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 , - the submodule interfaces 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 , - 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 , 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 , 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 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 , - the submodule interfaces 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 , - 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 , 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 , 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 are arranged serially in the collective message . <EOS>