For example, if you have a mouse on COM1, avoid moving it to COM2 if at all
`possible. If you move it, you may need to change the configuration of each ofyour
`application programs that use a mouse.
`
`To attach a serial device to your computer, you need a serial port. DOS can access
`them only if they are physically present on your computer. As explained earlier, a
`serial port comprises electronic circuitry that contains a UART or equivalent chip that
`performs the necessary data manipulation for serial data to be sent from the port.
`
`Most PCs, made after about 1985, include two serial ports as standard. These may be
`built into the computer‘s main system board or may be in an expansion board. Earlier
`PCs typically had only one serial port. However, you can purchase inexpensive expan-
`sion boards that contain a serial port {or two), or you can purchase an internal
`modem that incorporates a serial port.
`‘
`
`All serial ports that you can plug serial devices into are revealed by a connector in the
`computer’s case. This connector will be either a 25-pin D-type connector (so called
`because it is D-shaped) or a 9-pin D-type connector. The typical PC standard is for this
`connector to be male (have pins rather than holes), but some early PCs had female
`connectors. (The serial port on an internal modem cannot be used to plug in a
`different serial device.)
`
`If your display adapter is an EGA or earlier, it may also have a 9-pin D-type connector.
`This must be used to attach your monitor and is not a serial port.
`
`To communicate by conforming with the RS-232 standard, you do not actually need to
`use all the wires in the 25-pin connector. In fact, all the required connections can be
`made with the 9-pin connector. IBM used the 9-pin connector in the IBM AT for space
`considerations (a 9-pin connector is much smaller than a 25-pin connector). Conse-
`quently, many PC compatibles also use the 9-pin connector.
`
`Note: The serial ports with a 9-pin connector cannot be used for synchronous commu-
`nication because this requires more than 9 wires for the signals. Unless you are using a
`leased line (covered earlier in this chapter), you will be using asynchronous communi-
`cation and can use the 9-pin connectors without a problem.
`
`You typically assign a serial port’s name, address, and interrupt number by altering
`switches or changingjumpers on the circuit board. Some serial ports can be adjusted
`by using configuration software instead of physically changing switches.
`
`If you are adding an internal modem, the serial port is incorporated into the expansion
`board, and you need to assign an available port name, address, and interrupt number.
`
`Chapter 4: Selecting Your Equipment
`
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`As with the serial ports, this may be achieved by physically altering switches on the
`board or by running software. The manufacturer’s documentation is essential reading.
`
`An external modem can be plugged into an available serial port in your computer.
`Whether you add an internal or external modem, you need to write down the serial
`port name, address, and interrupt number. You need this information for your
`communications software. If any of your hardware or software does not work after
`installing a modern, the first question asked by technical support will be “Do you know
`your serial port name, address, and interrupt number?”
`
`As more and more PCs include extra devices, such as sound boards and network
`
`adapters, the port address and interrupts become an important issue. There are a
`limited number of addresses and an even more limited number of interrupts. Because
`you only want one device to respond at a time, the assignments can become tricky.
`
`For example, most network adapters are configured to use interrupt 3 when supplied.
`If you then try to add a second serial device, you may experience conflicts because the
`second serial device may be configured to also use interrupt 3. These conflicts may not
`occur when you are not actually using the second serial port but do occur when you try
`to add a modem. In this case, you may need to change the interrupt number assigned
`to your network adapter or to your second serial port.
`
`Several programs — commercial, shareware, and free — are available that will interrogate
`your computer hardware and teH you what devices are assigned to which ports. Microsoft
`Diagnostics (MSD), shipped as part of DOS 6 and later, is particularly convenient, but
`your computer or modern may be supplied with a utility program. Many Gateway 2000
`computers, for example, are supplied with QA Plus, and Qualitas‘ 386MAX memory
`management program is supplied with ASQ, a system analysis program.
`
`You can use a serial device on COM1 and another on COM3, and sirnilarly a serial device
`on COM? and another on COM4, if you usually only use one serial device at a time.
