Modules design

I have always wanted to use small modules. As much as possible and reasonable in terms of placing the components of a complete functional assembly on them. Ideally 50x50mm or even smaller.

The electrical connections between the modules must be reliable, the modules must be securely fastened together so that the structure does not fall apart from another careless movement at the table and the contacts should not fail.
As a result, I came to this format of the printed circuit board of the module.

Expanding the assembly vertically is, of course, good. But expansion in the horizontal plane is still necessary, otherwise, with a large number of modules, we will get a narrow and unstable rack of modules. Horizontal expansion is also possible using hub boards. Those. in one of the tiers, a larger hub board is placed, which is a multiple of the module dimensions (for example, 2x1, 1x2, 2x2, 3x2, etc.). It has all the same inter-module connectors at the top and bottom and mounting holes as conventional modules. Such hubs allow you to parallelize modules from several racks and connect them mechanically.

The obvious disadvantages of this solution are: the hub board consumes 1 tier in the assembly, and the mechanical connection between the racks is much lower than in one rack without hubs.

Features:

• For inter-module connection, 2 connectors 2x10 with a pitch of 2.54 mm are used. Surface mount connectors. Mothers above, fathers below.
• Inter-module connectors determine the distance between the boards of two adjacent tiers - 12 mm. Brass stands of this height are common and inexpensive. Stands are optional, though, because the inter-module connectors hold the modules together quite well.
• Module board size - 40x40 mm. The corners of the board are rounded with a radius of 6 mm, which allows you to make a similar rounding in
• Connectors for external connections, as well as means of control and indication can be output from the front or rear (the sides are covered with intermodule connectors).
• The distance between the pins of the two inter-module connectors is a multiple of 2.54 mm pitch, so the bottom module can be inserted into an arbitrary size breadboard and soldered there. Or solder 2-row connectors on the breadboard and insert the bottom module into them. This allows you to neatly expand the assembly of modules using standard breadboards:

Modules placement in the case

Connectors for external connections are located with a 2 mm protrusion beyond the board. This is necessary so that the connectors of the device are not recessed into the case and do not stick out of it. A case with a wall of 1.6 mm can be printed on a 3D printer, the remaining 0.4 mm is the gap between the board and the wall of the case.

Brass stands used for fastening the modules to each other can also be used to fasten the assembly of modules in the case. The M3 screw can be used to fasten the module rack to the chassis. Another option is to use M3 injection nuts:

then the first level module is applied, the first level stands are screwed in, the second level module is applied, etc. In a cut it looks like this:

Pinout of intermodule connectors

The task of choosing the pinout of a common bus in a modular design is very non-trivial. If you forget to distribute some important signals among the pins, then in the future you will either have to put up with it, or redesign some of the modules (or even everything!).

But let's take it step by step...

Inter-module connectors are located symmetrically and mechanically nothing prevents connecting the next module by turning it 180 degrees. This, in my opinion, is more bad than good. I've tried using this feature when designing modules, but it's confusing most of the time.

However, if connected incorrectly, nothing should definitely burn out. Therefore, the pins responsible for supplying power are located so that, in any orientation of the module, power is supplied correctly.

Also, as you can see from...