FMC - Predefined Matrix

Question

I've spent three nights to find the problem I have with my Zibo's First officer FMC. Now I hope to get help from the community.

My captains FMC, which ist configured as a matrix works fine. I have mounted a connector to the captains FMC to plug in the F/O FMC. But as soon as I connect this second FMC all the switches in the captains FMS slip to the left by 1 collumn.
Does anybody have an idea where I can continue my search for the fault?

I use the predefined matrix for the 737 FMC as described. I absolutely don't understand why the rows shift when conecting a cable to the CPT FMC - altough this cable has an open end. Can there be any interferences which disturb?

regards,

Johann

Answer 1

Johann,
In you case it can be the common ground wiring issue or too long cables (but why?). How do you power your matrix and how reliable is the common GND?  Have you placed the matrix board (Nano) near to the matrix keys?

All inputs in the firmware code are set as inputs with internal pull-up resistors, which resistance sometimes can be  not low enough  to avoid the signal/power interference on longer wires...   I didn't see any problems with quite long wires though...

But you can try to put additional pull-up 2-5K resistors to each input line #2-12 (closer to the key matrix, not to the board).

Vlad

Comments

Hello Vlad,

Your suggestion to add resistors to the matrix board was a bullseye! After adding 11x 4,7K resistors from D2 - D12 to VCC my problem with the shifting rows was gone! Seems this is the solution if anybody has the same problem.

Good night - and thank you for your work!

Johann