Issue: When connected via serial to my device and booting up, I can receive data, but I can not send anything, i.e. no keyboard input possible during boot process.
Root causes:
Faulty serial adapter
No physical connection between the RX serial pin/pad on the board and the CPU
intended by the OEM (resistor or jumper not soldered)
unintended by user (bad soldering of pins/wires, lose plug contact, pcb trace cut/delaminated, ...)
Solutions:
Open putty and test serial adapter by connecting RX and TX
If what you type on the keyboard is echoed back to the screen → OK
If not → faulty serial adapter, get a new one.
Check the serial connection visually for continuity. In case of interruptions detected
Solder missing resistors/jumpers → see devicepage of your device (use the wiki search or the ToH to find the devicepage)
Make sure pins/wires are soldered correctly. Re-solder if necessary.
Repair any cut pcb traces / delaminated pads
Links: