Circuit arrangement for serial data transmission between a plurality of subscriber stations

Claim 1

In a circuit arrangement for serial data transmission between two subscriber stations each connected to a respective transmission/receiving device, and each of said subscriber stations including means for transmitting data and means for receiving data, each means for transmitting data and each means for receiving data connected two leads of a data bus, each of said means for transmitting data normally assuming a passive condition and including a data input and a control input connected to the transmission/receiving device at the subscriber station containing the means for transmitting data, said control input responsive to control signals such that only one of the means for transmitting data is driven to an active condition at a time to output a voltage of a first polarity or of a second polarity dependent on the state of a data signal applied to its data input by the respective transmission/receiving device, each of the means for transmitting data having a higher internal impedance in the passive condition than in the active condition, and each of the means for receiving data operable in response to the data bus to output data to the respective transmission/receiving device, the improvement comprising: first and second recognition circuits each in a respective one of said subscriber stations, each of said recognition circuits comprising first and second data receivers, including inputs connected in opposite polarity fashion to the two leads of the data bus and an output, said data receivers assuming one of two prescribed signal states at their outputs dependent on the polarity of their input voltage and assuming one of the two prescribed signal states at their outputs in a defined manner given a signal-free input; and logic means in each subscriber station including inputs connected to said outputs of the first and second data receivers in a subscriber station and an output connected to the transmitting/receiving device at the subscriber station containing said logic means, for producing at said output a free signal having a first logic state in response to identical logic levels at its input and producing a busy signal having a second logic state in response to unequal logic levels at its inputs.