Modular multilevel converter having an electronic interface circuit and submodules (1_1 to 6_n), which submodules each have at least a first electronic switching element, a second electronic switching element, an electrical energy store, a first galvanic power terminal, a second galvanic power terminal and an optical communication terminal, and having a control device for the submodules, wherein - the control device is connected to the electronic interface circuit by means of a first communication connection, - the interface circuit has a multiplicity of submodule interfaces (450, 452), - the submodule interfaces (450, 452) are each optically connected to the communication terminal of one of the submodules (1_1, 2_1) by means of an optical submodule communication connection (456, 461), - the interface circuit has a distribution circuit which is configured to distribute a message, which originates from the control device and is intended for one of the submodules (1_1), to that submodule interface which is connected to the one submodule (1_1), and - the first communication connection is a first optical communication connection, characterized in that - at least one of the submodules is a half-bridge module in which the first electronic switching element is electrically connected in series with the second electronic switching element, the first galvanic power terminal is arranged at the connection point between the two switching elements (202, 206), the second galvanic power terminal is arranged at that terminal of the second switching element which is opposite the connection point, the second power terminal is connected to a first terminal of the energy store, and a second terminal of the energy store is electrically connected to that terminal of the first switching element which is opposite the connection point, and - the interface circuit has a collector circuit which is configured to forward messages (530, 532), which originate from a plurality of the submodules (1_1, 2_1) and are each intended for the control device, together as a collective message to the control device via the first optical communication connection, wherein the messages (530, 532) are arranged serially in the collective message. - the control device is connected to the electronic interface circuit by means of a first communication connection, - the interface circuit has a multiplicity of submodule interfaces (450, 452), - the submodule interfaces (450, 452) are each optically connected to the communication terminal of one of the submodules (1_1, 2_1) by means of an optical submodule communication connection (456, 461), - the interface circuit has a distribution circuit which is configured to distribute a message, which originates from the control device and is intended for one of the submodules (1_1), to that submodule interface which is connected to the one submodule (1_1), and - the first communication connection is a first optical communication connection, characterized in that - at least one of the submodules is a half-bridge module in which the first electronic switching element is electrically connected in series with the second electronic switching element, the first galvanic power terminal is arranged at the connection point between the two switching elements (202, 206), the second galvanic power terminal is arranged at that terminal of the second switching element which is opposite the connection point, the second power terminal is connected to a first terminal of the energy store, and a second terminal of the energy store is electrically connected to that terminal of the first switching element which is opposite the connection point, and - the interface circuit has a collector circuit which is configured to forward messages (530, 532), which originate from a plurality of the submodules (1_1, 2_1) and are each intended for the control device, together as a collective message to the control device via the first optical communication connection, wherein the messages (530, 532) are arranged serially in the collective message.