Hello,

First post on the forum and looking for some guidance on a LED matrix 
display problem.

I have a P3 32x64 RGB HUB75 display board (p3-6432-2121-16s), as 
pictured here: https://ibb.co/GF5zk2D
Note; I'm assuming that the IDC pin layout is correct as pictured. 
There is no silkscreen on the board that clearly identifies each input.

I also have a working P5 32x64 RGB HUB75 display driven from an Arduino 
Uno (5v logic) as pictured here: https://ibb.co/MGHC38L

I've connected the p3 board into the working p5 display and nothing 
lights on the p3 board.

1. The IDC is connected to the correct end of the RGB board (see 
arrows).
2. The p3 board has an 'extra' address line (E) where the ground pin is 
on the p5 board, so this is being grounded when connected to the Uno, 
but this shouldn't (?) cause a problem as the p3 board has a 1\16 scan 
rate and 4 address lines (ABCD) is sufficient.
3. If I leave the power connected to the p3 board and remove the IDC 
cable to the Uno, then run a finger across the IDC input on the p3, the 
board lights up at random - so the leds are working.
4. The Uno refreshes at about 250 screens\second.

Could be a defective board, but would like to check that I've understood 
the HUB75 interface before I return it.

It was my understanding that HUB75 boards with an identical scan level 
and pixel count should be interchangeable (?). If this is not the case, 
then what am I missing?

The p3 board I'm talking about has been discussed on here before in 2018

Beitrag "64x32 RGB LED Panel (p3-6432-2121-16s-d1.0)"

The last post talks about 'An appropriate made connection' (Google 
Translate) but doesn't state how the connection was made. Can't see how 
you'd use address line E either. On a 16 line scan system the E line 
would always be grounded unless you're attempting to address past 16.

Only thing I can think of is that address line E has to be pulled high. 
This would work if the 16 led rows have been mapped to rows 17-32 on the 
display.

Thanks,