Serial connection can receive but can not send

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:

  1. Faulty serial adapter
  2. 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, ...)


  1. 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.
  2. 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


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  • Last modified: 2019/09/11 15:36
  • by tmomas