PURPOSE: To obtain an economical data signal conversion system by extracting a terminal data, converting the speed into a terminal data speed and detecting a synchronizing pattern at a high speed data signal sent from an exchange while using a data signal frame pattern stored in a pattern memory of a data exchange circuit.

CONSTITUTION: A pattern corresponding to a terminal data and a synchronizing pattern or the like is stored in an address corresponding to 80-bit of a data signal frame in a pattern memory 226. When a counter 227 receiving a clock generated from a control signal generating circuit 228 generates an address synchronously with the transmission of the data signal frame, a control signal S3 is sent from an address relating to the terminal data of the data signal frame in the pattern memory 226 and a control signal S4 is sent from an address relating to the synchronizing pattern. A terminal data extracting circuit 223 receives the control signal S3 to extract the terminal data from the data signal frame and a synchronizing pattern detection circuit 224 receives the control signal S4 to detect the synchronizing pattern.