So TIA-232 connects two DTE! and the sender must be connected to the receiver and vis-versa meaning the cables must be crossed! Today as modems are seldom used with TIA-232 all practically all our devices are computer or microcontroller (e.g. So the straight connection cable from DTE to DCE uses a female connector (DTE (PC) side) and a male connector on the DCE side. Later the DB-25M connector (no more used today) was replaced with the smaller DE-9M connector (sometimes mistakenly called DB-9).įor computer the male connector was used (DB-25 female was the parallel port) and for the modem the female connector. The first computer and modem used the D-subminiature 25-pin connector recommended by the first revisions of the standard. On the microcontroller side we get the opposite! Connectors and cables The RS-232 control and message lines have a positive logic (3-15 V = HIGH), and the data lines a negative logic (3-15V = LOW). We get two data lines, two control lines (DTE to DCE) and 4 message lines (DCE to DTE). TIA-232 Signals and connectors Data and control signalsĬommonly used RS-232 signals and pin assignments for DB-25 and DB-9 connectors: DB-9ĭTE is ready to receive, initiate, or continue a call.ĭTE requests the DCE prepare to transmit dataĪll the signals are named from the standpoint of the DTE. RS-232 is because of its simplicity still used widely on microcontroller, but also in networking equipment, industrial machines, and scientific instruments where a point-to-point, low-speed and wired data connection is adequate. USB to serial adapters and cables are used today to connect serial devices to a computer. Later personal computers got an RS-232-compatible port for serial communications and used the standard for mice and keyboards. When computer terminals began to be used, they often had to be interchangeable with teletypewriters, and so supported RS-232. With the limits of seven-bit hardware from that time, ASCII represents each of the 128 characters with a numeric value. It consists of 32 control characters like linefeed ( LF, 0x0A) or carriage return ( CR, 0x0D) to control the teletypewriter or terminal) and 96 characters-letters, numbers, or punctuation marks. The American Standard Code for Information Interchange ASCII was released to serve as a common language among computers. An IBM engineer ( Bob Bemer) contacted the American National Standards Institute ( ANSI) to develop a single code for computer communication. 1 kHz for LOW and 2 kHz for HIGH) to represent the two binary states.Īt this time (1961) over sixty different ways of representing characters were used, and machines from different manufacturers could not communicate with one another. The simplest method is a frequency-shift keying FSK modulation, where we use 2 sine tones (e.g. The modulator-demodulator (modem) was necessary to transmit digital signals which need a huge bandwidth (see Fourier in ELEFU) over the telephone line witch had only 3.4 kHz of bandwidth. The standard was needed to connect electromechanical teletypewriters (or dumb computer terminals), called data terminal equipment ( DTE) to a data communication equipment ( DCE, also data circuit-terminating equipment) like a modem. The Recommended Standard 232 ( RS-232) was originally introduced in 1960 as telecommunication standard. The terms MARK and SPACE are still used in the RS-232 standard. Telegraph operators noticed that the sound the needle made when scratching the paper was enough to get the message ans so the drum was replaced by a speaker. With an electromagnet it was possible to lift the needle away from the paper creating a space. Morse's first system had a needle contacting a rotating drum of paper that made a continuous mark. Morse made a first practical fully serial binary system with his Morse Code (first only uppercase letters). Sometimes a little history helps to better understand why things are what they are. This will give us a basic knowledge that we can transfer to other protocols. We will try in this chapter to understand one protocol in depth. Song of this chapter: Gordon Lightfood > Summertime Dream > Protocol
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