ST‑LINK V3 Minie connects to STM32F100RBT6B Discovery but not to my custom board

[saran]

I’m using an ST‑LINK V3 Minie with an STM32F100RBT6B Discovery board (which also has an STM32F103C8T6), and it works perfectly. However, when I connect the same ST‑LINK V3 Minie to my custom board that uses the same STM32F100RBT6B chip, it fails to connect.

I’ve wired VCC (VTref), GND, NRST, TMS/SWDIO, and TCK/SWCLK exactly as specified.

Black - Ground

White - Vcc

Yellow - Reset

Blue - Clock

Purple - TMS

On the Discovery board this setup works fine, but on my custom board the debugger cannot establish communication.

Could anyone suggest what additional checks I should perform on the custom board?

[Ozone]

Doesn't this suggest the problem is with your custom board ?

I would suggest to connect the VDD /T_Vcc pin as well, as it is used to sense of the target is powered :

Have you checked the voltage at the supply pins of you custom board (at the MCU pins), and the power consumption ?
Only then would I connect an external tool like a debug adapter.