Hi,
I'm trying to connect to the G431 on a Nucleo G431 board and ST gives two options.

Using an external debug tool to program and debug the on-board STM32 There are two basic ways to support an external debug tool:
1. Keep the embedded STLINK-V3E running. Power on the STLINK-V3E at first until the COM LED turns red.Then connect the external debug tool through the CN4 STDC14/MIPI-10 debug connector ug
2. Set the embedded STLINK-V3E in high-impedance state: when jumper JP1 (STLK_RST) is ON, the embedded STLINK-V3E is in RESET state and all GPIOs are in high-impedance; then, connect the external debug tool to debug connector CN4.

When I try option #1 it does work but the J-tag see's two devices on the chain, a G431 and an "unknown".
When I try option #2 then +3.3V on the debug (Vt) connector goes away and the J-link then can't connect.

Is the J-Link EDU supplying the 3.3V Vt or is the J-link just sampling that pin? I don't see on the Nucleo board where any 3.3V is generated. They do have a 3.3V_ST-LINK but it does not connect to the +3.3V Vt pin as far as I can tell. Any ideas why Vt goes away?

~thanks

Replying to my own question.

I believe what is happening is that when the STLINK is held in reset it does not write the port bit to enable the 3.3V switch that supplies Vdd to the G431. I moved the jumper on JP5 to 5V_CHRG and left the reset jumper on JP1. Now the G431 gets it's 3.3V enabled and the hardware is happy.

The JTAG chaining is a separate issue to be addressed elsewhere.