It's not easy. You don't get that far with a scope because the encoding on the line is so hard to observe. However you can scan for basic activity. Note that on 1000BaseT (from my experience) you often see continuous carrier activity, while on lower speeds you only see activity when you send some data. This at least lets you know if there is actually ...
TRST(N) is optional for JTAG (https://en.wikipedia.org/wiki/JTAG).
However, you might choose to use SWD as opposed to JTAG. SWD is more modern and supported by the STM32L4 and STLINKV2 via that same connector. See this document (https://documentation-service.arm.com/static/5fce6c49e167456a35b36af1).
but I don't know what '\252' means.
This is an octal value. It's equal to 2 + 5*8 + 2*8*8 = 170.
Another example: '\123' is octal for 3 + 2*8 + 1*8*8 = 83.
Next I changed the variable to a char instead of an unsigned char and ran the same print test_var, I got an output of $10 = -86 '\252'
170 is greater than 127, so when represented as a char (which is a ...