In-Situ RDO PRO Process Optical Dissolved Oxygen Sensor Manual - Manual (Page 15)

5.5 Modbus Master with Built-in RS232 (Converter Required)
Signal Color
Ground/Return Black
External Power
(12 VDC, voltage limited by converter)
Red
RS485 (-) Green
RS485 (+) Blue
 Cable between converter and master must not exceed 60.96 m (20 ft.)
 Cable between master and slave must not exceed 1219 m (4000 ft.)
5.5.1 Converter
Page 15 Controller Requirements and Connections