Measurement & control devices (distributed I/O units) are located at the field control level of the HVDC control & protection system in the converter stations. As the interface to the primary system the I/O units take important part in the HVDC transmission. The development of a PROFIBUS DP based I/O unit is introduced. The hardware of the unit is composed up around closely coupled multi-processors and adopts modular structure which ensure very high processing speed and overall capacity and also the I/O resources can be flexibly configurable according to the actual needs. The software system of the device provides visualized MMI functions for hardware/software configuration and maintenance. With multi-level cyclic task structure of the software the time critical tasks and emergency events can always be handled. Features for accurate AC/DC sampling, automatic synchronous measuring and local interlock functions, duplicated PROFIBUS channels and power supplies, as well as comprehensive self-diagnosis and fault blocking functions, the performance of the device has been fully upgraded under the premise of ensuring the safe and reliable operation. Results derived from simulation tests and operation of real projects have proved that the PROFIBUS DP based I/O unit developed is fully meet the requirement of various HVDC applications.