I am currently testing Max7219 7-Segment displays from Amazon. They are obviously fake-clones from what they look like.

I tried hooking three of them up to a panel for testing, using the direct-connection method on pins #30, #31 and #32. Apparently the wire lengths are too long (about 1m to 1.5m) because the first display only shows values sometimes, and goes blank after a while. The other displays either show "88888888", flicker or go completely dark.

I tested it with shorter cables, still didn't work. I had to go down to about 10cm of cable length to get it working...

My question is, what would be the longest possible wire length for genuine Max7219 displays? Or what am I doing wrong?
Jul 25 in General Questions by

1 Answer

I don't know what the "genuine" Max module is, there is no such.
All they are just boards with chip and couple of resistors.

The length can be any: 



Check you power and make sure you have correct wiring.

Vlad

Jul 25 by
Thanks for the picture, that's very helpful to know those lengths are possible.

I have read in other Arduino-Forums that a lot of people have had problems with "cheap China" MAX7219 units, exactly like the ones I have. My wiring should be correct, as the same units start working once I cut the exact same wire down... however I am using non-shielded 0.14mm (26AWG I blieve) wires for testing.

Why are you using 8 signal wires? We have used +5V, GND, D (#28), L (#27) and the CLK which goes straight to Pins #30.
I read in the forums that people have had problems with the termination of the signal and ground wires... do I have to run those lines back to the board to create a "closed circuit"? When daisy-chaining the D and L lines, can I use the outputs on the other side of the PCB? They are labled DOUT, and LODA. I have read all your documentation and it makes perfect sense to me, however we still run into these problems. Hope you can tell me what's the best practice to wire those up. Will be using Multiplexers later on, for now I am using direct connection to get a "Quick and dirty" MCP running for testing

Thanks Vlad, you are doing an awesome job here :)

I always use only "cheap China" electronics, why pay more for the chips made on the same factory?
Only if the PCB itself has bad soldering, contact or passive components (not chip).

I have worked with electronics for 40 years and I will say that in most cases when the circuit does not work it's because either there is no contact where it should be, or a contact where it should not be.

Cable length and wire thickness are irrelevant in such simple and low frequency circuits. I always use thin wires.
In any cockpit, the wire is barely (and rarely) 1.5 meters long.  What requires more attention is the power supply and common ground.

8 lines  - 4 bus for Mux address, D,L,S, Pin.

I suppose you have read this:

https://simvim.com/tips_power.html

https://simvim.com/tips_wiring.html

Some pictures :

  

Wiring option:

         


NOTE: the "golden tubes" on my diagrams are NOT the grounded shields, just some "binding" tube to make the whole wiring  cleaner.

Vlad

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