Replacing LCD flat ribbon cable with longer cable gives white screen
 
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RE: Replacing LCD flat ribbon cable with longer cable gives white screen

@rpbrandon

It is 100% chance - length has an effect only because the unterminated nature of the lines induced horrendous ringing; but be warned, if the data changes, so does the effects of changing lengths. You might have gotten just as good a result adding 30 cm to the cable.

The real risk is the SD card data --- and now I understand why the file folder data I see on my printer is often scrambled - only fixed by r&r or reset.

I think (hope) reading data is a 'little bit' more reliable than writing data (requesting SD info or writing to lcd), but it's more than iffy both directions. I honestly can't believe Prusa used the LCD panel as is. But these are the same guys who think linear bushings don't need grease.

Posted : 20/05/2021 6:11 pm
d0ug
 d0ug
(@d0ug)
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RE: Replacing LCD flat ribbon cable with longer cable gives white screen

There is going to be a huge difference between a character LCD and graphic LCD when extending the cable length. 

A character LCD is only going to be sending a few bytes occasionally at low bitrate to update the parts of the text that needs to be updated. Even if the entire text content is updated, on every update you are talking about 80 bytes plus maybe a few control characters.

A graphic LCD is going to be sending all that graphic data for the 320x240 or whatever that screen's resolution is at 15/30/some other frames per second, this is a lot of data. There is a reason that HDMI cables used twisted pairs and shielding and we don't use flat ribbon cables to connect a display device to the device outputting the video.

Looking at the schematic for the buddy board, it looks like it is using SPI for the LCD. This might give you some pointers for extending the cable length. Protocols like SPI really aren't designed to go board to board over cables between the boards, they are designed for inter board communication between multiple chips/sensors/etc.  https://www.reddit.com/r/AskElectronics/comments/aalbxh/spi_communication_over_long_distance/

Posted : 01/06/2021 11:20 am
d0ug
 d0ug
(@d0ug)
Trusted Member
RE: Replacing LCD flat ribbon cable with longer cable gives white screen

The fact that it works at all in the shipped configuration means they got lucky, or knew about the issue and determined what was the max length they could reliability send the SPI data and that then became a constraint to the placement of the LCD/Buddy boards and even the size of the printer.

Posted : 01/06/2021 11:31 am
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