What is the best System Design and or Block Diagram software and/or Assembly and/or Sub-assembly inter-connection drawings?
May show something like...

• Sub-Assembly A as a box with 3 connectors
• Sub Assembly B as a box with 2 connectors
• Sub Assembly C as a box with 1 connector
• All Inside a "box" labeled Main Assembly
• Which may have a power LED, power switch, AC power connector, etc

Another way to think about it is... for instance...

• a PC showing a box for the mother board
• a box for each daughter board
• a box for the power supply
• and the main case's front panel and rear panel
• etc

Thanks for any help.
PS - I have used with InkScape and LibreCad but wanted to hear what you guys use.

 

I happen to use AutoCad but pricey.

 

I don't like any of them .........
( at least the ones I've seen )
You can also use any decent Photo-Editing Program,
( not the cheezy kind that just "fixes "Red-Eye" and lets You Crop the Picture and adjust the Contrast ).

I use an old program called "Microsoft-Picture-It",
( difficult to find now, and even more difficult to get it working on a modern Box ).

An example of what You could learn to do with "Photo-Editing" Software ..........
You can make it as simple or detailed as You prefer.
.
.

 

Have a look at diagrams.net (previously known as draw.io) it's the best replacement for Visio that I have found. You can use online or install an application.
Or kicad.
All Linux - Windows - and probably Mac.

 

As far as i could see the OP did not mention schematic drawing, for which I use (free) Kicad.
It is not that suited to mechanical drawings however, as the OP mentioned his need was for a AutoCad style pgm.?

 

Kicad:
I've ended up using Kicad with various connector symbols for this project. Making it a little more detailed than a set of boxes interconnected with lines.

LibreCAD:
I experimented with using LibreCAD. I made my own connector parts library for it.
Their electronics library came with a warning about malware so I decided not to download it.
Down side to me was LibreCAD apparently doesn't let you "weld" a line to the side of a shape... like connecting a line to the side of a rectangle representing a sub-assembly connection. (Can't make a "T" connection as I understood it.)
I do like LibreCAD however.
However I do wonder how big their active userbase actually is?

Inkscape:
Inkscape is is a another alternative and a big favorite of mine. I use it often for a wide variety of things.

These are all open source choices. For very high level system diagrams I've seen people use Microsoft Power Point.
Google has a free alternative to this included with the Google Apps you get when you get a gmail account.

Thanks for all the suggestions folks!