V.28
Recommendation V.28 of the ITU-T describes possible electrical properties of an interface . The main application is the connection of a data terminal device and a data transmission device in remote data transmission . In the private sector, this is the connection of a PC with a modem . It was adopted in Geneva in 1972 and has been changed several times since then.
V.28 describes, among other things, two voltage ranges: +3 to +15 V and −3 to −15 V on an interface line. These represent the binary value of a data bit or the switching status of control, message and clock lines. All lines have a common return conductor as signal ground. With older devices, the open circuit voltage can increase to +/- 25 V.
The range from −3 to +3 V is called the transition range. It can occur due to a faulty transmitter or when the power is off. The interpretation of this state is left to the recipient. A possible reaction would be to accept binary bit 1 on the data lines and the off status of the other lines.
The originally achievable data transfer rate was 20 kbit / s. An inserted Appendix A of this recommendation describes speeds of up to 64 kbit / s. The COM interface of a PC proves that it can also be used at higher data transmission rates.
Today used integrated circuits with V.28 level are short circuit protected. Very often ICs with the designation 1488 (transmitter) and 1489 (receiver), which are available from different manufacturers, are used for this purpose.
The ITU-T often combines the functions of the V.24 , the electrics of the V.28 and the mechanics of a connector according to ISO 2210 ( D-Sub , 25-pin). This combination is used, for example, with serial modems and with the central station of a modem for parallel transmission .
See also: RS-232
Individual evidence
- ↑ Data sheet 1488 ( Memento of the original from March 12, 2017 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Data sheet 1489
