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วันอาทิตย์ที่ 26 ตุลาคม พ.ศ. 2551

USB Hubs and Ports Explained

The USB standard has become the universal means of connection peripheral devices and equipment to the computer. Computers have either two or four USB port connectors. Each of these can support a large number of connections but to be able to attach more than two or four devices requires a multi-connection hub. There are many different types of hubs and this article seeks to explain some of the important considerations in selecting the best choice of hub for your system. The USB bus system in computers was introduced in 1996 and has gone through a number of developments and enhancements. It started out as USB 1 which was very slow and had many bugs. Version 1.1 brought a higher bandwidth of 12 MBits/second. The latest version of 2.0 gives a bandwidth of 480MBits/second and is fully backward compatible with USB 1 and 1.1. Link to a table showing differences between USB speeds (http://www.usb-products.com/usb-speed.aspx) Port Connections and Power All USB standards support up to 127 connections (or devices) per computer port. It is unlikely that this would be achievable as the port can only supply 0.5 Amps maximum. Some devices consume a full 0.5 Amps, so are powered from a power adaptor from the mains supply. Bandwidth Sharing Generally, the bandwidth of the port (http://www.usb-products.com/usb-port.aspx) is shared between all the devices that are attached through a hub. This means that if you want to add two USB 2 devices with a USB 1.1 device, the bandwidth is shared between all three devices. However, there is an extra complication. Most hubs (http://www.usb-products.com/usb-hub.aspx) have a single chip controlling the bus. This means that the hub will work at the lowest bandwidth determined by the connected devices resulting in the USB 2 devices working at much lower transfer speeds. Nearly all USB hubs on the market today (passive and powered) operate like this. Multiple Chip Hubs The transaction translator (TT) is the name for the controlling chip that assigns bandwidth to each connected device. In hubs that have one TT all devices share the same bandwidth and operate at the speed of the slowest. In hubs with a TT per socket, each receives a share of the bandwidth, but this is not restricted to the slowest device. Examples of this type of hub is the Belkin Tetra hub (http://www.usb-products.com/Products/Hi-Speed-USB-2-0-4-Port-TetraHub.aspx) This has 4 ports each with its own TT. What this means in reality is that a multiple TT hub will provide much higher bandwidth per port when several devices are connected from one hub. Maximum Performance and choice of Where to connect a device If you are after maximum performance for each of your USB 2 devices, each device should be plugged into its own USB port in the computer. Additional plug in cards can provide a vast number of USB ports for most needs. However, this situation might be going too far except for the most demanding of situations. Although several USB devices might be connected at the same time it is unlikely that they will all be in use together. A useful compromise is to plug small bandwidth devices together into a single hub. These would include: mice; keyboard; Speakers; little used devices. For devices that require a high bandwidth and are used often, direct connection in to a USB port is the best option. Devices would include: Flash memory drives; external disks; Web cams; Other frequently used devices. If you don't have sufficient ports then either more ports should be added with a plug-in card or a good quality hub with multiple transaction translators should be used. The devices placed on the hub are those that are used less often. Those devices placed directly on the port are used most often. Link to a selection flow chart that helps in selecting a hub (http://www.usb-products.com/USB-Hub-Selection.aspx) Access Considerations What type of configuration and mix of port and hubs will depend on the number of devices that you wish to attach. Desktop computer cases usually have ports in the front of the case for easy access. These can be used for devices that are often plugged / unplugged and require the highest bandwidth. These are flash memory drives and portable external drives and MP3 players. USB hubs tend to clutter up the area with cable connections and power lead. These naturally go out of sight if possible or behind the monitor. For mobile computers, a small compact hub can be obtained. These may have there own wind-in or tidy away cables. Powered and Passive Hubs Hubs may be restricted in their use without a power connection as passive hubs rely on the power of the USB bus. Powered hubs have a mains power adaptor to provide additional power for devices. These devices range in the power they require. It is possible to run a number of different devices all from a single passive hub running of a USB port as long as they have low power requirements. With only 0.5 Amps current it only takes one of the devices requiring the full power requirement for the hub to be overwhelmed. Nothing serious is going to happen though except that the single will degrade and many or all the devices will loose connection with the computer. To avoid this situation a powered hub should be used. Nigel Stephens heads an online business at <a target="_new" href="http://www.usb-products.com">http://www.usb-products.com</a> supplying computer hardware products. Nigel's emphasis is in providing useful information so that consumers can make more informed choices.

