Why can't I drive this LCD panel?

[KugelBlitz]

I am trying to drive an LP101WX2 panel with the Chalkboard Electronics HDMI-dualLDVDS converter but all I am getting is a black screen.

I tried to flash the board's firmware with both the demo and the FullHD+ firmwares and it makes no difference. Also, I wrote the panel's EDID (from the datasheet) to the board but no dice. I also tried with the original EDID the converter came with. Again, no difference.

This is how I initially wired up the converter to the panel:

Converter	Converter	LCD Panel	LCD Panel
Desc	Pin No.	Pin No.	Desc
+3V3	1	2	VCC
+3V3	2	3	VCC
GND	3	10	GND
GND	4	16	GND
O_RX0-	17	8	ORX0-
O_RX0+	18	9	ORX0+
O_RX1-	19	11	ORX1-
O_RX1+	20	12	ORX1+
O_RX2-	21	14	ORX2-
O_RX2+	22	15	ORX2+
O_RX3-	23	20	ORX3-
O_RX3+	24	21	ORX3+
O_CLK-	25	17	ORXC-
O_CLK+	26	18	ORXC+

For now, I am controlling the backlight separately (VLED, GND, LED_ENABLE and LED_PWM) to ensure the converter isn't doing anything funky.

Since the panel is using just 4 twisted wire pairs I did some digging around and found out that the converter is using the DS90C387A to drive the LVDS panels. I thought that maybe it's not working because the DS90C387A is set up to work in dual mode. According to the datasheet, pin 23, named DUAL, is responsible for configuring the mode:

Three-mode select for dual pixel, single pixel, or single pixel input to dual pixel output operation. Single pixel mode when input is low (only LVDS channels A0 thru A3 and CLK1 are active) for power savings. Dual mode is active when input is high. Single in - dual out when input is at 1/2 Vcc. (Note 10)
[...]
To configure for single pixel or dual pixel application using the DS90C387A/DS90CF388A, the “DUAL” pin must be set to Vcc (dual) or Gnd (single)

According to note 10, the DUAL pin has an internal pull down so all I had to do was ensure there was no pull-up resistor connected to the pin. According to the converter's schematics, the DUAL pin is connected to VCC through the R17 resistor. After unsoldering it, the screen still did not show anything. No noise, no nothing. Just black.

After taking a closer look at the DS90C387A datasheet I noticed that the pinout shows that pins 50 through 38 are for the odd rows and pins 37 to 26 are for the even rows, excluding some VCC and GND pins. It is also mentioned that the even row pins are disabled in single mode.

According to both the converter's schematic and its support page, the even pins are connected to pins 5 to 16 of the converter's connector:


However, looking at the traces of the actual board, the connections seems to be the other way around. The odd pins are connected to the connectors 5-16 pins:


I don't know if I misunderstood something or if they truly mislabeled the pins but I went ahead and swapped around the pins in the connector housing like so:

Converter	Converter	LCD Panel	LCD Panel
Desc	Pin No.	Pin No.	Desc
+3V3	1	2	VCC
+3V3	2	3	VCC
GND	3	10	GND
GND	4	16	GND
E_RX0-	5	8	ORX0-
E_RX0+	6	9	ORX0+
E_RX1-	7	11	ORX1-
E_RX1+	8	12	ORX1+
E_RX2-	9	14	ORX2-
E_RX2+	10	15	ORX2+
E_RX3-	13	20	ORX3-
E_RX3+	14	21	ORX3+
E_CLK-	15	17	ORXC-
E_CLK+	16	18	ORXC+

Still, the image remained black.

I'm also not sure why the converter pins are labeled as RX. This implies that whatever connects to it should have a matching TX pin and that the converter is the receiving end. The LCD panel datasheet also marked the pins as RX. Are they both receiving?

Is there something I missed? Is the converter incompatible with the LCD panel? Should I not at least get some random incoherent image instead of just black?

And if anyone is wondering why I am posting the question here instead of writing the support team an email, it's because I did drop them an email on the 24th of January. I am yet to receive a reply.

[amyk]

Do you have a scope or some other way to check if there is some semblance of signal? Have you checked the input to the converter board?

[KugelBlitz]

Unfortunately, no. I don't have access to a scope. It is on my wish list, though.

As for the input, what exactly should I check? The devices I plug into the converter all have working HDMI outputs, as in the HDMI works with other screens, and when I connect the board I do get the correct resolution in Windows, meaning that the EDID is retrieved correctly.

Also, the converter has a status LED which lights up when it supposedly detects a video signal. In practice, the LED is turned on when I connect the converter to the HDMI of any device and turns off when I disconnect it.

[Berni]

Yep a scope would be useful for seeing what the LVDS output looks like.

A screen doing nothing is typically the sign of there being no clock or no sync signal. It waits for vsync before clocking in pixels, so as soon as you have those two present you tend to get at least some garbage on the screen.

There is also more than one LVDS format that arrange the bits around differently, but the sync bits should be in the same place, so should still display garbage. For Dual Link LVDS it seams the sync bits are present in both even and odd lines, so it again should show something.

Another possibility is that the clock speed is too far from what the display wants.

[KugelBlitz]

Is this something I could check quick and dirty with a multimeter?

If I get just a solid 0v on those lines I can assume I don't have the signals, right?

Regarding the clock speed, isn't this something I can lookup in the datasheets I mentioned? If so, what should I check? I see a lot of clock skew, clock frequency, clock rates. Not sure which is which.

[Berni]

The levels are pretty small, but should be able to tell apart from a output that is not moving.

As for the clock speed you can convince windows to output custom display timings on HDMI. The Nvidia drivers have a setting tab somewhere that lets you set any resolution, blanking, refreshrates etc.. so you can use this to force a higher or lower pixel clock speed.

Yes the datasheets should have all the timing info about how fast you can run the display.

[KugelBlitz]

I set up a series of custom resolutions with different settings, including following the parameters from the datasheet to the letter, but still no output of any kind. As for measuring the pins, I got the same voltage of about 1.6V regardless of whether the HDMI was plugged in or not.