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The only connector specified in the original RS-232 standard was the 25-pin D-subminiature, however, many other connectors have been used to save money or save on physical space, among other reasons. In particular, since many devices do not use all of the 20 signals that are defined by the standard, connectors with fewer pins are often used.
Presence of a 25-pin D-sub connector does not necessarily indicate an RS-232-C compliant interface. For example, on the original IBM PC, a male D-sub was an RS-232-C DTE port (with a non-standard current loop interface on reserved pins), but the female D-sub connector on the same PC model was used for the parallel "Centronics" printer port .
The DTR signal is present on pin 20 of the 22-wire RS-232 interface using a DB-25 connector, and on pin 4 of a newer DE-9 serial port. The signal is asserted (logic "1") by raising the voltage of the pin from negative to positive. Dropping the signal back to its negative state indicates to the modem that the communications session shall be ...
DCE devices are just the opposite: pin connector number 2 receives and pin connector number 3 transmits the signals. When two devices, that are both DTE or both DCE, must be connected together without a modem or a similar media translator between them, a crossover cable must be used, e.g. a null modem for RS-232 or an Ethernet crossover cable/body
MAX232 chip in DIP-16 package The die of a MAX232 MAX232 pinout: Red: power, Yellow: charge pump capacitors, Blue: outputs, Green: inputs, Pins 9–12: TTL/CMOS I/O voltages. The MAX232 is an integrated circuit by Maxim Integrated Products, now a subsidiary of Analog Devices, that converts signals from a TIA-232 (RS-232) serial port to signals suitable for use in TTL-compatible digital logic ...
A null modem adapter. Null modem is a communication method to directly connect two DTEs (computer, terminal, printer, etc.) using an RS-232 serial cable.The name stems from the historical use of RS-232 cables to connect two teleprinter devices or two modems in order to communicate with one another; null modem communication refers to using a crossed-over RS-232 cable to connect the teleprinters ...
Later Macintosh models use 8-pin miniature DIN connectors instead. On PCs, 25-pin and (beginning with the IBM PC/AT) 9-pin plugs were used for the RS-232 serial ports; 25-pin sockets were used for parallel ports (instead of the Centronics port found on the printer itself, which was inconveniently large for direct placement on the expansion cards).
Connectors on this cable would be designated: 1A1W35P1; 1A1W35P2; ASME Y14.44-2008 continues the convention of Plug P and Jack J when assigning references for electrical connectors in assemblies where a J (or jack) is the more fixed and P (or plug) is the less fixed of a connector pair, without regard to the gender of the connector contacts.