Rear Projection TV Facts - Understanding the Pros & Cons of Rear Projection in the Home Theater

Rear Projection Television - An Affordable Option A great deal of consumer appeal for Rear Projection TV systems arise out of the shear simplicity that this product offers as an immediate solution to getting a bigger TV. No mess, no fuss, if you have the space, either visit your local big screen retailer - or better still, check at your favorite online electronics superstore - to order your product and get it delivered in just a few days; unpack the product and there you have a big screen TV in your living room ready for immediate use! Rear Projection TV Facts: As already stated, rear projection offers a most immediate solution to getting a bigger TV. Probably, this is also one of the main drivers behind rear projection television sales. Yet the real 'culprit' behind the popularity of rear projection TV systems does not arise out of some particular benefit associated with rear projection, but out of the fact that most big screen retailers seem to give the impression that rear projection systems are cheaper than a front projection setup. This may be true in retail stores, but not necessary so when buying online. The reality is that for a given budget level, prices online are such that front projectors will deliver a much more cinema-like experience for the same price bracket. Therefore, do not base your decision on price alone to decide between a front projection setup and a rear projection TV box. Clearly, there is a market for both - the primary decisive factor should be your room size. If you don't have a large viewing room, a 40" to 60" diagonal TV will probably be more than adequate rendering a rear projection TV the ideal affordable solution - as long as it fits in the available space. Size - or rather unit depth - is becoming less of a problem with modern LCD and DLP rear projection TV units. A typical 52" diagonal widescreen DLP or LCD rear projection TV set requires no more than 15-inches in depth; this contrasts heavily with a similar size CRT rear projection model which would normally require between 22 and 24 inches in depth. What's more, considering that a similar size Plasma TV is still out of reach of most average household budgets, today's slim-styled LCD and DLP rear projection TV sets, with their lower prices yet high performance, are becoming the affordable 'immediate' big screen TV option in the television mass-market. However prior to committing yourself to a rear projection TV, it is important to be fully aware of a few limitations associated with rear projection systems, namely: limited screen size, limited viewing angle, glare problems, poor aspect ratio management, poor use of floor-space, etc. We take a look at each of these limitations in further detail below: Screen size: Rear projection TV systems come in screen sizes ranging from typical 42" up to a maximum of just over 70". This may or may not be a limitation. It is true that you can get a 100" projection with a home theater projector for the price of a high quality digital 50" rear projection TV, yet the screen size should be dimensioned to suit your room. If your room size does not support such big projections, rear projection is probably the way to go. Viewing angles: Rear projection TV systems used to have a rather limited viewing angle - with the optimum viewing position being one directly in front of and eye-level with the unit. Move away to either side, and color, contrast, and brightness will degrade substantially. A narrow viewing angle will limit the number of people who can watch the set due the lowering in picture quality at the extreme viewing angles. Most modern systems support a viewing angle of circa 150 degrees - which should be adequate for normal home theater use. However, it is always best to check on this prior to your purchase as some products are worse than others. Reflections: It is common that any light source at a complementary angle to your viewing-angle will result in glare - in particular if the unit makes use of a screen-saver (a clear protective material that covers the fragile screen itself). Glare can seriously degrade the picture quality. The only real solution is to take away the offending light source; in some cases the situation can improve if one removes the screen-saver ? BUT remember that an unprotected screen is fragile and expensive to replace if damaged. Floor-space: Any rear projection TV is literally a large box with a relatively large footprint. It is true that modern slim-type models do exist that are no more than 15 to 18 inches in depth - depending on the screen size, yet the cheaper CRT-based rear projection TV sets will stand out by at least 24 to even 30 inches to allow for the necessary air-space between the back of the unit and the wall. Remember to take this into your calculations when planning your home theater as these two feet or so will have to be deducted from your available viewing distance. Rear Projection TV Speakers: Forget all about them! Do not give any weighting to the speaker system coming with your rear projection unit. You would not be using them as you will surely want to replace these with your dedicated home theater surround receiver speaker system. Do not even think of using the build-in speakers of your rear television set as a center channel replacement. They will just interfere with the sound coming out of your dedicated system - hence do not pay anything extra for this as you will surely be switching off your TV sound completely during a movie show. Aspect ratios: We have already mentioned a number of limitations associated with rear projection television, yet in comparison, these are just minor issues. The real serious limitation with a rear projection TV is aspect ratio management. This is the trickiest of it all. Standard television comes only in 4:3 but rear projection TV systems come in both standard 4:3, and in the 16:9 widescreen format. Once you choose your format however, you have to live with it - so once again, you have to choose wisely. The 4:3 (1.33) or 16:9 (1.78) referred to as the aspect ratio, is the ratio of the screen width with respect to the height of the image. All standard non-HDTV material is in the 4:3 format while most modern films come in one of the many widescreen formats - the most common being the 2.35, which in itself is not compatible with any of the fixed aspect ratio TV systems. There are various ways to deal with this - including: <ul> <li>Image stretching to fill the available screen. <li>Use of black or gray bars on top and bottom of a 4:3 screen to show the movie in its correct aspect ratio as originally filmed, but then the effective film display will be smaller. <li>Pan and scan editing where only the most important portion of each frame is shown with the rest being discarded. </ul> Image stretching and horizontal bars can be extremely irritating while in the 'pan and scan' you are giving up film information to have a full screen view. Worst of all, prolonged use of horizontal bars - especially black bars - leads to tube burn-out in CRT based systems at huge costs to you. The incompatibility between screen formats renders the decision on aspect ratio a rather complicated issue when choosing a rear projection TV set. Surely, there is no such dilemma with a front projection setup, but if your only way forward is rear projection, then you will have to choose wisely. Here no one can help you in your decision - it is simply a matter of preference. The best way to decide on aspect ratio is by first determining what you will be viewing most. Making the Choice: Surely, there is a market for both front and rear projection TVs ? it is all a question of knowing what are the advantages and limitations of each with respect to your specific needs. Andrew Ghigo ? A Telecoms/Electronics engineer by profession, with specialization in digital switching and telecoms fraud management systems. Editor and publisher of <a target="_new" href="http://www.practical-home-theater-guide.com">http://www.practical-home-theater-guide.com</a> - a site dedicated to all home theater enthusiasts with the scope of serving as a comprehensive home theater guide to home theater systems, product reviews and home theater design. This article is an excerpt from a series of guides appearing under the <a target="_new" href="http://www.practical-home-theater-guide.com/Projection-Tv.html">Projection Television</a> section of the site.