`
`For example, you may have a mouse on COM1 and a plotter or serial printer on
`COM3. Because you use the printer only when printing and the mouse to move the
`cursor around the screen, you can probably make both devices work successfully.
`
`Problems arise when you try to use both devices at once. For example, ifyour program
`moves the mouse around while printing to a serial device, you can end up sending
`incorrect information to the mouse and the serial printer, and neither will work
`correctly. (This does not occur with parallel printers and mice, because the mouse and
`printer are configured as different devices with different port addresses.)
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`This conflict is particularly important for modems and mice. As a general rule, ifyou
`are using a mouse on COM], avoid COMB for your modem; and if you are using a
`mouse on COME, avoid COM4 for your modem. Read the documentation that comes
`with your modem to help avoid problems.
`
`Choosing a Modern
`
`Like PCs, modems are advertised with lots of buzzwords, and different manufacturers
`
`emphasize different features to make a particular modem stand out from the crowd.
`
`Also like PCS, your particular needs will be different from other people’s require-
`ments, and selection really is a matter of personal preference. You can buy very
`inexpensive modems that will probably operate under most conditions but may lack
`the extra features, technical support, warranty, or company reputation, and not be
`tolerant ofslightly noisy phone lines or work with all other modems.
`
`In contrast, you also can buy expensive topof—t.he4ine modems from well-known compa-
`nies with every conceivable feature, excellent technical support, and good warranties.
`However, you may not need all these features and may be wasting your money.
`
`One important difierence between modems and PCs is that you need two modems to
`communicate, but many PCs never have their compatibility with other PCS tested. Any
`modem you buy, including fax modems, must be able to establish, maintain, and discon-
`nect with another modem, which may or may not be of the same brand or standard.
`
`Choosing a data modem, fax modem, or
`combined unit
`
`Your first choice is relatively easy. Decide whether you need or will need a data
`modem, a fax modem, or a modem that combines data and fax capabilities. (Many
`new model modems automatically include a fax modem.)
`
`If you know you want to transfer files between computers, call such online services as
`CompuServe or BBSs, or link your computer to another computer through telephone
`lines, you need a data modem.
`
`Chapter 4: Selecting Your Equipment
`
`l.-:35"
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`If you want a substitute for a star1d—alone fax machine and want to send documents
`(equivalents of pieces of paper), consider a fax modem. As the concept of the elec~
`tronic desktop becomes more of a reality, where nothing is handed around on pieces
`of paper, the desirability of PC fax modems increases. As discussed in a later section,
`choosing a fax modem requires careful consideration.
`
`Choosing the modem’s form factor
`
`.«
`I\
`
`After deciding whether you need a data modem or combined data modem and fax
`modem, you need to assess your current computer system, because this may limit your
`options. If you are going to use the modern with a desktop or tower computer, you
`have two main choices: internal or external.
`
`An internal modem fits into an expansion slot inside your computer and incorporates a
`serial port. You need an available slot in which to place it, and you need to assign it an
`unused serial port name. If you already have four serial ports on your computer, you
`need to remove one to use an internal modem.
`
`If you have a PS/2 computer and want to use an internal modem, you may need to
`select one that uses the Micro Channel Architecture (MCA) bus connection rather
`
`than the more common Industry Standard Architecture (ISA) or PC bus connector.
`(Not all PS/ 2 computers have the MCA bus; some low-end computers, such as the
`Model 25, use the ISA bus.)
`
`If you want an external modem, you have two general choices. The typical box style is
`about 6 inches wide, 1% inches high, and 10 or 1] inches deep, although some
`companies offer futuristic shapes rather than the boring box. Alternatively, you can
`choose a small modem, often known as a pocket modem, that can be used with a desktop
`or tower computer or (because of its small shape) taken on the road with a laptop.
`
`If you want to use a modern with a laptop or notebook, you can use an external
`modem and attach it to a serial port Some laptops come with built~in modems or
`modems that are available from the computer manufacturer as upgrade options.
`Alternatively, if your computer has a PCMCIA type 2 slot, you can purchase a PCMCIA
`modem that plugs into the expansion slot in your notebook. (These may be products
`offered by the computer manufacturer or a third-party company.) These modems are
`also small and lightweight for relatively easy travel.
