STLINK/V3MODS VCP loopback test fails with no target attached

Suti71 · ‎2026-01-27

I get no message if i send some bytes about my terminal tool. Because i have no schematic i dont know waht going wrong. Do i need an external supply( target ) or do i need pullup resistor or someone else? A target board isnt still connected.

TDK · ‎2026-01-27

What are you using to send data? Some terminal programs buffer until a newline is sent or received.

Can you view the TX line using a logic analyzer or scope?

Where and how specifically did you connect the TX and RX pins? Show a picture.

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Andrew Neil · ‎2026-01-27

@Suti71 wrote:
Do i need an external supply( target ) .

Quite possibly.

It may well have internal buffers, which are driven or enabled by the Vtarget reference.

Just try it!

A complex system that works is invariably found to have evolved from a simple system that worked.
A complex system designed from scratch never works and cannot be patched up to make it work.

Suti71 · ‎2026-01-27

I have try out with Realterm and Hterm. I dont belive that we have CR/LF problem. I have a scope - I could check if a signal was transmitted.

Suti71 · ‎2026-01-27

I have connect my osci probe on jumper( soldered bridge ) and Pin 5+7(GND), but nothing happends. Do it works with a target board only? I want to integrate the board into my hardware and therefore wanted to check beforehand whether the UART VCP communication is working. As I see it, I will probably have to connect a target board. I really dislike such incomplete documentation.

Suti71 · ‎2026-01-27

Or could it be, that on my board( red ) are electronic components missing (see below)?