What is RS-232?

By: CableOrganizer®


Recommended Standard 232 — best known by its acronym RS-232 — was created in 1960 to define telecommunication signal connections. The standard reflects signals that link from data terminal equipment (DTE) like computers to data communication equipment (DCE) such as a modem. RS-232 characterizes the signals’ characteristics and the connectors’ pinouts.

The Electronic Industries Alliance (EIA) amended RS-232-C (the third version) definitions in 1969 to encompass:

  • • Electrical signal characteristics — It outlined voltage levels, signaling rate, timing and slew-rate of signals, voltage withstand level, short-circuit behavior and maximum load capacitance.
  • • Interface — Mechanical characteristics, pluggable connectors and pin identification were among topics.
  • • Circuit Functions — Each one addressed in the interface connector.
  • • Standard Subsets of Interface Circuits — Subsets were presented for selected telecom applications.

Some elements in the RS-232-C standard were not defined, including:

  • • Character encoding (for example, ASCII, Baudot or EBCDIC)
  • • The framing of characters in the data stream (such as bits per character, start/stop bits and parity)
  • • Protocols for error detection or algorithms for data compression
  • • Bit rates for transmission, although the standard says it is intended for bit rates lower than 20,000 bits per second.
  • • Power supply to external devices.

Details of character format and transmission bit rate are controlled by the serial port hardware, often a single integrated circuit called a UART (universal asynchronous receiver-transmitter) that converts data from parallel to serial form. A typical serial port includes specialized driver and receiver integrated circuits to convert between internal logic levels and RS-232 compatible signal levels.

The early DTEs were electromechanical teletypewriters, while the first DCEs were frequently modems. When both smart (complex) and dumb (without capabilities to process) electronic terminals emerged onto the scene, they supported RS-232 and were compatible with teletypewriter (TTY) machines. The C revision of the standard was issued in 1969 in part to accommodate these devices.

The process for interpreting the RS-232 analysis of computers and peripheral devices was random. Standards were not followed systematically with issues cropping up that included absent or erroneous control signals and pin assignments on connector circuits that did not comply with the guidelines. This deviation led to the creation of equipment to compensate for the inconsistences. Signals, for example, were created at lower voltages (such as +5V and -5V instead of +12V and -12V) without identifying them, while still earmarking them “RS-232 compatible.”

Computers and other items that were produced in later years until the late 1990s adhered to RS-232 to prevent equipment compatibility issues, with RS-232 compatible ports a serial communications standard for the modem ports on personal computers. Other standards have since superseded it, but it is still actively applied to some products that need to connect with earlier peripherals.

RS-232 has evolved since its first version in 1960. There was no new version again after the 1969 one until 1986, with EIA-232-D. The Telecommunications Industry Association (TIA) began overseeing the standards in 1988, issuing EIA-232-E in 1991 and TIA-232-F in 1997 (with reaffirmation notices through 2012). The name of the most current publication through 2012 is Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange.


Power is the rate a circuit’s energy flows past a given point. “Real power” manifests as a purely resistive load when the voltage and current in alternating current (AC) circuits are in phase. With purely reactive loads (a load where no circuit power is spent, with “capacitive” for stored energy and “inductive,” which modifies currents into magnetic waves), power is returned to the generator as the phase completes its cycle. No real power is drawn for the load with “reactive power.” When a load is combined of resistive (one that only absorbs real, “active” or “true power”) and reactive power types, it is referred to as “apparent power.”

Voltages and currents are no longer simultaneous because of generators, capacitors and inductors that release and collect energy at various periods in AC power systems. Coils keep power as magnetic fields like “electrical flywheels,” which move and become kinetic energy that causes delays in the current flow. On the other hand, capacitors hold onto power in a way that creates electric charges, initiating current flow similarly to electrical springs.

Real power is the average of a portion of power flow during a full cycle of an AC waveform, concluding with a final energy transfer in one direction. Reactive power is created from reserved energy, which ends at the source after every cycle. AC power transmission lines experience losses when power loads are entirely reactive, not pulling from real power but wasting energy instead as it takes power because of supplied and reflected types dissolving on the power lines. AC power loads should have minimal reactive power as a result. Extra charges could apply when the power factor (the percentage of “working power” or real power within apparent power) does not reach a particular threshold (usually between 90% and 99%) because of wasted power documented on the power meter.

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