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`If you are not limited in your choice, consider the following advantages of external
`modems:
`
`E
`8
`in Most external modems have LEDs (li ht-emitin diodes) on the front that
`
`give an indication of the current modem status.
`
`5 You can use the serial port for other purposes, such as attaching a plotter.
`
`fir You can move the modem from one computer to another. This is good if
`you own multiple computers or decide to upgrade your computer.
`
`it External modems have a separate power supply and do not use PC power.
`(This is only a factor with older PCs that don’t have a power supply sufficient
`to power the modem along with everything else.)
`
`Consider the following advantages of internal modems:
`
`it
`
`Internal modems do not need a data cable connecting the serial port to the
`modem.
`
`in Internal modems come with a built-in serial port.
`
`it You don’t need extra room for the modem on the desk.
`
`it You don’t need an extra electrical outlet for the 1nodem’s power supply.
`
`Apart from the physical shape of your modem, you need to consider its features. Some
`modems can transfer data faster than others. The majority of buzzwords, in particular
`the ones that are not acronyms or abbreviations, used by communications people are
`related to the communications standards. These standards dictate the speed at which
`data can be transferred from one computer to another. In some standards, more data
`is sent at once. In others, the data is compressed before transmission and
`uncompressed by the receiving modem, which results in more data being sent in a
`certain amount of time.
`
`Selecting compatibility
`
`For PCs, the first consideration is to buy only a Hayes-compatible or AT-compatible
`modem. (As explained in Chapter 5, the AT is not the same as IBM’s AT but refers to
`the two characters that are used as the start of modem commands.)
`
`The analogy with PCs is worth reiterating here. The term PC-compatibie has come to
`mean PCs that run and support all software and hardware designed for the IBM PC. In
`some cases, the PC- compatible standard has been expanded and enhanced by other
`
`Chapter 4: Selecting Your Equipment F
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`manufacturers, and these extras may be desirable features. In other cases, increasingly
`less significant nowadays, PC compatibility may mean supporting almost ali software
`and hardware designed for the IBM PC. This is where name brand —~ not necessariiy
`IBM, but a major computer manufacturer — plays an important part in the support
`you will get from the computer manufacturer and software vendors when you find an
`incompatibility. They are more willing to support a computer that has brand recogni-
`tion, because there are more likely to be many customers with the same computer.
`
`The term AT-compatible is comparable. Buying a modem that claims to be AT-
`compatible is essential for you to use PC-communications software easily and conse-
`quently make a connection with another modem easily. However, it does not guaran-
`tee full compatibility and/or it may buy you extra features that are not available on all
`other AT-compatible modems.
`
`For most PC. users, there is only one type of telecommunication, and you use an AT-
`compatible modem to link with other modems. This type of communication is called
`more formally asynchronous communication. Asynchronous (as opposed to synchronous)
`communication is communication that occurs between two computers without regard
`to the precise clock timing sequences of the connected computers.
`
`Synchronom cornmunication is used between two computers when the events at each end
`of the phone line must occur in sync with (or in step with) the devices and computers
`attached to each other. People who need synchronous communication are usually
`aware of it, because they either have specialized computer knowledge or are supported
`by people who do. The most common implementation is linking a terminal to a
`mainframe computer. You can purchase specialized adapter boards for your PC that
`perfonn this type of communication.
`
`However, if you need synchronous communications, you can purchase modems that
`will do both asynchronous and synchronous communication. You can do synchronous
`communication on dial-up phone lines and don't need a specialized synchronous
`communication adapter.
`
`Note: The Hayes Autosync protocol handles the situation this way. The synchronous
`application taiks to the Autosync driver, and the Autosync driver sends special
`formatting signals to the modem so that the modem can convert the data. The
`modem has firmware in it that converts the asynchronous data into synchronous and
`puts it out on the phone line. Between the computer and the modem, the connection
`is asynchronous, but to the software, it appears to be a synchronous channel, and the
`data going over the phone line is properly formatted synchronous data. It’s a clever
`way of tricking the hardware to do what the software needs to do.
