I can connect my FC to betaflight as normal and everything works as it should, but when i press the boot button and connect USB to flash firmware it fails to open serial port and can't connect to my FC. I have tried the guide below but the problems he solves doesn't seem to be the same.

Are you seeing DFU in the upper right of BFC when you boot into bootloader mode? You have to at least see that. If you are not seeing it, then there is something wrong with your driver or FC. Can you also show me what settings you have clicked when you flash? Eg. No boot sequence, full chip erase etc. Also what FC do you have.

Not sure what DFU is?
if i connect USB it detects the COM7, but if i hold boot and connect USB nothing happens in BFC. not

I have a F4 magnum board, OMNIBUSF4 firmware.

Have you tried running the ImpulseRC driver fixer? If not then try doing that next. It can be downloaded from the following web page...

https://impulserc.com/pages/downloads

You will need to install the Zadig driver to flash for your FC. Follow the tutorial below.

Thanks guys,
this is really strange.. when i connect the USB - FC windows detects a USB device, zadig finds it and i can install the right driver just like in the video above. But when i press boot and plug in the USB nothing happens, windows doesn't detect anything and the device cant be found in zadig.. the only thing that works as it supposed to do is the blue LED on the FC in bootloader mode.

When in DFU/boot mode, do any unrecognised devices with a yellow exclamation mark against them show up for it in Windows Device Manager?

Also, I take it you've tried a different USB cable to eliminate any possible dodgyness with the cable itself?

You can force DFU mode through BFC. Go to setup tab. Toggle expert mode top right. Restart BFC. Go to Setup tab again. You should see a button top miide for bootloader mode. Click that. Hopefully that will get you into DFU/bootloader mode.

(09-Jan-2019, 10:13 PM)SnowLeopardFPV Wrote: When in DFU/boot mode, do any unrecognised devices with a yellow exclamation mark against them show up for it in Windows Device Manager?

Also, I take it you've tried a different USB cable to eliminate any possible dodgyness with the cable itself?

Nothing shows up in device manager and zadig cant seem to find any devices in DFU mode. Tried different cables and they all work when not connecting in DFU mode.

(09-Jan-2019, 10:38 PM)voodoo614 Wrote: You can force DFU mode through BFC. Go to setup tab. Toggle expert mode top right. Restart BFC. Go to Setup tab again. You should see a button top miide for bootloader mode. Click that. Hopefully that will get you into DFU/bootloader mode.

I dont have this button in BFC even if i follow your instructions.. i have version 10.4.0

Tried to enter DFU in BFC by typing BL like in the instruction below. The blue LED lights up as it should and i hear windows disconnect the USB device and then BFC crashes..

Solved!

Okay, while forcing the board in CLI to enter DFU mode windows somehow just detected the usb device and i could replace the driver with zadig and now i can access the FC in DFU mode latest firmware now successfully flashed and everything seems fine

thanks for the help, i would not have figured this out on my own. forcing into DFU mode through BFC did the trick

(16-Jan-2019, 07:55 PM)Andreas-sa Wrote: I dont have this button in BFC even if i follow your instructions.. i have version 10.4.0

BFC 10.4.0 definitely has the DFU/Bootloader button.  The screenshot I posted is from 10.4.0 BFC.

Good you got it to work.  I found for some reason the button works more consistently than "DFU" in CLI.

(16-Jan-2019, 09:19 PM)voodoo614 Wrote: BFC 10.4.0 definitely has the DFU/Bootloader button.  The screenshot I posted is from 10.4.0 BFC.

Good you got it to work.  I found for some reason the button works more consistently than "DFU" in CLI.

That's strange, I did not have the button until I upgraded the firmware, now it's there. Maybe it depends on what BF version the FC have?

(17-Jan-2019, 08:02 AM)Andreas-sa Wrote: That's strange, I did not have the button until I upgraded the firmware, now it's there. Maybe it depends on what BF version the FC have?

Yes, that's correct.

Having just looked at the Betaflight Configurator 10.4.0 source code, the "Activate Boot Loaded / DFU" button only gets displayed on the Setup tab if the "MSP API Version" of the Betaflight firmware on the Flight Controller is version 1.40.0 or greater. Looking at the the source code for the Betaflight firmware itself, the "MSP API Version" was incremented to 1.40.0 in Betaflight 3.5.0.

So, you need Betaflight firmware version 3.5.0 or greater installed on your Flight Controller for that "Activate Boot Loaded / DFU" button to appear in Betaflight Configurator 

(17-Jan-2019, 10:26 AM)SnowLeopardFPV Wrote: Yes, that's correct.

Having just looked at the Betaflight Configurator 10.4.0 source code, the "Activate Boot Loaded / DFU" button only gets displayed on the Setup tab if the "MSP API Version" of the Betaflight firmware on the Flight Controller is version 1.40.0 or greater. Looking at the the source code for the Betaflight firmware itself, the "MSP API Version" was incremented to 1.40.0 in Betaflight 3.5.0.

So, you need Betaflight firmware version 3.5.0 or greater installed on your Flight Controller for that "Activate Boot Loaded / DFU" button to appear in Betaflight Configurator 

That explains it, and if you have Betaflight firmware prior to 3.5.0 you can type "BL" in CLI which does the same thing.