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RS-232

 

 

The Electronic Industries Alliance (EIA) standard RS-232-C as of 1969 defines:

  • Electrical signal characteristics such as 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.
  • Functions of each circuit in the interface connector.
  • Standard subsets of interface circuits for selected telecom applications.

The standard does not define such elements as

  • character encoding (for example, ASCII, Baudot or EBCDIC)
  • the framing of characters in the data stream (bits per character, start/stop bits, 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. Many modern devices support speeds of 115,200 bps and above
  • 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 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 original DTEs were electromechanical teletypewriters and the original DCEs were (usually) modems. When electronic terminals (smart and dumb) began to be used, they were often designed to be interchangeable with teletypes, and so supported RS-232. The C revision of the standard was issued in 1969 in part to accommodate the electrical characteristics of these devices.

Since application to devices such as computers, printers, test instruments, and so on were not considered by the standard, designers implementing an RS-232 compatible interface on their equipment often interpreted the requirements idiosyncratically. Common problems were non-standard pin assignment of circuits on connectors, and incorrect or missing control signals. The lack of adherence to the standards produced a thriving industry of breakout boxes, patch boxes, test equipment, books, and other aids for the connection of disparate equipment. A common deviation from the standard was to drive the signals at a reduced voltage: the standard requires the transmitter to use +12V and -12V, but requires the receiver to distinguish voltages as low as +3V and -3V. Some manufacturers therefore built transmitters that supplied +5V and -5V and labeled them as "RS-232 compatible."

Later personal computers (and other devices) started to make use of the standard so that they could connect to existing equipment. For many years, an RS-232-compatible port was a standard feature for serial communications, such as modem connections, on many computers. It remained in widespread use into the late 1990s. While it has largely been supplanted by other interface standards in computer products, it is still used to connect older designs of peripherals, industrial equipment (such as based on PLCs), and console ports, and special purpose equipment such as a cash drawer for a cash register.

The standard has been renamed several times during its history as the sponsoring organization changed its name, and has been variously known as EIA RS 232, EIA 232, and most recently as TIA 232. The standard continues to be revised and updated by the EIA and since 1988 the Telecommunications Industry Association (TIA). Revision C was issued in a document dated August 1969. Revision D was issued in 1986. The current revision is TIA-232-F Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange, issued in 1997. Changes since Revision C have been in timing and details intended to improve harmonization with the CCITT standard V.24, but equipment built to the current standard will interoperate with older versions.

Power is defined as the rate of flow of energy past a given point. In alternating current circuits, voltage and current only remain in phase if the load is purely resistive. When this happens the power is said to be 'real power'. If instead the load is purely reactive (either Capacitive or Inductive), all of the power is reflected back to the generator as the phase cycles. The load is said to draw zero real power, instead it draws only 'reactive power'. If a load is both resistive and reactive, its will have both real and reactive power, resulting in total amount of power called the 'apparent power'.

The currents and voltages are forced out of synchrony by attached devices that store and release energy in different ways at different times. In AC power systems, attached loads that store energy behave like combinations of coils and capacitors. Coils store power as magnetic fields, behave something like "electrical flywheels" and delay changes in the current. Capacitors store power as electric charge, behave something like "electrical springs" and therefore advance changes in currents.

The portion of power flow averaged over a complete cycle of the AC waveform that results in net transfer of energy in one direction is known as real power. The portion of power flow due to stored energy which returns to the source in each cycle is known as reactive power.

In reality there are losses along AC power transmission lines, meaning a purely reactive load, while drawing no real power itself, consumes power because the supplied and reflected power dissipate away on the transmission line, and energy is wasted. For this reason an AC load should be designed to have as little reactive power as possible. In most jurisdictions the power factor (percentage of apparent power that is real power) must be at least a certain percentage (typically 90%, 95% or 99%), otherwise extra charges may apply above what is recorded on a power meter, as power as been reflected back up the transmission line and wasted.

 

 


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