I sometimes need to make diagrams(not circuit details) to explain the system(how the devices are interconnected as a big picture):
Here a primitive one in MSPaint:
Is there a better adhoc program for this purpose?
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A free and pretty nice vector drawing tool is GravitDesigner. It can handle all sorts of graphics. You can try it on-line or download it for almost any device including Windows, MacOS, Android, ChromeOS, and Linux.