When programming the NetBurner Mod5213, I ran into the problem where I would get the error "Failed to Connect to Serial Port" and I could no longer download any programs (even ones that worked previously) to the board.
Apparently the code I was trying to run did something to make the 5213 very unhappy - such as a pointer to a bad location or something along those lines. So, it crashed and entered a state where it resets itself every few seconds which makes it impossible to program. A very helpful tech support person just walked me through the simple (haha) procedure to get out of this loop and back to programming. I'm posting the procedure here because it is apparently a very common problem.
-With power on and serial cable connected, Open MTTTY
-Connect to the board (File -> Connect)
-Cycle power to the board. (You should get the message "Waiting 5sec(s) to start 'A' to abort")
-Type in "A". (You should get a message with valid commands)
-To make sure it is working, load the original factory program from SerialLoad...
-Open Serial Load (Start -> Programs -> Netburner NNDK -> Serial Update Tool)
-Browse to the file Nburn -> MOD5213 -> original -> MOD5213FApp_APP.s19
-Go back to MTTTY and connect (if disconnected)
-Hit the reset button on the board. (You should get the Factory Demo program Main Menu)
-Fix whatever the heck caused this problem in the first place before trying to re-load the program