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 Overview of RS-232 and V.24 The Information Factory is providing you the first 5 topics of our video training course "Overview of RS-232 and V.24" so that you may get a feel for the content and technical level of the course.
One of the big mysteries in data communications is why does everything use a different cable to connect to a modem? We have all heard that there are supposed to be standards for connecting computers and other devices to modems but, in reality it never seems to work like the book says. Or, to put it another way "if there is an industry standard for connecting things to modems, why isn't there a standard cable?" This textbook and its associated videotape will provide an introduction and overview of the EIA RS-232 and CCITT V.24 standards. The purpose of these standards is to provide manufacturers with a common interface for the electrical connection between modems, computers and terminals. One problem is that the use of the labels "RS-232" and "V.24" have become generic terms to describe almost any part of the connection between a computer and a modem. You will hear or see references to "RS-232 cables," "V.24 cables" and "RS-232" or "V.24 ports" on computers. We need to begin our course by defining what RS-232 and V.24 standards recommend and where they fit in when connecting a computer and modem. Modems and computers are usually built by different companies. There are several reasons for this including:
The modem manufacturers had to insure that it would be easy to connect modems and data communications systems or terminals, so they worked with the telephone companies to provide a common interfacing method. In the United States a recommendation was released through the Electronic Industries Association or EIA. This recommendation was given the designation RS-232 and described the methods of providing an electrical interface between modems and data communications equipment for speeds up to 19,200 bits per second. Internationally a recommendation was developed by the Consulting Committee on International Telephone and Telegraphy or CCITT. The CCITT released their interface as Recommendation V.24 and it is almost identical to EIA RS-232. Throughout this course we will refer to EIA RS-232 and only reference CCITT V.24 when there may be a difference to explain. As shown in Figure 1, RS-232 defines the modem and the telephone company as the "Data Communications Equipment" or DCE. The standard then defines the computer or terminal as the "Data Terminal Equipment" or DTE. According to RS-232 the DTE and the DCE meet at the rear of the modem in a standard 25 pin connector. The use of the terms DTE and DCE to define the two sides of the interface are extremely important in understanding the RS-232 standard.
RS-232 also defines the connector on the rear of the modem and the various electrical signals that will appear on its twenty-five pins. As shown in Figure 2, the standard defines the interface on the modem but, it does not define the other end of the cable where it connects to the computer or terminal.
The EIA is a non-profit organization of manufacturers and other parties that develop standards like RS-232 for connecting different types of electrical equipment. The EIA standards are also known as "recommendations" or "recommended standards" which means that a manufacturer doesn't have to adhere to them when building computers or modems and they are not required to use all of the signals or pins that are defined in the interface. Because RS-232 is a "recommendation" and because it only defines the interface at the modem, it means that whenever we change computers or terminals, we may have to change the cable leading to our modem. By not defining the connection at the computer end of the cable, the standard allows flexibility for the computer manufacturer. As an example most personal computers use a small 9 pin connector for their modem port which gives them enough room to provide multiple ports on one vertical card slot. This ability to implement different parts of the specification might sound confusing but, it allows manufacturers flexibility when incorporating an RS-232 interface into their equipment. At the end of the textbook we will give further information on the Electronic Industries Association (EIA) and the Consulting Committee on International Telephone and Telegraph (CCITT) including how to contact them and purchase copies of their standards. THE RS-232 AND V.24 INTERFACES THE MODEM CONNECTOR The RS-232 standard defines the connector on the rear of the modem as a twenty-five pin female connector. The type of connector used here has become known by several nicknames including the "RS-232 Connector." "Modem Connector," or "Data Connector." The proper designation is a twenty-five pin miniature D Shell connector. The name D Shell comes from the outside shell of the connector being shaped like the letter D as shown in Figure 3.
The RS-232 interface allows the connecting cable between the computer and the modem to be up to 15 meters in length or the equivalent of about 50 feet. You will hear objections to this distance parameter from people who will say they have run RS-232 interface cables in lengths of over 30 meters (or more than 100 feet). These objections can be correct because the standard has some qualifying points, the most important being the operating speed. What the standard actually defines is a limit of 15 meters when operating at the maximum bit rate of 19,200 bits per second as shown in Figure 4. This means that if you lower the operating speed, you can run a cable longer than 15 meters.
Technically the reason that RS-232 cables have a distance limitation has to do with the definition of the electrical signals on the cable. RS-232 defined a signal with a "fast rise time." This meant when a signal switches from positive to negative, it happens in about 2 microseconds. Signals with fast rise times tend to generate more electrical noise and require a cable with low capacitance. Fast rise time signals also tend to deteriorate along the length of a cable. Looking at the standard it is easy to ask why did they limit it to 15 meters? In retrospect though, the standard was originally intended to connect data devices to a modem a short distance away. The designers never expected RS-232 to become a standard way of connecting terminals and computers throughout buildings without modems. Some vendors sell low capacitance or "low cap" cables which allow longer distances for RS-232 interfaces. These types of cables change the electrical or transmission characteristics of the wires in the cable to offset some of the deterioration of the signal. Remember though that speed is still the main key to distance even with low cap cables. If you lower the speed, you can increase the distance. DEFINING AND GROUPING THE SIGNALS Now that we have defined the connectors and cables in RS-232 and V.24, we need to begin looking at what signals these cables and connectors carry. RS-232 has a number of electrical signals which carry control functions, binary data and timing. It is not difficult to understand these signals because RS-232 and V.24 are put together in a logical fashion which allows us to arrange the signals into a few basic groups and functions. The first group of signals are the "Control" signals (Figure 5) which perform two functions in the interface. The first function of the control signals is to turn things "on" and "off." So when the computer wants to turn on a function in a modem, it will use one of these control signals to perform the task.
The second function of the control signals is to tell whether something is "on" or "off." When the modem wants to let the computer know if a function is on, it will use a control signal. The second group of signals are the "Binary Signals" as shown in Figure 6. These are the actual binary data streams of ones and zeros going into or out of the modem.
The final group of signals we encounter in the interface are called "Timing" signals. These signals are used by certain modems to synchronize the binary data flow between computer and modem.
RS-232 has signals defined for 22 of the 25 pins on the D Shell Connector at the rear of the modem. This number of signals shouldn't discourage you because many of them are redundant or duplicate. As we will see most modems operate with only about ten to fifteen of the signals.
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