Electronics > Repair

HP 54540C LCD panel replace

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Halberd:
My HP 54540C has been experiencing some problems with the LCD display, so I have attempted to replace the panel.
The genuine Toshiba LTM08C015KA component was quite old and difficult to obtain, and there was no detailed data available, so I replaced it with one from another vendor after analyzing the behavior of the actual device.
The results are approximately satisfactory, although we still have some problems. However, one question remains, and I would like to resolve it.

As far as actual use is concerned, HP54540C uses only 8 total display colors (include black). Is there a mode for displaying gradations?
If not, the connection will be simpler.

If you have any information, please let me know. Thanks.

m k:
No info, just guesses.

Since combined two colors of RGB will give you CMY it's quite possible there are only on/off style software levels for individual colors.
But blue is quite dark so some trimmings are also quite possible.

000 black
001 blue
010 green
011 cyan
100 red
101 magenta
110 yellow
111 white

So even if levels are fluctuating all colors can be easily differentiated, even if not as brightly as meant to be.

Palette colors are also a possibility.
If so then there can be a setting for every individual color, excluding white I guess.
Is there any hidden menus?

Easiest for manufacturer would be direct software solution, where bitmask from above is commanding the actual pixel color selector subroutine.
Then the following and possibly changing hardware is only affecting that selector subroutine.

Since the panel is digitally controlled anyway there's hardly any analog hardware for colors, if not panel's supply voltage.

Halberd:
Thanks for your valuable input.
I had forgotten about the existence of the palette technique.

The 54540C has a 3-bit color depth (TOTAL 512color) for each RGB color, if we only look at the wires to the LCD. However, as long as it is observed in test mode, etc., it always uses R0=R1=R2, G0=G1=2, and B0=B1=B2, in other words, only 8 colors are used.
The original LCD panel LTM08C015KA is 3-bit RGB each, while the NL6448BC26 used for this replacement is 6-bit RGB each, so it is necessary to convert the color depth if necessary.

The hardware for color depth conversion is already ready, but I did not want to add extra circuitry for a situation where it will never be used.

If there is a mode such as eye pattern measurement that overlays waveforms like later generation oscilloscopes, there is a possibility of using more than 7 colors at the same time.
If I can get information from someone that the 54540C uses something other than the basic 8 colors, I will incorporate a color depth conversion circuit, otherwise I will just use the current simple connection.

Thank you.

james_s:
Is this one of the old digital RGB parallel interfaces? If so, I've driven panels like that from FPGA boards lots of times and all I do is tie the unused LSB color lines low, it doesn't seem to make much difference with the displays I've used.

Halberd:
As you say, I can get nearly problem-free images that way. The maximum brightness will be 56/63, but it is not a practical problem at this time.

Furthermore, if the effective output is 1 bit, you can get a maximum of 63 (100%) or 0 (0%) and a perfect display by just putting the same signal in all 6 bits with a driver.

As far as I have observed on the actual device, the effective output is 1 bit. However, after completing the modifications and fully assembling the 54540C, I found a mode that uses many bits and did not want to open the lid again, so I sought the information first.

The screenshot I mentioned above uses that simplified method. Also, the full method I have already prepared uses an FPGA (I use a CPLD) to do the table conversion. I plan to add a few more components and an EXT VGA output.


As a side note, there was a major mishap in this renovation.

The same 8.4inch catalog spec LCD was used, but the NEC one had a slightly larger display area than the original Toshiba one. I am currently shaving the frame orz

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