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`Modem communications standards
`
`Modems can operate at a variety of speeds and with different options that affect the
`effective data rate and insensitivity to data errors. A modem's data transfer rate is
`specified in bits per second (bps). However, this is not the only number you need to
`consider. When choosing a modem, you want to pick the most advanced standard you
`can afiford.
`
`Remember, however, that you must communicate with a second modem. The fastest
`and the highest standard that you will be able to communicate to second modem is
`the highest common denominator. For example, if you are only going to communicate
`with a 1200 bps modem, you do not need the fastest modem on the market.
`
`The history ofthe modem provides insight into the apparently random numbering of
`communications standards and the apparent duplication of standards for certain data
`rates. You simply need to understand that from the time of the very first modem in the
`19205, people have wanted to send more data faster.
`
`As a consequence, when a national or international standard for faster transmission
`had not been defined but public demand existed, manufacturers created proprietary
`standards to accommodate their customers. However, in most cases, the manufactur-
`
`ers also supported existing standards as fallback positions. If a modern was not con-
`nected to a modem that could support the faster standard, a fallback, and probably
`slower, standard was agreed on by the modems and used instead.
`
`You don’t have to worry about whether the modem you are calling has a particular
`standard. You set your modem to communicate with all its features, and it negotiates
`with the other modem automatically to find the best common denominator. However,
`when you buy a modem, you need to know which features you want, because its top
`feature will be the best at which you can communicate.
`
`In the U.S., early microcomputer modems conformed with the Bell 103 standard. This
`transmits data at 300 bps. Although now rarely used, it is still supported by most high-
`speed modems as the final fallback standard. The next standard in the U.S. that is still
`used is Bell 212A. This transmits data at 1200 bps.
`
`However, in Europe and the rest of the world, the two equivalent standards that were
`adopted were specified by the United Nations agency Consultative Committee on
`International Telephony and Telegraphy (CCITT) and were V21 for 300 bps trans-
`missions and V22 for 1200 bps. CCITT is now renamed ITU~T (International Tele-
`communication Union).
`
`Chapter 4: Selecting Your Equipment
`
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`.
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`.
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`..
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`.
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`..
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`.
`
`Unless you purchase a very old modem, most modems support both Bell 212A, Bell
`103, as well as V21 and V22. Bell 212A or V22 are likely to be the lowest standards
`your modems will ever use.
`
`For speeds above 11200 bps, U.S. manufacturers tend to adopt the international
`standards as they become available. However, there are some modems that also
`incorporate proprietary standards that you can use only when calling a modem of the
`same brand.
`
`The 2400 bps standard is V.22bis (pronounced bizz or bis: depending on who you talk
`to), and the 4800 bps and 9600 bps standard isV.32. The 14,400 bps standard is V.32bis.
`
`So far, apart from slightly strange numbers, the bigger the standard number, the faster
`the modern, and the addition of iris to the standard number means a faster modem. As
`an additional help, most manufacturers use the data speed as part of the product’s
`name to indicate its maximum speed.
`
`Two more important standards are commonly available on newer modems: V.42 and
`V.42bis. Both of these standards may be available on any 1200 bps or faster modem.
`
`A modem that supports V.4‘2 can do-error-checking on your data to help ensure that
`the data received is the same as the data sent. (Error—checking is explained in more
`detail in Chapter 6.)
`
`A modem that supports V.42bis can compress and uncompress the data you are
`transmitting. As a consequence, you send more data between the modems in a given
`amount of time. Although manufacturers clearly tout the V.42 and V.42bis support,
`they do not usually make the potentially faster data-throughput speed with data
`compression part of the product name. The amount of data compression you actually
`get depends on the data that you are sending.
`
`The ITU—T will agree on a further communications standard called V.FAST in 1994
`and will probably name it V.34. This was intended to be the fastest possible standard
`possible on typical analog phone lines. In fact V.34bis is already being discussed.
`However, eventually further speed advancements beyond a standard will require
`specialized telephone lines and other techniques as yet not considered.
