Abstract: A passive ISDN adapter card includes a serial controller (1) as is normally used to connect an external data transmission device; an ISDN data transmission controller (2) connected to the serial controller and controlled by the CPU of a computer (4); and an ISDN connection controller (3) controlled by the CPU of the computer (4) and connected to the data transmission controller (2). The ISDN adaptor card is compatible with a modem connected to the serial port without an INT-14-H interface for the evaluation process.

Abstract: A serial data stream of control commands is sent to a shift register having a delay time corresponding to a shift frequency which is a multiple of the maximum expected bit rate, each control command beginning with a uniform binary character having a start sequence with a maximum duration. The bit rate is calculated from the duration of the start sequence which is measured by activating a counting circuit during the time in which the data stream is passing through the shift register. The bit rate is programmed into an integrated serial data transmission component, preferably a UART, and the delay time of the shift register is determined according to the maximum duration of the start sequence, plus the time required to program the bit rate into the UART.