Not the person you're asking but I use StarURL, it's not perfect but it's good enough. I bought it when I was doing a lot of courses involving it, but I think you can use it for free if you're ok with it nagging you when you open it.
Yes, Since this is "ADHD_Programmers", I'm assuming you are a programmer, therefore there is this thing called UML ( Unified Modeling Language) that defines a collection of diagram types and its interfaces, that can be used to describe the behavior of the software ... And example of this are state machine diagrams, class diagrams, use case diagrams. Google it there is a lot of information about this online.
Regarding the tool that you could use if you don't want to spend money ... give this one a try, no need to buy license if it is not for commercial use.
SysML is kind of the same as UML, most of the diagrams used on UML are used on SysML, but SysML is to describe systems parts and its interactions (Mechanical/Software/Electrical)
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I hope this helps !!
Try StarUML http://staruml.io/ It's a free program like Visio in a way but geared towards software architecture. You'll have to learn UML to make any sense of it, but it helps keep our projects in order.
I used StarUML in college last semester and it is pretty solid ^(^and ^free).
From their site:
>Over 3,000,000 Downloads StarUML is one of the most popular UML tool in the world. It has been downloaded over than 3 millions and used in more than a hundred of countries.
Maybe you should give it a try. It is available for Windows and Mac OS X.