`
`However, V.FAST has been very slow in being defined and agreed upon, and many
`people are clamoring for faster modems now. As a result, several U.S. manufacturers,
`headed by Multi—Tech Systems Inc., AT&T Paradyne, and AT8cT Microelectronics,
`created a standard known as V.32terbo as an interim solution. This standard has a
`maximum speed of 19,200 bps. (As with memory capacities, 19,200 bps is usually
`represented as 19.2 kbps and is often spoken of as nineteen dot two.)
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`Understanding Modulation
`
`Although you don’t need to know the intimate
`details of the communications standards. a concep-
`tual understanding of modulation is valuable. It will
`help you understand commonly confused terms.
`such as speed and baud, and understand why the
`current analog phone line cannot transmit data
`infinitely faster. These topics are covered in
`Chapter 5.
`
`Acovered in Chapter 3. the modem isa modulator!
`demodulator. When sending data. the modem
`modulates digital data into an analog form, and
`when receiving data.
`it recovers the data and
`changes it into a digital form again.
`
`In frequency modulation. which is like FM radio. the
`carrier signa|'s frequency is altered to indicate the
`data bits. Low-speed modems use frequency modu-
`lation.
`
`The third type of modulation is called phase modu-
`lation. This requires two sine waves with the same
`frequency to be sent. A sine wave is an oscillating
`wave: at one point in time.
`its signal
`level
`is a
`minimum; at another,
`it
`is at a maximum. The
`number of times per second the signal
`is at a
`maximum (or at a minimum) is the signa|'s ire»
`quency. Phase modulation delays or advances the
`second sine wave relative to the first sine wave.
`
`There are many different ways of modulating data.
`and each communications standard uses one or
`several different methods. To understand the ba-
`
`Depending on the communications standards. the
`modems use one or more of the modulation
`methods to transmit data. The details of what
`
`sics. consider that steady alternating signal, known
`as a carrier signal,
`is transmitted at a particular
`frequency with a particular amplitude. The data
`signals are detected as changes in the carrier signal.
`The carrier signal
`is a sine wave. which you may
`remernberfrom high school mathematics. All modu-
`lation schemes use amplitude. frequency. or phase
`modulation, or a combination of them.
`
`Consider first amplitude modulation. which is similar
`to AM radio. The amplitude. or signal level, of the
`carrier is changed to indicate the data. A carrier
`signal with a normal signal level may represent a
`zero. and a carrier signal with a slightly lower signal
`level may representa one. The carrier's signal level
`goes up and down to represent the data bits.
`Amplitude modulation on its own is not used in data
`modems.
`
`represents a one and what a zero are part of the
`communications standard. However.
`in many of
`the standards. you are sending more than one bit of
`data at a time.
`
`Using amplitude modulation again as the example.
`because it is the easiest to visualize. a normal
`
`carriersignal amplitude may representtwo zeros in
`a row. A slightly lower signal may represent a zero
`followed by a one. A slightly lower signal may
`represent a one followed by a zero. and a slightly
`lower signal still may represent two ones in a row.
`Consequently, each change in the carrier signal
`represents more than one bit of data. Sending two
`data bits at a time rather than one means that twice
`
`as much data can be sent in a given time.
`
`111 3. separate effort, a different group of manufacturers, including Rockwell Interna-
`tional Corp. and Hayes Microcomputer Products, has created another standard called
`V.Fast Class or V.FC. Rockwell has developed a modem chip that is available to all
`modern manufacturers as well as computer manufacturers. Computer manufacturers
`may include this chip on a system board so that the modem is an integral part of the
`computer. This standard has a maximum speed of 28,800 bps.
`
`Chapter 4: Selecting Your Equipment .
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`Many manufacturers have announced support for this V.FC standard, but it should be
`considered an interim standard. lfyou need to communicate at 28,800 bps now,
`rather than waiting for V.FAST standard to be finalized, V.FC is of interest. Ifnot, it is
`worth waiting for the new standard modems.
`
`Table 4-2 summarizes the specification numbers in increasing speed order. Remember
`that any modem 1200 bps or faster may include V4? and V.42bis features. Both of
`these standards are desirable features for your modem to support.
