UART if. uart0 splitter & Level shifters w/o internal pullups

In some cases like using max3232 or similar level shifters that do not have internal pullups on ttl side it may be important to add external pullup resistor to U1 leg 11 to keep that input '1' state.
If you have problems with declare/flight dl from uart0 please measure voltage level from anode leg of one of splitter diode. Measure points are shown in picture.
If you see voltage(close to zero) like first picture, pullup resistor is needed!

Our next pcb version have place for additional pullup. in this picture voltage is measured after pullup is added.  Value of pull up is not critical, you can use example 100kohm or 470kohm or something between these values...

If you have older pcb version, add pullup resistor between U1 legs 16 and 11.