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Just documenting what I did to save the next person some time. This is all for Windows 10.
Things you'll need..
-MicroUSB breakout board (you can get 10 for like 4 bucks on amazon)
-dfu-util for windows from here.. http://dfu-util.sourceforge.net/releases/ (I used dfu-util-0.9-win64.zip)
-Micropython DFU file for the H743 board from here.. https://micropython.org/download/stm32/ (I used NUCLEO_H743ZI-20210202-v1.14.dfu)
I believe there are two versions of the H743 board. Mine has JP3 on the main board, and not the little board with the ST-Link on the end. The following worked on my board.
Move the jumper on JP3 to E5V from U5V. I found this in the Nucleo documentation on STs website. You change this jumper if you want to power the board from 5V from outside the board (not through its normal USB connector)
Connect VBUS from MicroUSB breakout board to E5V on CN11 (pin labels are on bottom of the board) This puts the USB 5V onto the boards 5V.
Connect GND from MicroUSB breakout board to GND pin on CN11 (right below E5V)
Connect D+ from MicroUSB breakout board to PA12 on CN11 (positive data line for USB connection)
Connect D- from MicroUSB breakout board to PA11 on CN11 (negative data line for USB connection)
On top of the board there is a 16 pin black female connector labeled SDMMC. It has a 3V3 pin labeled on it. Connect that pin to CN11 BT0 (this is the BOOT0 pin). BT0 pin is right next to the GND pin we are using. Connecting 3V3 and BT0 together will cause the board to enter dfu mode when we power it on.
Plug in the USB cable from the computer to the MicroUSB breakout board. In device manage under USB devices you should see "DFU in FS mode".
Open a command prompt and navigate to the dfu folder that you downloaded earlier. The command I ran to upload the firmware was...
C:\Users\me\Downloads\dfu-util-0.9-win64>dfu-util --alt 0 -D C:\Users\me\Downloads\NUCLEO_H743ZI-20210202-v1.14.dfu
Once download is complete, disconnect the USB cable from the MicroUSB breakout board, remove the jumper between 3V3 and GND, and put the JP3 jumper back to U5V from E5V. You'll also probably want to disconnect all the wires coming from MicroUSB breakout board. It won't hurt anything to leave that breakout board connected as long as there is no MicroUSB cable plugged into it.
You can now plug your MicroUSB cable into the main MicroUSB cable on the board (labeled CN1). In device manager you can look in ports to see what COM port it gets assigned, then you can connect to it at that COM port at 115200 b/s. Hit enter, and you have a REPL prompt.
For more dfu-util info see https://learn.sparkfun.com/tutorials/ho ... -board/all and http://dfu-util.sourceforge.net/