The RS232 Standard

The RS232 Standard Information being transferred between Interfaces Selecting a Cable data processing equipment and In addition to communications between The major consideration in choosing an peripherals is in the form of digital data computer equipment over telephone RS232 cable is, what devices are to be which is transmitted in either a serial lines, RS232 is now widely used for connected? First, are you connecting or parallel mode. Parallel direct connections between data two DTE devices (null modem cable) or communications are used mainly for acquisition devices and computer a DTE device to a DCE device (modem connections between test instruments systems. As in the definition of RS232, cable)? Second, what connectors are or computers and printers, while serial the computer is data transmission required on each end, male or female, is often used between computers and equipment (DTE). However, many 25-pin or 9-pin (AT style)? Usually, it is other peripherals. Serial transmission interface products are not data recommended that the user obtain the involves the sending of data one bit at a communications equipment (DCE). Null two devices to be connected, and then time, over a single communications line. modem cables are designed for this determine which cable is required. In contrast, parallel communications situation; rather than having the pin- to- require at least as many lines as there pin connections of modem cables, null RS232 Specifications are bits in a word being transmitted (for modem cables have different internal TRANSMITTED SIGNAL an 8-bit word, a minimum of 8 lines are wiring to allow DTE devices to VOLTAGE LEVELS: Z needed). Serial transmission is communicate with one another. Binary 0: +5 to +15 Vdc Z beneficial for long distance (called a “space” or “on”) communications, whereas parallel is Cabling Options Binary 1: -5 to -15 Vdc designed for short distances or when RS-232 cables are commonly available (called a “mark” or “off”) very high transmission rates are with either 4, 9 or 25-pin wiring. The required. 25-pin cable connects every pin; the RECEIVED SIGNAL 9-pin cables do not include many of the VOLTAGE LEVELS: Standards uncommonly used connections; 4-pin Binary 0: +3 to +13 Vdc One of the advantages of a serial cables provide the bare minimum Binary 1: -3 to -13 Vdc system is that it lends itself to connections, and have jumpers transmission over telephone lines. The to provide “handshaking” for those serial digital data can be converted by devices that require it. These jumpers DATA FORMAT: modem, placed onto a standard voice- connect pins 4, 5 and 8, and also pins Start bit: Binary 0 grade telephone line, and converted 6 and 20. The advent of the IBM PC Data: 5, 6, 7 or 8 bits back to serial digital data at the AT has created a new wrinkle in RS232 Parity: Odd, even, mark or receiving end of the line by another communications. Rather than having the space (not used with 8-bit modem. Officially, RS232 is defined as standard 25-pin connector, this data) Stop bit: Binary 1, the “Interface between data terminal computer and many new expansion one or two bits equipment and data communications boards for PC’s feature a 9-pin serial equipment using serial binary data port. To connect this port to a standard Transmission Example exchange.” This definition defines 25-pin port, a 9-to-25-pin adaptor cable 111110110000101111 data terminal equipment (DTE) as the can be utilized, or the user can create computer, while data communications his own cable specifically for that CHARACTER TRAILING IDLE "A" BITS LEADING equipment (DCE) is the modem. purpose. IDLE BITS A modem cable has pin-to-pin STOP BIT connections, and is designed to connect PARITY BIT START BIT Data Flow a DTE device to a DCE device. Pin Assignments 25-Pin Style 9-Pin "AT" Style PIN PIN PIN NUMBER NUMBER NUMBER 1 DATA CARRIER DETECT 1 PROTECTIVE GROUND PIN NUMBER SECONDARY TRANSMITTED DATA 14 2 TRANSMITTED DATA DCE TRANSMITTER SIGNAL ELEMENT TIMING 15 DATA SET READY 6 3 RECEIVED DATA SECONDARY RECEIVED DATA 16 2 RECEIVED DATA 4 REQUEST TO SEND RECEIVER SIGNAL ELEMENT TIMING 17 5 CLEAR TO SEND 18 REQUEST TO SEND 7 6 DATA SET READY SECONDARY REQUEST TO SEND 19 7 SIGNAL GROUND/COMMON RETURN 3 TRANSMITTED DATA DATA TERMINAL READY 20 8 RECEIVED LINE SIGNAL DETECTOR SIGNAL QUALITY DETECTOR 21 CLEAR TO SEND 8 9 +VOLTAGE RING INDICATOR 22 10 - VOLTAGE 4 DATA TERMINAL READY DATA SIGNAL RATE SELECTOR 23 11 DTE TRANSMITTER SIGNAL ELEMENT TIMING 24 RING INDICATOR 9 12 SECONDARY RECEIVED LINE SIGNAL DETECTOR 25 5 SIGNAL GROUND 13 SECONDARY CLEAR TO SEND Z-10.

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