Learn to Find Cheap Laptop Computers on the Internet

Cheap laptop computers are coming to a store near you. In fact, there are many there already. That's because, like any newfangled electronics device, laptop computers were once the cream of the computer crop, which made them super expensive. Nowadays, however, laptops are a dime a dozen. Held-held personal digital assistants and Bluetooth cell phone devices are the new rage. And so laptops are cheap. This does not mean that they are cheaply made, though. Thanks to the wonders of the technological cycle, laptop computers are decreasing in price as their capabilities rise. Of course, there are the latest, greatest laptops out there-monsters made for gaming, for instance, or for high-powered multimedia applications-and these still cost a pretty penny. But the average user can find themselves cheap laptop computers that provide all that they need in power, connectivity, and then some. Every major manufacturer out there, such as Dell, HD, and IBM, has their lower end and intermediate models that come with more than enough speed and usability. The key to finding the right model for you is to figure out exactly what you need it for. Will you be using it mainly for surfing the Web or word processing? Do you need it to work from home or mainly to save your MP3s and digital pictures? Then consider the different facets of computer, starting with its hard drive, memory, and processor. Your hard drive is essentially the brain of your computer, where all of your applications, files, and folders are saved. Your memory, or RAM, involves the "thought processes" of a computer, and gets used whenever you have a program open, such as Word or your Firewall. Your processor essentially does as its name suggests, processing the inner workings of how each program runs. Some other aspects to consider for your computers are also the ports involved, which can connect your computer to your camera or music player. Many cheap laptop computers today also come with wireless and broadband connectivity. Mike Long is the successful web publisher of Laptop-SuperGuide.com providing valuable tips, advice, and info about a multitude of relevant topics including <a target="_new" href="http://www.laptop-superguide.com">cheap laptop computers</a>.

