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Serial Cable Tutorial
Serial Cable Defined

A serial cable is a cable that can be used to transfer information between two devices using serial communication, often using the RS-232 standard. Serial cables may use D-subminiature connectors, but other connectors are used. A specially wired cable used for connecting two similar computer serial ports directly is known as a null modem.


Maximum Cable Lengths

The maximum working length of a cable varies depending on the characteristics of the serial port transmitters and receivers, the bit rate on the cable, and the capacitance and resistance of the cable. The RS 232 standard states that a compliant port must provide defined signal characteristics for a capacitive load of 2500 pF. This does not correspond to a fixed length of cable since varying cables have different characteristics. Empirically tested combinations of bit rate, serial ports, cable type, and lengths may provide reliable communications, but generally RS232-compatible ports are intended to be connected by at the most a few tens of meters of cable. Cable construction and workmanship becomes important when patching long cable runs.


Construction of a Serial Cable

RS-232 cables may be built with D-subminature connectors or other forms depending on the application. The number of wires used depends on the subset of RS-232 circuits required. Shielded bulk cables attached to the D-Sub shells may reduce electrical noise radiated by the cable.  Cable ends may be overmolded or assembled using mechanical backshells.


Where Used

Serial-connected peripherals often use non-standard connectors and device manufacturers will supply their own proprietary cables to connect to a standard connector on a PC. Many such devices may have additional pins to connect to an external power supply.
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