Systems for transmitting data via telephone lines

Claim 1

In a system for transmitting data via a telephone line, which system includes a data communications device connectable to the telephone line for transmitting the data, at least one other communications device being connected to the telephone line and each device being switchable between an inactive state, in which the device is not in use and presents a high impedance to the telephone line, and an active state, in which the device reduces the telephone line voltage and signals can be transferred between the device and the telephone line, the improvement wherein: said system further comprises telephone line state detection means associated with said data communications device and connected to the telephone line for electrically monitoring the telephone line to provide an indication of the state of another communications device connected to the telephone line, and control means connected for controlling the connection of said data communications device to the telephone line; and said detection means comprise a switch which is controlled by said control means to be closed when said communications device is connected to the telephone line, a first circuit having an input connected to the telephone line via said switch and having means for supplying an output signal to said control means when said switch is closed and the voltage on the telephone line is below a selected value, and a further circuit having an input connectable to the telephone line via said switch and having means for supplying an output signal to said control means when said switch is closed and the voltage on the telephone lien decreases by a predetermined amount in a selected time interval.