The RS-232 interface is the Electronic Industries Association (EIA) standard for the interchange of serial binary data between two devices. | RS-232 INTERFACE Introduction The RS-232 interface is the Electronic Industries Association EIA standard for the interchange of serial binary data between two devices. It was initially developed by the EIA to standardize the connection of computers with telephone line modems. The standard allows as many as 20 signals to be defined but gives complete freedom to the user. Three wires are sufficient send data receive data and signal ground. The remaining lines can be hardwired on or off permanently. The signal transmission is bipolar requiring two voltages from 5 to 25 volts of opposite polarity. Communication Standards The industry custom is to use an asynchronous word consisting of a start bit seven or eight data bits an optional parity bit and one or two stop bits. The baud rate at which the word sent is device-dependent. The baud rate is usually 150 times an integer power of 2 ranging from 0 to 7 150 300 600 . 19 200 . Below 150 baud many system-unique rates are used. The standard RS-232-C connector has 25 pins 21 pins which are used in the complete standard. Many of the modem signals are not needed when a computer terminal is connected directly to a computer and Figure 1 illustrates how some of the spare pins should be linked if not needed. Figure 1 also illustrates the pin numbering used in the original DB-25 connector and that now commonly used with a DB-9 connector normally used in modern computers Specifying compliance to RS-232 only establishes that the signal levels in two devices will be compatible and that if both devices use the suggested connector they may be able to be connected. Compliance to RS-232 does not imply that the devices will be able to communicate or even acknowledge each other s presence. Table 1 shows the signal names and functions of the RS-232 serial port pinout. Table 2 shows a complete pin description Table 1. RS-232 Serial Port Pinout Name Pin Signal Name Function AA 1 PG Protective Ground This line is connected to the chassis
