Serial Communication Vs Parallel Communication PathwaysSerial Port and Modem Basics.Next.Previous. Contents You dont have to understand the basics to use and install a.But understanding it may help to determine what is wrong if.After reading this section, if you want to.How Modems Work in this document not yet.More details on the serial port including much of this.Serial HOWTO.Most all telephone main lines are digital already but the lines.Such a voltage.If viewed on an oscilloscope it looks like a.Serial Communication Vs Parallel Communication' title='Serial Communication Vs Parallel Communication' />A parallel cable connection is a method that allows data to be transferred from one computer to another.The parallel cable required is slightly complicated as it has.A digital signal is.For example 3 v volts might be a 1 bit and 0 v.For most serial ports used by external modems 1.To send data from your computer over the phone line, the modem takes.It.MODulating.Since the result still represents digital data, it could also be.But it looks something.At the.DEModulates this signal.Put together the mod and.A modem is thus a MODulator DEModulator.Just.Modulation Details.Intro to Serial The UART serial port or just serial port for short is an IO InputOutput device.Since modems have a serial port between them and the computer.Most PCs have one or two serial ports.Each has a 9 pin connector.Computer programs can.The other pins are for control purposes and.The serial port is much more than just a connector.It converts the.Inside the computer, data bits flow in parallel using.Serial flow is a stream of bits over a.For the serial port to create such a flow, it must.Most of the electronics of the serial port is found in a computer chip.UART.For more details on UARTs.What are UARTS in the Serial HOWTO.But you may want to finish this section first so that you will.UART fits into the overall scheme of.Pins and Wires Old PCs used 2.Each of the 9.Besides the two wires used for.The voltage on any wire is measured with respect to this ground.Thus.Except that it has been known to work with no signal ground wire.There are still more wires which are for control purposes signalling.All of these signals could have been.Some or all of these control wires.Modem control wires are either in.One of these wires is to signal the computer to stop.Conversely, another wire.If the attached device is a modem, other wires may.See the Serial HOWTO.Pinout and Signals for more details.Internal Modem Contains Serial Port For an internal modem there is no 9 pin connector but the behavior.Instead of a a 1.The internal modems serial port looks just like a real.It even includes the speed limits that.Unfortunately for Linux, many internal modems today use software.CPU to do much of the modems work.Unfortunately.MS Windows OS it hasnt.Linux.Thus you cant use some of these modems with.Software based Modems winmodems.Since the computer needs to communicate with each serial port, the.IO address.It also needs to know which wire IRQ number.CPU.It requests service by sending an interrupt voltage on this wire.Mcafee Virus Scan Enterprise 8 7 Eternal License Rar File here.Thus every serial port device must store in its non volatile memory.IO address and its Interrupt Re.Quest number IRQ.See.Interrupts. The PCI bus has its own system of.But since the PCI aware BIOS sets up these PCI interrupts.IRQs, it seemingly behaves just as described above.Except.PCI interrupts is allowed 2 or more devices may use.IRQ number.IO addresses are not the same as memory addresses.When an IO.This both tells main memory to ignore the address and.IO addresses such as the serial port.If the address matches, then the IO device reads the data.The IO address of a certain device such as tty.S2 will actually be a.The lower address in this range is the base.The serial ports are named tty.S0, tty.S1, etc.COM1, COM2, etc.DOSWindows.The dev. Lady Gaga The Fame Monster Meaning . Type ls devtty. S to.Just because there may be for example a tty.S3 file.Which one of these names tty.S0, tty.S1, etc.The serial driver.IO address corresponds to.S.This mapping of names such as tty.S1 to IO addresses.IRQs may be both set and viewed by the setserial command.What is Setserial.This does.IO address and IRQ in the hardware itself which is.Thus which physical port.S1 depends both on what the serial driver thinks.If a mistake has.S2 and thus it cant be used.See.Serial Port Devices devtty.S2, etc.Bytes come in over the phone line to the modem, are converted from.When the serial port receives a number of bytes may be set to 1, 4.FIFO buffer, it signals the CPU to fetch them by.Thus the FIFO waits until it has.However, this interrupt will also be sent if there is an unexpected.Thus if the bytes are being received slowly such as from someone.For some UART chips the rule is like this If 4.FIFO.Of. course, if the FIFO is empty, no interrupt will be issued.Each interrupt conductor inside the computer has a number IRQ and.For.S0 normally uses IRQ number 4 known as IRQ4 or IRQ 4.A.Configuring Serial Ports.Interrupts are issued whenever the.CPUs attention.Its important to do.If the CPU fails to remove such received.For an external modem, there may be no way such as flow control to.For an.FIFO buffer is on the same card and most.Thus it will not overrun.Modem to Modem Flow Control to prevent the modem itself from.This is one advantage of internal modems over an.Interrupts are also issued when the serial port has just sent out all.FIFO buffer out the external.It then has space for 1.The interrupt.CPU of that fact so that it may put more bytes in the.Also, when a modem control.The buffers mentioned above are all hardware buffers.The serial port.This will be explained later.Interrupts convey a lot of information but only indirectly.The.The interrupt controller.CPU.The CPU then runs a special program to service.That program is called an interrupt service routine.It tries to find out what has.This program can easily find out what has happened since the serial.IO addresses known to the serial driver.These registers contain status information about the serial.The software reads these registers and by inspecting the.Before continuing with the basics of the serial port, one needs to.In.CPU but unfortunately at present, such software only works.MS Windows.The discussion here will be for the case where the.Linux.In order to send data faster over the phone line, one may compress.The encoded data is smaller than the original less bytes and.Internet in less time.This process is called.If you download files from the Internet, they are likely already.Your modem may sense that what is passing thru has.If you are receiving data which has been compressed by the.Thus the flow of data from your.The ratio of this flow is called the compression ratio.Compression ratios as high as 4 are possible, but not very likely.Similar to data compression, modems may be set to do error.While there is some overhead cost involved which slows.For the serial ports interface with the external world, each 8 bit.Without error correction, these extra start and stop bits usually go.But when error.This is more efficient and results in.Data bytes representing letters, pictures, etc.Flow rates such as 5.But almost everyone says speed.If there were no data compression the flow.Actually there are two different speeds to consider at your end of the.The speed on the phone line itself DCE speed modem to modem The speed from your computers serial port to your modem DTE speedWhen you dial out and connect to another modem on the other end of the.CONNECT 2.
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