Computer FAQ Scanner USB Firewire Defrag dot pitch

General FAQ

My computer came with one USB (Universal Serial Bus) port, and my printer uses it. I would like to add a USB scanner to my machine, but how do I hook it in? Assuming that I can add it, can USB handle both a printer and a scanner?

Just about every peripheral made now comes in a USB version. A sample list of USB devices that you can buy today includes:
Printers
Scanners
Mice
Joysticks
Flight yokes
Digital cameras
Web cams
Scientific data acquisition devices
Modems
Speakers
Telephones
Video phones
Storage devices like Zip drives
Network connections like Intel's AnyPoint home network.
Most computers that you buy today come with only one or two USB sockets. With so many USB devices on the market today, you easily run out of sockets very quickly. For example, on the computer that I am typing on right now, I have a USB printer, a USB scanner, a USB web cam and a USB network connection. My computer has only one USB connector on it, so the obvious question is, "how do you hook up all the devices?"
The easy solution to the problem is to buy an inexpensive USB hub. The USB standard supports up to 127 devices, and USB hubs are a part of the standard.

A typical USB 4-port hub accepts 4 "A" connections

A hub typically has 4 new ports, but may have many more. You plug the hub into your computer, and then plug your devices (or other hubs) into the hub. By chaining hubs together, you can build up dozens of available USB ports on a single computer.
Hubs can be powered or unpowered. The USB standard allows for devices to draw their power from their USB connection (All USB cables contain two wires for +5 volts and ground). Obviously a high-power device like a printer or scanner will have its own power supply, but low-power devices like mice and digital cameras get their power from the bus. The power (up to 500 milliamps at 5 volts) comes from the computer. If you have lots of self-powered devices (like printers and scanners), then your hub does not need to be powered -- none of the devices connecting to the hub need additional power, so the computer can handle it. If you have lots of unpowered devices like mice and cameras, you probably need a powered hub. The hub has its own transformer and it supplies power to the devices that connect to the hub so that the devices do not overload the computer's supply.
The Universal Serial Bus can easily handle both a scanner and a printer, even if you are scanning and printing at the same time. The bus supports up to 12 megabits per second, and the maximum that any one device can consume is 6 megabits per second of it.
My computer and video camera have a FireWire connection. How does that work?

The designers of the Universal Serial Bus (USB) had several particular goals in mind when they created the USB standard:
Low implementation cost, so that USB could be used in cheap peripherals like mice and game controllers
Low cabling cost
Lots of devices on the bus
Good speed characteristics for things like printers
The idea was to create a system that would replace all of the different ports on computers (parallel ports, serial ports, special mouse and keyboard ports, etc.) with a single standard. USB achieved all of these goals very effectively, and there will come a day in the not-too-distant future when computers will have nothing but a set of USB connectors on the back.
FireWire, originally created by Apple and later standardized as IEEE-1394, actually preceded USB and had similar goals. The difference is that IEEE-1394 was originally intended for devices working with lots more data -- things like camcorders, DVD players and digital audio equipment. IEEE-1394 and USB share a number of characteristics and differ in some important ways. Here's a summary:
Like USB, IEEE-1394 is a serial bus that uses twisted-pair wiring to move data around.
However, while USB is limited to 12 megabits per second, IEEE-1394 currently handles up to 400 megabits per second.
USB can handle 127 devices per bus, while IEEE-1394 handles 63.
Both USB and IEEE-1394 support the concept of a isochronous device -- a device that needs a certain amount of bandwidth for streaming data. This mode is perfect for streaming audio and video data.
Both USB and IEEE-1394 allow you to plug and unplug devices at any time.
Most digital video cameras have an IEEE-1394 plug. When you attach a camcorder to a computer using IEEE-1394, the connection is amazing. With the right software the computer and the camera communicate, and the computer can download all of the scenes on the tape automatically and with perfect digital clarity.
How does disk defrag work?
The word "disk defrag" is typically used to refer to the Microsoft Windows utility called Disk Defragmenter. It is designed to solve a problem that occurs because of the way hard disks store data.
Hard disks store data in chunks called sectors. If you imagine the surface of the disk divided into rings (like the rings of a tree), and then imagine dividing each ring into pie-slices, a sector is one pie-slice on one ring. Each sector holds a fixed amount of data, like 512 bytes.
The hard disk has a small arm that can move from ring to ring on the surface of the disk. To reach a particular sector, the hard disk moves the arm to the right ring and waits for the sector to spin into position.
Hard disks are slow in computer terms. Compared to the speed of the processor and its memory, the time it takes for the arm to move and for a sector to spin into place is an eon.
Because of fact #3, you want to minimize arm movement as much as possible, and you want data stored in sequential segments on the disk.
So let's imagine that you install a new application onto an empty hard disk. Because the disk is empty, the computer can store the files of the application into sequential sectors on sequential rings. This is an efficient way to place data on a hard disk.
As you use a disk, however, this efficient technique becomes harder for a disk. What happens is that the disk fills up. Then you erase files to reclaim space. These files that you delete are scattered all over the surface of the disk. When you load a new application or a large file onto the disk, it ends up being stored in hundreds or thousands of these scattered pockets of space. Now when the computer tries to load the scattered pieces, the disk's arm has to move all over the surface and it takes forever.
The idea behind the disk defragmenter is to move all the files around so that every file is stored on sequential sectors on sequential rings of the disk. In addition, a good defragmenter may also try to optimize things even more, for example by placing all applications "close" to the operating system on the disk to minimize movement when an application loads. When done well on older disks, defragmenting can significantly increase the speed of file loading. On a new disk that has never filled up or had any significant number of file deletions, it will have almost no effect because everything is stored sequentially already.
As you might imagine, the process of individually picking up and moving thousands of files on a relatively slow hard disk is not a quick process -- it normally takes hours.

