V.31
V.31 is a recommendation of the ITU-T . It describes possible electrical properties of an interface . The intended application is the connection of a data terminal device and a data transmission device in remote data transmission over the telephone network . A characteristic of these interface lines is the maximum transmission speed of 75 bit / s, which can be implemented using switches or teleprinters . Relays are often used in practice.
Each interface line consists of two contact points for a separate forward and return conductor. All lines are electrically isolated from one another . A common return line can be used for individual connected groups of interfaces.
In the case of data lines, a closed contact counts as a binary 1 ; in the case of control, message and clock lines, the status is On .
One application was the outstations of a modem for parallel transmission. With these data transmission devices, the entire interface consisted of functions of the V.24 , the electrical properties of the V.31 and the mechanics of a plug according to ISO 2110 ( D-Sub , 25-pin).