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A simulation, not a measurement. A browser cannot open a serial port, so the numbers are written in advance per scenario — healthy port, degrading above 115200, missing plug, dead port. What it faithfully shows is the sequence and the states, including the loopback probe that runs before any sweep.

A serial port diagnostic tool that sweeps a COM port across 11 baud rates with a loopback plug attached, and tells you which rates the port can actually carry — continuously, with a live pass/warn/fail table.

Shipped in two implementations of the same tool, for two different worlds.

The Python version is the comfortable one — live matplotlib graphs, modern widgets, quick to change.

The C# version exists because the machines that actually need serial diagnostics are often the oldest machines in the building: industrial PCs and instrument controllers running Windows XP, where you cannot install a Python runtime and would not be allowed to if you could. So it targets .NET 3.5 / x86 and is written under that constraint throughout — no default parameters, no volatile double, no string interpolation, since none of those exist in that compiler. It renders its own table and gauge by hand with GDI+ rather than depending on any modern control, and falls back from Segoe UI to Tahoma and from Consolas to Courier New when running on an XP SP2 box that has neither.

It compiles to a single ~40 KB executable with no installer and no dependencies. Copy it onto the machine, run it, delete it.

• Enumerates COM ports via WMI, so ports appear with their real device description rather than a bare COM3
• Probes for the loopback plug first — writes A5 5A F0 0F at 9600 and requires at least 3 of 4 bytes to echo. Without this, a missing jumper looks identical to a dead port, and the whole sweep reports garbage. When no plug is found the tool says NO LOOP and waits instead of producing a table of meaningless failures.
• Sweeps 11 baud rates from 1200 to 921600, scoring each independently as PASS / WARN / FAIL
• Live status table and gauge, both owner-drawn, plus a scrolling log
• CSV export of results, timestamped per port
• Releases the port properly on stop — the worker thread is joined before the button re-enables, and the log says so explicitly: -- Port released -- safe to open Device Manager --

That last one sounds minor and isn't. A diagnostic tool that keeps a handle on the port after you press Stop makes the next tool you reach for fail, and you end up debugging the debugger.

Python:

pip install pyserial customtkinter matplotlib
python serial_loopback_pro.py

C# — build with the compiler already present on any Windows machine:

C:\Windows\Microsoft.NET\Framework\v3.5\csc.exe /target:winexe /platform:x86 /out:SerialLoopbackPro.exe /r:System.Management.dll SerialLoopbackPro.cs

No SDK, no Visual Studio, no NuGet — csc.exe ships with Windows.

You need a loopback plug: a DB9 connector with pin 2 (RX) shorted to pin 3 (TX). Two centimetres of wire will do. Everything else the tool handles.