What does it mean when someone says I need a monitor with a .28 dot pitch or better?

The dot pitch rating of a monitor tells you just how sharp the displayed image will be. Dot pitch is measured in millimeters (mm), and a smaller number means a sharper image. How you measure the dot pitch depends on the technology used.
In most CRT's you measure dot pitch as the distance between holes in the shadow mask. The shadow mask is a metal screen filled with holes through which the three electron beams pass that focus to a single point on the tube's phosphor surface.
Monitors based on the Trinitron technology, developed by Sony, use an aperture grill instead of a shadow mask. The aperture grill consists of tiny vertical wires. The dot pitch of one of these monitors is measured by the horizontal distance between wires.
In LCDs and the majority of other display technologies, dot pitch refers to the distance between subpixels of the same color in pixel triads.
In computer displays, common dot pitches are .31mm, .28mm, .27mm, .26mm, and .25mm. Traditional televisions often use a larger dot pitch, about .51 mm, and large screen TVs or projection devices can go up to 1 millimeter in pitch.
The smaller and closer the dots are to one another, the more realistic and detailed the picture appears. When the dots are farther apart, they become noticeable and make the image look grainier. You will usually want a .28mm or finer. Anything larger than that on a typical monitor will begin to appear grainy.
The dot pitch translates directly to the resolution on the screen. If you were to put a ruler up to the glass and measure an inch, you would see a certain number of dots, depending on the dot pitch. Here is a table that shows the number of dots per square centimeter and per square inch in each of these common dot pitches:

Dot Pitch Approx. number of
pixels/cm² Approx. number of
pixels/in²
.25 mm 1,600 10,000
.26 mm 1,444 9,025
.27 mm 1,369 8,556
.28 mm 1,225 7,656
.31 mm 1,024 6,400
.51 mm 361 2,256
1 mm 100 625

Is it better to turn my computer off when I am not using it, or to leave it on all the time?