`
`
`Table 4-2 Summary of Modem Specification Numbers
`
`,
`
`Speed
`
`300
`
`I200
`
`2400
`
`4800
`
`9600
`
`I 4,400
`
`I 9,200
`
`Standard
`
`I03 or V.2I
`
`2| 2A or V.22
`
`V.22bis
`
`V32
`
`V32
`
`V.32bis
`
`V.321:erbo
`
`
`
`28,800 V.FC or V.FAST (V34)
`
`Note: VA? and V.42bis are available on modems 1200 bps or faster. V42 adds
`error-detection and V.42bis adds data-compression.
`
`Selecting extras
`
`Besides communications standards and physical shape, you should consider the extras
`that come with your modem when considering price. The warranty, technical support,
`or any money—back guarantee offered may be important to you.
`
`Ifyou buy a modem that supports a new or less commonly used standard, examine the
`upgrade path being offered. The biggest complaints I hear on bulletin boards are
`from modern buyers who thought they would be able to upgrade their modems
`inexpensively, but when the time came, the upgrade cost a couple of hundred dollars.
`
`The analogy with PCs applies here again. In most cases, PC upgrades are not as
`smooth or inexpensive as you may think. A modern is not the same as software, and
`upgrading costs the modem manufacturer more than the shipping costs.
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`If you are purchasing a data modem and fax modem combination, take special care to
`read the product’s specifications. Fax modems follow different communications
`standards than data modems. As a result, you may think that you are purchasing a
`modem with a particular speed when you are actually reading the fax modem’s
`specification. Consider your data modem needs and your fax modem needs sepa-
`rately, and then look at products that can fill both needs. The following section on
`choosing a fax modem gives details on what to look for in fax modems.
`
`The other extras that come with modems are the cables and communications soft-
`ware. The cables, typically a data cable for external modems and a phone cable for all
`modems, are fairly inexpensive to buy for yourself, but you do need to remember to
`purchase them. Check with the supplier regarding the length of the phone cable in
`case you need a longer one to reach the phone outlet.
`
`For external modems, check whether a data cable is supplied and the type of connec-
`tor on the end. You may need to buy a different cable to fit your serial port. Remem-
`ber that serial ports on the PC may have a 9-pin or 25-pin connector. You can purchase
`a 25-pin to 9-pin adapter cable or complete replacement cables if necessary. These
`cables are called a variety ofnames, including serial cables, RS-232 cables, and DTE to
`DCE cables.
`
`“Do I need as null modem cable?”
`
`You sometimes hear about a null modem cable, and people get confused about
`whether this is the cable they need to connect their modem. A null modem cableis a
`cable that allows you to link two pieces of equipment together without using modems.
`It is also known as a crossover cable, a modem eliminator cable, or a DCE-to-DCE
`cable, or DTE-to-DTE cable. You don‘t need one for connecting your modem to a PC.
`
`You also do not want to purchase a specialized plotter cable to use with a modem
`because, like the DCE-to-DCE cable, the connectors on the ends may be right, but it
`may not have the correct wires going to the correct pins. This specialized plotter cable
`may be a DCE~to-DCE cable, which will not work correctly connecting a modem to a
`PC, or it may be a completely custom cable for a specific plotter configuration.
`
`Chapter 4: Selecting Your Equipment
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`1
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`071
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`Facebook/Instagram Ex. 1009
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`071
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`Facebook/Instagram Ex. 1009
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`; Understanding Flow‘-Control or Handshakmg. __
`_The concept of flow control is fairly simple. The
`_. Hardvvare handshaking involves tvvo "signal . lines
`called clear to send
`and request to send
`flow control settingslestablish “vvho talks, vvhen". A
`"ham radiooperator. for'ex'aLrnp|e'. uses the term.
`(RTS). When a serial device (modem or computer.
