This paper presents the design and implementation of a digital control system for modular multilevel converters, based on digital signal processors. To achieve higher system control reliability and multi-functionality, the proposed architecture has been built with an effective split of the control tasks involved between a master controller and six slave controllers, one for each of the six arms of the converter. The master controller handles energy flow within the converter and user communication using Ethernet and universal serial bus. Each slave controller handles capacitors voltages balancing within the arm. Intercommunication between the different boards is achieved mainly through optical fiber and inter-integrated circuit bus. Experiments have been carried out to verify the effectiveness of this architecture.