in a data transmitting / receiving device having parallel / serial and serial / parallel character conversion , particularly for data exchange between communicating data processing systems , and being arranged between a local data processing system and a data modem terminating a data communication line for the purpose of controlling data transmission and / or reception procedures and containing a procedure store , and in which a random access storage is provided as a data buffer for intercepting data jams is provided along with an interrupt control for interrupting processing operations in a microprocessor in respect of higher processing priority , and in which a timer is provided for setting optimum transmission speed , first and second modem connection lines are provided and data messages to be transmitted are converted bit - oriented or character - oriented by the microprocessor from data groups supplied thereto by means of corresponding input switching commands . in addition , the optional transmission of bit - oriented data messages according to the synchronous data link control method or according to the high - level data link control method is provided .