Build your own serial USB to audio jack cable for the Pinebook Pro

construct-serial-usb-audio-jack-cable.md

How to Construct a Serial USB to Audio Jack Cable

for the Pinebook Pro (and others?)

I bought a CH341 USB Serial to Audio cable at the same time I ordered the Pinebook Pro. What I discovered was that the CH341 chip had difficulty keeping up with the 1.5Mbit speed the Pinebook Pro communicates at.

I had a FT232RL USB UART device available and built a new cable, this gist describes how.

Cable Materials

The minimum materials required are:

• 24 guage stranded wire, about 30 inches (approx 75 cm)
• 1/8 inch (3.5mm) 3-pole male audio plug
• 3.3V FT232rl USB Serial (UART) breakout board with female mini USB connector.
• male mini USB to male type A USB cable (assuming the host has type A USB ports)

What makes it nicer:

• heat shrink tubing or electrical tape

Required Tools and Materials

• soldering iron
• lead free solder
• braided desoldering wick
• multimeter or continuity tester

Nice to have but not required:

• wire cutter/stripper

Construction

Audio Plug End

Audio plug Pinout

    /---- Rx
   / /--- Tx
  / / /-- Gnd
<=:=:==[

Instructions

1. Unscrew the collar from the plug.

2. Cut 3 equal lengths approx. 6 inches or 15 cm in length.

3. Strip approx. 3/8 or 1 cm from each end of each wire.

4. Insert a wire into an eyelet, fold the wire in half onto itself and solder.

5. Using a multimeter or continuity tester, place one probe on the Rx end of plug and find it's matching wire. Mark this wire with one line at the end.

6. Do the same thing with the Tx end of the plug. Find it's matching wire and mark it with 2 lines at the end.

7. Confirm the last wire is attached to Gnd. You do not need to mark this wire.

8. If using heat shrink tubing or electrical tape, apply it to the wires from the plug connection leaving about 2 inches or 5 cm from the end of the wires.

9. Thread the collar over the wire and back onto the plug.

FT232rl USB UART (Serial) end

1. Carefully desolder any connectors attached to the serial end of the board. Usually labeled:

• GND
• CTS
• 5V
• TXD
• RXD
• DTR

2. Solder the Rx wire (marked with one line) to the TXD hole of the FT232rl board.

3. Solder the Tx wire (marked with two lines) to the RXD hole of the FT232rl board.

4. Solder the Gnd wire to the GND hole of the FT232rl board.

Test

IMPORTANT: Most of these boards can be configured for 3.3V or 5V using either a switch or a jumper. Ensure it is set to 3.3V.

1. Plug the mini USB connector into the FT232rl board and the opposite end into an open USB type A port.

2. Make sure the UART switch on the Pinebook Pro motherboard is set to on, towards the hinge.

3. Insert the audio plug into the audio jack on the Pinebook Pro.

4. Start your serial communications program on the host. If using linux, and picocom:

picocom /dev/ttyUSB# -b 1500000

where # is the number shown in /dev/

This is usually /dev/ttyusb0 if you only have one of these attached.

5. Turn on the Pinebook Pro, and you should see u-boot messages, and then kernel messages.

Post test - and After use

Remember to turn the UART switch off, towards the touchpad.