few volts above ground, is used to make sure the lines are deselected when they are not to be programmed. So to program row 500 of 1000, we'd make that row -Vy and all the others Vscn. This is the functionality of the buffer boards and panel COF drivers.

The programming is at too low a voltage at this point to cause light emission. Instead, the programming creates a wall charge on the panel's pixels. Each pixel is kind of like a capacitor, storing an on or off state.

When you apply a sustain waveform, you cause the pixels with a wall charge to conduct. Those without a wall charge do not conduct. Current flowing through the gas, at the breakdown voltage, causes the gas to emit light (sustaining.) The current causes the wall charge of each pixel to flip, so you must reverse the sustain waveform on each cycle. (Vs and COM are switched on each cycle.) The amount of light varies with the sustain frequency, panel efficiency, type of gas, etc. This is how an AC panel gets its name, the pixels receive an effective AC wave, varying from +Vs to -Vs.

Since the display has inherent memory, you need to erase it. This is what the priming SetUp/SetDn waveform does. In addition, it liberates electrons from the plasma gas, allowing a much lower sustain voltage to be used, and increasing the chances of a successful discharge.