วันเสาร์ที่ 25 ตุลาคม พ.ศ. 2551

The Right Way To Discharge Capacitor In Switch Mode Power Supplies

Switch mode power supply contain some large filter capacitors that can hold dangerous charges even if the unit has not been used for several days. Typical values of this type of filter capacitor are 220uf 250volt and 330uf 400volt! If you are going to work on the power supply circuits, it's a good idea to discharge that capacitor first. There are three ways to discharge the big filter capacitor in a power supply. 1)Discharge it with a screw driver (not recommended). The reason for not using the screw driver to discharge a capacitor is because -the printed circuit board or circuitry can be damage due to the spark generated while discharging the high voltage capacitor. I once blowed the power section using this method. However, if you know that the capacitor stored voltage is not too large, you can easily discharge it with a small screw driver. -if the capacitor holds a heavier charge of electricity. Discharging the capacitor may melt the tip of the screw driver and the copper on the printed circuit board. -Sometimes a heavier spark may cause small disintegrated solder lead or copper to fly out from the circuit board and may injured your eyes. 2)Place the leads of a socketed 100 watt electric bulb on the leads of the capacitor. This method had been used by many technicians around the world for the light bulb will act as an indicator to see if the capacitor still holds the charge. If there is a charge the light bulb will light and after discharged the light bulb will goes off. 3)Place the leads of a high wattage resistor on the leads of the capacitor. I use a 2.2k ohm 10 watt resistor to discharge the high voltage capacitor in a switch mode power supplies. It is simple to use and very effective .It takes only couple of seconds to fully discharge the capacitor. Conclusion I strongly suggest for those who use screw driver to discharge a capacitor to swift to the second and third method as these is the safest method to protect the circuit and most importantly is yourself. Jestine Yong is a electronic repairer and a writer. For more electronic repair information please visit his website at <a target="_new" href="http://www.noahtec.com/electronic-repair-articles.htm">http://www.noahtec.com/electronic-repair-articles.htm</a>

Your Printer Is Out Of Ink -- Now What?

We've all been there. You're in the middle of printing out an important document and your printer suddenly stops. It's the dreaded "out of ink" message. So you pull your backup ink cartridges out of your desk drawer and replace them. Wishful thinking, right? Most of us just don't plan ahead -- no matter how many times this has happened to us. When we buy ink cartridges, we tend to just get one set of black and color cartridges. The cost of ink is so high that buying more than one set is just too expensive for most consumers. Thankfully, there are less expensive options out there ? and you don't have to sacrifice quality. Option 1 ? Avoid Staples, Best Buy, and other large retailers altogether. You're never going to get a good deal on ink cartridges since they only sell name brand replacements. Instead, do your homework on the internet. There are many reputable websites out there that charge up to 80% less for cartridges. How is that possible? Rather than selling name brand replacements, online ink retailers sell compatible and remanufactured cartridges. There cartridges are technically just as good as the originals, and can be purchased for half as much money. As an added precaution, however, we recommend buying ink cartridges at sites that offer 100% guarantees. In the online world, you never know what you're getting. My advice -- run a search on Yahoo for "ink cartridges." Visit the first 5 sites. Find the web site that offers the best prices, lowest shipping costs, and a 100% guarantee. Option 2 ? Refill your ink cartridges. It's not as bad as you might think. And best of all, this is definitely the cheapest way to go. You can fill your ink cartridges up to 10 times with a single ink refill kit. There is obviously some work to this. You basically use a syringe to inject ink back into your original cartridges. If you do this correctly, this is no spillage, and you'll be up in running in no time. It takes a bit of practice to master the art of refilling ink cartridges. Don't get discouraged if it takes a couple of tries before you get it down. Bill Smith is a printer expert who has specialized in ink cartridges and refill solutions for over 5 years. Visit his site at <a target="_new" href="http://www.buythebestbuy.com">http://www.buythebestbuy.com.</a>

