This information was collected from - https://communities.intel.com/message/208514 

Two parts are needed to make the cable:

• DB9 Male connector
• 5mm Male stereo audio connector

You may find cheaper or higher quality alternatives than the linked parts

The spec is included in the open source schematic (pg17/28)

Pinouts: 

• For a 3 pin audio jack:
  • Sleeve - Ground
  • Ring - Input RxD
  • Tip - TXD
• on the DB9:
  • 1 - No connect
  • 2 - Input RxD
  • 3 - Output TxD
  • 4 - Output DTR
  • 5 - Ground 
  • 6 - Input DSR
  • 7 - Output RTS
  • 8 - Input CTS
  • 9 - Input RI