OK, I did a bit more work today, with some interesting findings:
1) It is absolutely NOT necessary to use Git to download the files from GitHub. Downloading the Zip file works just fine. That is how I downloaded Marlinfirmware/Marlindev.
2) The correct version of PlatformIO.ini is the one that thawkins shared above (shows as 10 August on my screen). I got the same error that KD6HQ did when I tried to use the one from GitHub.
3) When I ran PlatformIO run -e printrboard the first time, it noticed that I was missing components from "teensy" so it asked if I wanted to install them. I answered yes, and it took about 10 minutes to accomplish that task. However, that was a one-time exercise; subsequent attempts to compile did not need to retrieve "teensy."
4) The compilation was apparently a success when in the directory Marlinfirmware/Marlindev. However, I have no idea where it put the hex file! It looks like it was named "printrboard.hex" instead of "Marlin.hex" - but no matter.
5) Then I copied the PlatformIOAddons directory into an un-modified copy of the Marlin Unified source code (which is basically what I am running on my printer, with a few small tweaks). However, when I ran "PlatformIO run -e printrboard" there, I got errors and it did not finish successfully.
PlatformIO result with Directory Structure 1.PNG
In fact, I then noticed that maybe I had copied the Addons folder into the wrong place, one level too far down in the tree.
So I moved it back up to be equivalent to the position in "Marlinfirmware/Marlindev" - but in that position it generated even more errors faster!
I do like the idea of working this way, but I have no idea how to fix the problems. (I'd really like to be able to stick with the "Unified" firmware, not getting messed up with probing and all that stuff.)
By the way, I've installed Notepad Plus Plus and I really like it. When you use Eclipse, you are entering a high-powered system that keeps track of file locations and names with internal metadata, so if you ever move anything or change a file name, Eclipse gets broken (unless you do it from within Eclipse). But with Notepad++, you are simply navigating in an ordinary file system. One thing I really like about Notepad++ is that you can search for a particular string wherever it might appear in the current directory or subdirectory. This is fantastic for tracking down references in the huge array of files that comprise Marlin. As far as I know, Arduino doesn't have that capability, and I haven't found it in Eclipse either!
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Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick
"My next printer is..." Prusa i3 MK3, upgraded to MK3S