The module is a 42x42 mm square PCB (printed circuit board). The corners are rounded and mounting holes are located at the corners. M3 screws and brass stands are used to mount the module through the mounting holes.
There are two 20-pin connectors. This connectors have sockets on top and pins on bottom. This allows the modules to be connected to each other directly into a vertical rack. This is a way to vertically scale the device.
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There is a way to scale the device horizontally. This can be done using backplanes, I call them HUBs. They can be of different sizes and allow you to connect several stacks together electrically and mechanically. For example: HUB 1x2, HUB 2x2.
An important feature is that all contacts of all connectors are located in the same grid with a pitch of 2.54 mm. This is very important and necessary so that several stacks with modules can be inserted into one large breadboard with a standard pitch of 2.54 mm.
There is another way to electrically connect several stacks. This can be done using cables with IDC connectors. While this method is not very convenient due to the intertwined loops, it is still possible.
All modules receive power through the intermodule connectors. 2 supply voltages are used - 3.3 V (usually) and 5V (sometimes). All power pins are located symmetrically relative to the center of the board, which is necessary to allow the modules to be rotated 180 degrees.
The intermodule connectors also have 2 UARTs, 2 SPIs, 2 I2Cs. They are also located symmetrically relative to the center of the board.
All other pins can be used as general-purpose input/output (GPIO) or as service signals for some modules (Chip select, Data/Command or Transmitter Enable signals).
To distribute the modules across the intermodule buses, configuration soldered jumpers are used on the bottom side of the modules.