RS232 pinouts, wiring, protocol, signals

Recommended Standard 232

RS232 serial communications, pinouts, wiring,
protocols and signals covered in detail

RS232 has been used with PC's (Personal Computers) practically since they've been around. The abbbrieviation RS232 actually stands for 'Recommended Standard 232' of The Electronics Industries Association (EIA).

It is a standard for serial (bit by bit) binary data signals communication between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment).

Even today the standard remains amidst the onslaught on USB devices, it's a legacy that's not easily dispensed with, it's serial and staying here... Albeit in one guise or another.

It is commonly used in todays PC's serial ports, it is defined and referred to (by techies) as asyncronous or syncronous.

Asyncronous means that the communication packets of data are not sent in defined time slots. Syncronous is the opposite of Asyncronous, the data packets are sent within defined time slots.

The Recommended Standard
The Electronics Industries Association (EIA) standard RS-232-C[1] as of 1969 defines certain points such as;

  • Electrical signal characteristics such as voltage levels
  • signaling rate
  • timing and slew-rate of signals
  • voltage withstand level
  • short-circuit behavior
  • maximum load capacitance
  • interface mechanical characteristics
  • pluggable connectors
  • 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 code 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
  • power supply to external devices
  • bit rates for transmission

Although the writing of the standard says that it is intended for data bit rates slower than 20,000b/s (b/s = bits per second). Devices these days support speeds of 115,200b/s and higher.

Details of the character set format and transmission bit rate are controlled by the serial port hardware, often a single integrated circuit (IC) called a UART (Universal Asyncronous Receiver Transmitter) that converts data from parallel to asynchronous start-stop serial form.

Details of voltage levels, slew rate, and short-circuit behavior are typically controlled by a line-driver electronic circuit that converts the UART's logic voltage levels to RS-232 compatible signal voltage levels, and a receiver that converts back from RS-232 compatible signal voltage levels to the UART's logic voltage levels.

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