DVD ROMs

DVD CD ROMs have become predominantly the most used CD drive for desktop and notebook computers. They are very reliable and now come as a standard in most computers. If you are looking for a laptop then make sure it has a DVD ROM, this will give you extra speed for normal Cds, and you will be able to watch your favourite DVDs while you travel. I often sit up late watching DVDs on my laptop after a hard working day. If you are interested in Desktop computers, then the DVD drive will enable you to watch your favourite DVDs on your monitor. I currently have a 21-inch monitor and a 5.1 computer surround kit. This brings DVDs to life and acts just like a home cinema system, however the quality is even better. Most DVD ROMs will come in speeds ranging from 4 to 10 speed. This is more than adequate to watch the latest DVDs, play the latest games, and use and install the latest software. Normal CDs can only perform at a certain speed, and it is a lot lower than what the DVD ROM offers. I hope you have a better understanding of why DVD ROMs are now used more than a normal CD nowadays. It is basically all part of bringing a home entertainment system at the best quality closer to everyone. This is one of the unique articles posted weekly on <a href="http://www.computer-and-printer-reviews.com" target="_new">http://www.computer-and-printer-reviews.com</a>. About The Author John Gibb is currently the webmaster of several guide websites. <a href="http://www.computer-and-printer-reviews.com" target="_new">http://www.computer-and-printer-reviews.com</a> is his latest addition.

What Is A Crystal? And How To Test It

Crystal are use to keep the frequency of the clock from drifting. If the signal from this clock stops, or is weak, or the pulses begin to vary, the electronic equipments might show intermittent faults or might stop altogether. The microprocessor pins that hold the crystal are usually called OSC IN and OSC OUT as shown in Figure 1 and the frequency is marked on the crystal. Typical examples of crystal oscillator frequency are 3.58MHZ, 4MHZ, 8MHZ, 24MHZ etc. Testing Crystal Crystals are quite fragile components because of their construction. Unlike a resistor or capacitor, if you drop one on the ground from a decent height, its 50-50 bet whether it will work again. Testing the crystal is not a breeze either. You cannot just take out your trusty multimeter and plug the crystal in it. In fact, there are three right ways to test a crystal: - (a) Using Oscilloscope A crystal produces a sine wave when excited. It is appropriate then, to see a waveform representative of a sine wave on the clock pins. If the clock is not functioning properly, replace the crystal. In most cases this should solve the problem since microprocessors are usually very reliable. Check the crystal with power on. (b) Frequency Counter Frequency Counter can be use to check the frequency of the crystal. The reading must be taken when the equipment power is switch "on". Place the probe of frequency counter to the crystal pin and read the measurement. Be sure that your frequency counter meter has the range that is higher than the crystal frequency you are measuring. (c) Crystal Checker With this method, usually the crystal is placed in the feedback network of a transistor oscillator. If it oscillates and the LED is lighten up, this mean that the crystal is working. If the crystal doesn't work, the LED stays off. Instead of using LED, some other crystal checker uses a panel meter to indicate if the crystal is working or not. Jestine Yong is a electronic repairer and writer. For more information about electronic repair please visit his website at <a target="_new" href="http://www.noahtec.com">http://www.noahtec.com</a>