Why on pin UART Data In of my XBee there is always high state?

I am trying to communicate STM32F072B-DISCO development board with XBee Pro S5 module. Unfortunately when I am connecting Tx from uC with UARTDataIn of my XBee, there always occurs high state on this connection. My oscilloscope shows that there is just a small voltage drop from 3.0V to 2.7V. uC seems to work correctly because it is able to operate with another devices. I disabled PullUp resistor, but there is no effect (vcc voltage is still present on this pin).

DI should be high. Driving DI low on power up will put the module in a command state. Your mC Data out should be connected to Data in on the radio.

TX from uC is connecting to Data input on my XBee. The issue is why during transmission voltage falls to 2.7V instead to 0V.

That is because your processor does not have have the ability to pull the line low. What is in between the two devices?

Right now I have nothing, I am just testing the setup.
What should I use between Xbee and uC?

If your processor runs at 3V, than nothing. But if your processor is 5V, then you need a level shifter between the two or you will damage the XBee module.

Ok, I know about level shifter, but what is the best solution to have the ability to pull the line low?

What voltage level does your processor run at?

My processor work at 3V. I added a non-inverting buffer 74LVC1G125 and it work correct. Is there any better solution?

I have the Xbee 900 HP connected to an MSP430 but when I plug the actual Xbee module on my PCB with the uC (MSP430) I see no activity on my UART lines? Is my setup wrong it is the hardware design on my PCB missing something.

If you can get the processor to talk to the Xbee via a bread board but not in your PCB using the same code, I would tend to think it is a PCB hardware design issue.