RS-232 Cable

The most widespread standard for serial communications in use is EIA/TIA–232–E specs. Developed by the Electronic Industry Association and the Telecommunications Industry Association (EIA/TIA), this standard is more commonly known as “RS–232” where “RS” means “recommended standard”.

An RS-232 interface was once a regular feature for pc, used for connections to modems, storage the information, continuously power supply, and other peripheral devices. Massive voltage swing, the low transmission speed, and huge standard connectors driven by development of the Universal Serial Bus, that has displaced RS-232 from most of its peripheral interface roles.

Many modern pc’s don’t have RS-232 ports and must use external USB to RS-232 converter or internal expansion card with one or more serial ports to attach to RS-232 peripherals. RS-232 devices are used particularly in industrial machines, networking equipment, and scientific instruments.


RS–232 is a “complete” standard. This means that the standard sets out to guarantee compatibility between the peripheral systems and host by specifying 1) common pin wiring configurations, 2) common voltage and signal levels, and 3) a lowest quantity of control data between the peripheral systems and host.


The electrical characteristics section of the RS–232 standard includes specifications on voltage levels, line resistance, and rate of change of signal levels.

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In 1962 was defined the first RS–232 standard, and at that point this standard doesn’t use 5V and ground logic levels. In place of that, a high level for the driver output was set as being +5 to +15V and a low level for the driver output was define between –15 and –5V. The receiver logic levels were set to supply a 2V noise margin. As such, a high level for the receiver is set as +3 to +15V and a low level is -15 to -3. Figure 1 shows the logic levels define by the RS–232 standard. it’s needful to notice that, for RS–232 communication, a low level (-15 to -3V) is set as a logic one and is historically mentioned as “marking”. As well a high level (+3 to +15V) is set as a logic zero and is mentioned as “spacing”.

The RS–232 standard additionally limits the maximum slew rate at the driver output. This limitation was enclosed to assist reduce the chance of cross–talk between adjacent signals. Maximum slew rate allowed is 30 V/µs. In addition, a maximum data rate of 20k bits/sec has been defined by the standard. For the initial RS–232 standard, the cable between the driver and the receiver was specified to be a maximum of 15m long.


RS–232 standard don’t include only specifications on electrical characteristics. The second aspect of operation that’s lined by the standard considerations the functional characteristics of the interface. This basically implies that RS–232 has defined the function of the various signals that are utilized in the interface. These signals are separated in four different categories: control, timing, data and common. Table 1 shows the signals that are defined by the RS–232 standard. As we can see from the table there’s  a overwhelming number of signals defined by the standard.

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The third area lined by RS–232 considerations the mechanical interface. RS–232 specifies a 25–pin connector and this is the minimum connector size that can accommodate all of the signals defined within the functional portion. In Figure 2 is shown the pin assignment for this connector. The connector for data communications equipment is male for the connector housing and female for the connection pins. The 9–position DB9S connector is the most popular and supply the means to transmit and receive the required signals for modem applications.

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Modem applications are one of the foremost common uses for the RS–232 standard. Figure 4 shows a characteristic modem application use the RS–232 interface standard. As we can see in the diagram, modem is the DCE and computer is the DTE. Communication between every PC and its associated modem is accomplished using the RS–232 standard. Communication between the two modems is accomplished via telecommunication. Several modem applications need only nine signals.

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As we saw in the section on RS–232 electrical characteristics, RS–232 doesn’t use the strandard 0 and 5V levels implemented in TTL and CMOS designs. Drivers need to provide +5 to +15V for a logic zero and for logic one -5 to –15V. This implies that additional power supplies are required to drive the RS–232 voltage levels. Usually, a -12V and a  +12V power supply are used to drive the RS–232 outputs. This is a good unpleasantness for systems that haven’t any different needs for these power supplies.

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