Apparatus for serial data transmission between a control unit and a position measuring device, having • a control-side interface unit and a measuring-device-side interface unit that are connected to one another for the purpose of transmitting serial data packets by means of a bidirectional data channel, • a control-side transmission unit, arranged in the control-side interface unit, for sending serial data packets via the data channel to a measuring-device-side receiver unit arranged in the measuring-device-side interface unit, • a measuring-device-side transmission unit, arranged in the measuring-device-side interface unit, for sending serial data packets via the data channel to a control-side receiver unit arranged in the control-side interface unit, wherein the apparatus is characterized in that • data packets sent by the control-side transmission unit contain an introductory clock sequence from whose time profile it is possible to derive the frequency of a control-side transmission clock signal (CU_TXC) on which the transmission is based, • the measuring-device-side receiver unit comprises a measuring-device-side clock recognition unit that evaluates the clock sequence by ascertaining the transmission data rate from the clock sequence and, for the purpose of reading in control-side output data (CU_DO) that the data packet contains, generating a measuring-device-side receiver clock signal (EC_RXC) whose frequency largely corresponds to that of the control-side transmission clock signal (CU_TXC), and • the supply of control-side output data (CU_DO), requiring sending, from a processing unit that the control unit contains to the control-side transmission unit and the forwarding of received control-side input data (CU_DI) from the control-side receiver unit to the processing unit each take place via a synchronous serial interface. • a control-side interface unit and a measuring-device-side interface unit that are connected to one another for the purpose of transmitting serial data packets by means of a bidirectional data channel, • a control-side transmission unit, arranged in the control-side interface unit, for sending serial data packets via the data channel to a measuring-device-side receiver unit arranged in the measuring-device-side interface unit, • a measuring-device-side transmission unit, arranged in the measuring-device-side interface unit, for sending serial data packets via the data channel to a control-side receiver unit arranged in the control-side interface unit, • data packets sent by the control-side transmission unit contain an introductory clock sequence from whose time profile it is possible to derive the frequency of a control-side transmission clock signal (CU_TXC) on which the transmission is based, • the measuring-device-side receiver unit comprises a measuring-device-side clock recognition unit that evaluates the clock sequence by ascertaining the transmission data rate from the clock sequence and, for the purpose of reading in control-side output data (CU_DO) that the data packet contains, generating a measuring-device-side receiver clock signal (EC_RXC) whose frequency largely corresponds to that of the control-side transmission clock signal (CU_TXC), and • the supply of control-side output data (CU_DO), requiring sending, from a processing unit that the control unit contains to the control-side transmission unit and the forwarding of received control-side input data (CU_DI) from the control-side receiver unit to the processing unit each take place via a synchronous serial interface.