If you want to use the four-in-a-row LED matrix modules, you may get the problem that the individual modules have a little too much distance to the next module.
Of course, this is not desirable for our application as a scrolling led bar. These modules are said to have existed in the past without this spacing.
In my case, the individual modules had a distance of 1-2mm. Therefore, I quickly separated them with the saw and then put them together with glue.
This is a bit of work, but the result is also very satisfactory. Alternatively, you can also buy the modules individually. In this case, this step is not necessary.
Here are the individual work steps:












I always soldered 4 individual modules, and repeated this for all modules.
At the end I connected the 4-pcs modules to each other, so that I finally connected all 21 modules in series.
It is advisable to check the connections with a continuity checker.
The connections from VCC, GND, CS, and CLK should be connected throughout, i.e. from the first module to the last.
Of course, there should be no connection between the individual lines (especially between GND and VCC).
DOUT is always connected to the DIN of the next module, i.e. like a chain, whereby there should be no connection between Dout and Din of the same module.
If you do everything right when soldering, then there should be no smoking 'surprise' later when connecting to the power supply :-)
If you are not sure, you should better check the contacts with the continuity checker once more than once!