WHAT IS RS-232?

RS-232 is a standard developed by EIA (Electronic Industries Association) for communication up to 15 meters using two voltage levels between +15V and -15V. It is used to transfer digital data to units over short distances, such as a modem, keyboard or terminal.

Data is usually transmitted in 8-bit characters (due to the 8-bit definition of ASCII characters). However, it can optionally be transmitted in bits as low as 7 and 5. Transmission is serial (bits are sent sequentially). Transmission is defined differently in the standard, both synchronous and asynchronous. In the case of asynchronous, the sender and receiver do not need to coordinate.

The sender transmits data prepared in a certain format. The receiver is constantly listening to the line, and after receiving the signal indicating the arrival of the data, it collects the incoming data and generates the characters.

In RS-232, minus voltage level means “1” and plus voltage level means “0”. The minus voltage level indicates that the line is free. Sending data is expressed by pulling the voltage to the positive value (0, the start bit) and then a series of characters is sent. The voltage for each bit (between -3,-25 V for 1; +3,+25 V for 0) is kept at the same level for a certain time. The transmitting and receiving units are set according to this time. An end bit “1” is added at the end of each character.