This is one of those questions where there is no single right answer. In other words, it depends on how you use your computer. What I can offer are some thoughts to keep in mind, and then you can decide which scenario applies to you.
Let's start with, "Three situations that force you to leave your computer on 24 hours a day." They are:
You are on a network, and the network administrators back up files and/or upgrade software over the network at night. If that is the case and you want your machine backed up or upgraded, then you need to leave it on all the time.
You are using your machine as some sort of server. It needs to be on 24 hours a day. If your machine acts as a file server, print server, web server, etc. on a LAN or the Internet, then you need to leave it on all the time.
If you are running something like computer resource sharing and you want to produce as many result sets as possible, you need to leave your machine on all the time.
If you do not fall into any of those categories, then you have a choice about whether or not to leave your machine on.
One reason why you might want to turn it off is economic. A typical PC consumes something like 300 watts. Let's assume that you use your PC for 4 hours every day, so the other 20 hours it is on would be wasted energy. If electricity costs 10 cents per kilowatt-hour in your area, then that 20 hours represents 60 cents a day. 60 cents a day adds up to $219 per year -- a pretty big chunk of money.
It's possible to use the energy-saving features build into modern machines and cut that figure in half. For example, you can have the monitor and hard disk power down automatically when not in use. You'll still be wasting $100 per year.
The argument for leaving your computer on all the time is that turning it on and off somehow "stresses the computer's components". For example, when the CPU chip is running it can get quite hot, and when you turn the machine off it cools back down. The expansion and contraction from the heat probably has some effect on the solder joints holding the chip in place, and on the micro-fine details on the chip itself. But here's 3 ways to look at that:
If it were a significant problem, then machines would be failing all the time. In fact, hardware is reliable (software is a whole different story, and there is a lot to be said for rebooting every day).
I don't know a single person who leaves the TV on 24 hours a day. TVs contain many of the same components that computers do. TVs certainly have no problems being cycled on and off.
Most vendors will sell you a 3-year full-replacement warrantee for about $150. If you are worried about it, spend some of the money you are saving by turning your machine off and buy a service contract. Over 3 years you come out way ahead!
You can decide which approach works best for you.
What do the computer error messages "fatal exception error", "invalid page fault", and "illegal operation" mean? What causes them?

When a program like Microsoft Word or Excel "crashes", it means that something has gone seriously wrong during the program's execution. The operating system often recognizes that there is a serious problem and kills off the offending application in a clean way. When it does this, the operating system will say something cryptic like "fatal exception error" (and often display a large collection of hexadecimal numbers that are totally useless to you, the user, but might be of some use to the original programmer). The other way for a program to crash is for it to take the operating system down with it, meaning that you have to reboot.
Even though there is nothing you can do with the cryptic error messages, it might be nice to at least know what they mean! So let's go through the three most common:
Fatal exception error - An application program like Microsoft Word is made up of many layers and components. There is the core operating system, an operating system services layer, perhaps an encapsulation layer on top of the system services, hundreds or software libraries, internal function/class libraries and DLLs, and finally the main application layer. Most modern operating systems and languages (like C++, Java, etc.) support programming concepts known as exceptions and exception handling. Exceptions allow different layers to communicate problems to each other. For example, say that a program needs some memory, so it asks the operating system to reserve a block of memory. If the operating system is unable to honor the memory request (because the requested block is too big, or the system is low on memory, or whatever), it will "throw a memory exception" up to the layer that made the request. Various layers may continue to throw the exception upward. Somewhere along the line one of the layers needs to "catch the exception" and deal with the problem. The program needs to say, "wow - the system is out of memory. I need to tell the user about this with a nice dialog box". If the program fails to catch the exception (because for some reason the programmer never wrote the code to handle that particular exception), the exception makes it all the way to the top of all the layers and the operating system recognizes it as an "unhandled exception". The operating system then shuts down the program. Well designed software handles all exceptions.
Invalid page fault - A program uses memory (RAM) to store data. For example, when you load a document into Microsoft Word, large parts of the file you are editing take up space in RAM. As the program needs memory, it requests blocks of memory of specific sizes from the operating system. The program remembers the location of each block it allocates using a "pointer". If the program tries to write data to a location beyond the end of a memory block, or if the program gets confused and tries to access a non-existent block of memory using an invalid pointer, the operating system can see that happening and it generates a "page fault" or a "segmentation fault". The operating system shuts down the program because the program obviously does not know what it is doing.
Illegal Operation - A microprocessor has a finite number of instructions it understands, and each instruction is represented by a number known as an "opcode". The opcode 43 might mean "add", the opcode 52 might mean "multiply", etc. If the microprocessor is executing a program and comes to an opcode that it does not recognize or that it cannot execute because of the current state it is in, then the microprocessor stops to complain. The operating system handles this complaint by shutting down the offending program. Illegal opcode's normally come from software jumping to a location in memory that does not contain valid program information.
All of these problems are caused by human error on the part of a programmer. The programmer is not diligent enough to catch an exception, or allows the program to access invalid memory. Sometimes the root cause is incompetence or inexperience, but in many cases it is the complexity of today's programs. There are hundreds of exceptions and often millions of blocks of memory that a program manages in an intricate layered environment. One false move and the application crashes -- software is very brittle. Testing finds many errors, but usually it does not find them all.