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 RS 232-compatible ports are intended to be connected by at the most a few tens of metres (yards) of cable. Other serial communications standards are better adapted to drive hundreds or thousands of metres of cable.

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. The cables may be between 3 and 25 conductors; often 4 or 6 wires are used. Flat RJ (phone-style) cables may be used with special RJ-RS232 connectors. Cables are often unshielded, although shielding cables may reduce electrical noise radiated by the cable.

For configuring and diagnosing problems with RS-232 cables, a "breakout box" may be used. This device normally has a female and male RS-232 connector and is meant to attach in-line; it then has lights for each pin and provisions for interconnecting pins in different configurations.

Serial-connected peripherals often use non-standard connectors and the manufacturers will supply their own proprietary cables for connecting the device to a standard DE-9M connector on a PC. Many such devices such as the Garmin eTrex and other GPS receivers use a 3-pin serial interface but have additional pins on the device itself for connecting to an external power supply. These pins either branch off from the serial cable to the power supply or can be used with a purpose built power cable.

See also

• Direct cable connection
• InterLnk
• LapLink cable (can be seen as a parallel equivalent to a serial null modem cable)
• Legacy port
• Network bridge

References

• Serial cables schemes
• RS232 Specifications and standard
• Serial Cables pinouts and specifications

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