Personal computer manufacturers replaced the DB-25M connector with the smaller DE-9M connector. Most devices only implement a few of the twenty signals specified in the standard, so connectors and cables with fewer pins are sufficient for most connections, more compact, and less expensive. The standard recommends the D-subminiature 25-pin connector up to revision C, and makes it mandatory as of revision D. Many terminals were manufactured with female connectors but were sold with a cable with male connectors at each end the terminal with its cable satisfied the recommendations in the standard. Other devices may have any combination of connector gender and pin definitions. According to the standard, male connectors have DTE pin functions, and female connectors have DCE pin functions. RS-232 devices may be classified as Data Terminal Equipment (DTE) or Data Circuit-terminating Equipment (DCE) this defines at each device which wires will be sending and receiving each signal. Used by the Data Set to indicate to the Data Terminal that a ringing condition has been detected. Used by the Data Terminal to signal to the Data Set that it is ready for operation, active high. Used by the Data Set to indicate to the Data Terminal that the Data set has detected a carrier (of another device).ĭata Terminal Ready. Used by the Data Set to signal to the Data Terminal that it is ready for operation and ready to receive data, active high.Ĭarrier Detect. The Data Terminal will not send out data with out this signal, active high.ĭata Set Ready. Used by the Data Set to signal the Data Terminal that it may begin sending data. The Data Set will not send out data with out this signal, active high.Ĭlear to Send. Used by the Data Terminal to signal the Data Set that it may begin sending data. Many receivers designed for RS-232 are sensitive to differentials of 1v or less. For instance, the definition for V.10 has a dead area from +0.3v to -0.3v. In the various RS-232-like pinout definitions this dead area may vary. The dead area between +3v and -3v is designed to absorb line noise. The output signal level usually swings between +12V and -12V. This means circuits powered by 5 VDC are capable of driving RS232 circuits directly, however, the overall range that the RS232 signal may be transmitted/received may be dramatically reduced. In fact, the ON state may be achieved with lesser positive potential. Some computer equipment ignores the negative level and accepts a zero voltage level as the OFF state. From +3 to +12 volts indicates an ON or 0-state (SPACE) condition while A -3 to -12 volts indicates an OFF 1-state (MARK) condition. Circuits driving an RS-232-compatible interface must be able to withstand indefinite short circuit to ground or to any voltage level up to 25 volts. The standard specifies a maximum open-circuit voltage of 25 volts, but common signal levels are 5 V, 10 V, 12 V, and 15 V. This is a reasonable assumption when a short cable connects the DTE to the DCE, but with longer lines and connections between devices that may be on different electrical busses with different grounds, this may not be true. The RS-232 interface presupposes a common ground between the DTE and DCE. RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol. The idle state (MARK) has the signal level negative with respect to common, and the active state (SPACE) has the signal level positive with respect to common. The RS232 pinout signals are represented by voltage levels with respect to a system common (power / logic ground). Personal computers are usually equipped with simplified version of RS-232 interface. The standard recommended but did not make mandatory the common D-subminiature 25 pin connector. RS-232 devices may be classified as Data Terminal Equipment (DTE) or Data Communications Equipment (DCE) - this defines which wires will be sending and receiving each signal. Both synchronous and asynchronous transmissions are supported, but asynchronous link sending seven or eight bits packets is most common configuration on PC. RS-232 data transmission consist of time-series of bits. RS-232 standard defines electrical signal characteristics (voltage levels, timing, signaling rate, short-circuit behavior, cable length), mechanical characteristics of RS-232 interface, RS-232 pinouts and connectors, plus some other details. It was introduced in 1960, and despite rumors for its early demise, has remained used through the industry. RS-232 is simple, universal, well understood and supported.
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