`“over" tosignal when it is time forthe listener to
`for example) is ready to receive data. it raises the
`sp_eak. A committee discussion mavuse rules ‘so
`' CTS line. When the serial device is ready to send "
`data, it raises the RTS line. When both the CTS and
`that the person_ speaking stands-up; when the
`RTS lines are raised, the data is se'nL'When the CTS
`"speaker sits down, another person can stand up and '
`or.R'l'-S line is. lowered, the data transmission" _
`speak; This controls the flow of inforn1_at_ion_.and
`prevenfi two" people Speaking. at_ once.
`betvveen"the two serial devices is stopped_._ 5
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`'_ The RS.‘-232.speci'fZcation, used by the serial ports in _.
`your computer, supports two forms of flow con-
`'trol, knovvnas hardware handshaking and software _
`i_ian'dshal<_ing. As the names vaguely suggest. contro!_-
`"ling data flovv vvith ‘software handshaking involves
`-sendinga signal withinthe data stream (comparable -
`vv_it_h. saying.‘-‘ove'r” in -the ham radio example);
`Controlling data "flow with hardware handshaking _
`involves using extra signals separate from the data" .
`stream {comparable with the committee discussion -
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`ina.typica|_coz'nrnunications connection. pyou-"have
`twoareas of flow control. The loco.‘ flow control
`' dictatesthe type ofhandshaking used l:__uetv_veen the .,
`serial port_=and the modern. The end-to-_end flow
`control dictates the type of haridshaking used be-
`tween the_tvvo'moder'ns. -
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`For example, ifthe data stream sirartslvvith XON
`control is determined'auto- '
`' The endetol-encl
`icha racter, the receiving program is e><pectirig'to see '
`matically "by the error-__control_ features in your_'_ _
`an XOFF to indicate the end of the transmission. if -_
`a file. contains an XON "character, the receiving.
`modem" orlby the =file_-transfer protocol frorn the‘
`communications softwareyou are using. (File tran s"-.
`program may senda message.-.‘1'ou' can..manuall)_r
`send an XON or XQFF character "as required. by I
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`'_'fer is covered in Chap_t_er'6.)_-
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`pressing the_re|evant'k_eyst%“okes.
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`You can alter the local flow control method and
`Because it is likely that XON or XOFF may_appear_ "
`' "choose between'hardware and softvvasfe ha_ndshak-
`-‘occasionally in afile. harfdware handshaking is.pref.--
`ing. ll.-ovver—speed_rnodems may only s_uppo_ri_: soft-
`' er-abieto software".-hand's¥1al<ing. However, apart
`ware handshaking,
`in which; case you have'.no'
`" choice.-Ho'vveve_r.'most highe_r—speed' modems,
`'_ from the local control,_ the handshaking type is
`' which include error—cor:fection features, support
`3 determined by the communications protocols and
`' "both hardvi-are. and ‘software handshaking. -When
`not bylthe user._
`.tra"nsn*iittin'g'data at higher"speeds,h'ar'dwar_e hand- '_ .'
`"shaking is pr_eferab|e._
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`Software handshaking involves characters added to _
`"the data stream that signal the beginning and end of _
`-the transmission. The)_(C>I_\.i_charac_ter (proriounced
`ex-on)" starts -the _transmission. and the XOFF
`lcharacterr stops the transmission. The X_ON'ch'ar-
`acter isthe character seritwhen you press Ct_i-I"-I-;S
`_' _and XOFF character is the character se_ntvvher_i you I
`- press Ctr|+Q.j
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`Qccasiolnally. the HXON or HXHOFF cha'ract_er ap-
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`peers in the middle ofa file transmission; dLieto_th_e__ _
`particular. combination of bits, and the rec_eiving"" "
`computer'ger.s'confused. it sends a message. or
`your coz‘nmunic_ation's_progmmissues a message.
`' depending on which program.detected'the XON
`or'XOFF in an unexpected location," that indicates _'
`' an )(_ON or XOFF character was found.
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`072
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`Facebook/lnstagram Ex. 1009
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`072
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`Facebook/Instagram Ex. 1009
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`

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`Table 4-3 lists the pin assignments needed to connect a 9-pin serial port connector to a
`modem. (The names of the signals are also included for reference.) A 25-pin serial
`port connector needs a straig