Digital key telephone set and digital key telephone system

A digital key telephone set used as a key telephone set in a digital key telephone system which transmits a serial signal of n-bit frame structure, wherein n is a desired integral number of frame structure bits, including real data of m bits, wherein m is a desired integral number of real data bits, and preset additional bits including a start bit arranged in the head portion of the real data, from a key telephone unit to the key telephone set via a data channel, and fetches the real data among the serial signal by a controller of the terminal comprising:

• a clock generating circuit which selectively generates one of a plurality of preset clock signals having different rates in response to a signal for specifying a rate;
• a start bit detecting circuit which detects the start bit in the serial signal transmitted to generate a detection signal;
• a counting circuit which counts the number of clocks of the clock signal in response to the detection signal and generates a fetch signal when the count reaches n;
• a parallel converting circuit which fetches the serial signal at preset fetch timing which is synchronized with the clock signal, and outputs preset m bits among the serial signal in response to the fetch signal;
• a latch circuit which fetches and latches preset m bits output from said parallel converting circuit in response to the fetch signal;
• and
• a reception state monitoring circuit which outputs a completion notifying signal indicating that reception of one frame of the serial signal is completed in response to the fetch signal, monitors the reception state of the serial signal based on the state of a preset bit output from said parallel converting circuit, and outputs a flag signal indicating the monitoring result to the controller.