Need help with a USB-Serial cable for F030

Hello.

Maybe somebody can tell me where I made a mistake with this.

I have a F030 (MTK6235).

When I connect it to the PC with its USB cable, the phone asks me to select the mode and I select serial connection. Windows starts up the default USB-to-Serial driver. So the USB cable works. I can communicate with the phone and look at its data.

Now I want to try another cable with a PL-2303 chip so I can backup the phone's firmware or whatever.

I install the latest PL-2303 drivers and I plug in the new cable. Windows recognizes it and installs the Prolific driver. So this cable works too.

The PL-2303 chip is inside a USB plug that has 3 wires on its other side. GND, -tx and -rx (identified by the vendor of the cable).

To connect the PL-2303 cable to the original phone's USB cable, I cut the phone's USB cable.
It has 4 wires. a -tx, -rx and two wires that both look like GND. (I used a multimeter on the wires with the phone ON).

I connect all the GND wires together and connected -tx and -rx of one cable to the -rx and -tx of the other. (I even tried connecting directly -tx to -tx and -rx to -rx).

Now when I plug in the modified wire, the Prolific chip is again recognized and installed.
When I connect the phone though, nothing happens.

I don't get a prompt from the phone to select a serial mode or anything else. Should I?

Also all the programs I used, don't see the phone and cannot communicate with the Prolific serial port.
I've tried SetupCOM 2.1.5.0, floAt's Mobile Agent 2.1.4.0, various MTK Flash Tools and some PhoneSuites.

I also tried two different cables with PL-2303 on them and different phone USB cables. No change at all.

Any idea why this setup is not working? Is there something else I have to do to get a working connection?

Thank you for any advice.
 
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