Im looking to connect a Omega PHH222 pH meter to the RPi GPIO through a serial connection. A 3.5mm jack connects into the device and it outputs serial data. The manual shows 2 wires coming out of the 3.5mm one bring the center and the other bring ground/shield.
It shows the connections for a 9 pin connector as follows:
Center Pin=Pin 4 (DTR on RS232 pin out)
Ground/shield=Pin 2 (RXD on RS232 pinout)
Pin 2 to Pin 5 with a 2.2k Resistor
RS232 FORMAT:9600, N, 8, 1
Im not quite sure on how to connect this to the PI. The manual is in the PDF below and the section for the connection is on page 19.