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CompTIA A+ Study Guide (220-601 and 220-602)

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Input Devices

Introduction:
A PC requires a variety of external devices in order to function - some of these are input devices and some are output devices. As the names imply, an input device provides information TO the PC while an output device provides you with data FROM the PC. Common input devices include mice, keyboards, webcams, digital cameras, and scanners (scanners are discussed in Domain 4.0). There are also specialty input devices such as bar code scanners, fingerprint scanners, touch screens, and others. Common output devices are monitors and printers, both of which are discussed in other sections of this guide. In this section, we are going to discuss the various common and specialty input devices and the ports and connectors that they use.

Serial:
Before we begin looking at devices, we need to discuss the ports that are available for connecting these devices. Serial ports are nearly extinct and have been largely replaced by USB and Firewire, but they do still exist. The most common places they are still seen today are the built-in modems on laptops and the communications port on many routers. Their other main function was for mouse connections.
 

Serial devices used a DB-9 connector (see left image) that plugged into a DB-9 port on the computer. Because the expansion bus uses parallel communications, the serial port on the computer has a chip called Universal Asynchronous Receiver/Transmitter (UART) that is responsible for converting between the parallel and serial data streams. RS-232 is the standard that defines all of the specifications of serial communications.

Nearly every device we work with in modern PCs is plug-and-play so we don't spend much time manually configuring ports anymore. The serial port is the exception, and in most cases, must be configured in the Device Manager. The exception to this rule are modems which have their own built in serial ports. They still use a COM port like other serial devices, but their settings are pre-configured internally. The other major problem with the serial port was its lack of speed which maxxed out at 115 Kbps.

Parallel Port:
Most PCs still have a parallel port, although it is being used less and less. The main function of the parallel port was for connecting printers and scanners. Both of these devices are largely connected via USB (some printers are now networked). The maximum speed of a standard parallel port was 115 Kbps, although the later EPP/ECP varieties increased this speed to 3 MBps.

Parallel connectors are DB-25 and use a male to connect to the PC and a female to connect to the device. These connectors are shown below.

USB:
Serial and parallel ports are very slow by modern standards and suffer from a host of other problems. This led to the development of the Universal Serial Bus (USB) which has become the standard for connecting peripherals (input/output devices). USB has the advantages of being much faster, it is plug-and-play compliant, the bus is expandable via the use of USB hubs, and devices are hot-swappable. USB comes in 3 basic speeds as follows:
 

  • Low Speed - Has a rate of 1.5 Mbps that is mostly used for Human Interface Devices (HID) such as keyboards, mice, and joysticks.
  • Full Speed - Has a rate of 12 Mbps. Full Speed was the fastest rate before the USB 2.0 specification and many devices fall back to Full Speed. All USB Hubs support Full Speed.
  • Hi-Speed - Has a rate of 480 Mbps and is known as USB 2.0. Though Hi-Speed devices are advertised as "up to 60 MB/s", not all USB 2.0 devices are Hi-Speed. Most hi-speed USB devices typically operate at much slower speeds. USB 2.0 devices will work on USB 1.x hubs, but will run at the lower speed.

USB utilizes 4 different connection types as follows:

  • USB A - The USB A connector is side of the cable that plugs into your PC's USB ports. Most input devices do not use this type of connector on their end - they use one of the 2 types below.
  • USB B - This connector is easy to spot because it is square. It is plugged into a variety of peripherals, but you'll never see this port on your PC.
  • USB Mini-A - This is the newest standard used for connecting smaller devices for USB On The Go (OTG). USB OTG allows consumers to connect mobile devices without a PC. For example, USB OTG lets consumers plug their digital camera directly into a compliant printer and print directly from the camera, removing the need to go through the computer. Similarly, a PDA keyboard with a USB OTG interface can communicatea with any brand PDA that has a USB OTG interface.
  • USB Mini-B - This small connector is common for connecting smaller devices such as digital cameras and phones. Again, you will never see this port on your PC.

USB runs off of a host controller that is usually an integrated circuit that is part of the motherboard chipset. The host controller can support up to 127 devices. Most PCs only have a few USB ports, so the other options are to install an expansion card with more USB ports, or use a USB hub (much more common option). Although there are probably very few instances where 127 would be used, there are a couple of important things to note about connecting too many devices. USB devices divide the USB bandwidth between them in a first-come first-served basis and it is not uncommon to run out of bandwidth if too many devices are using. Along the same lines, many USB devices are bus-powered which means that they get their power or battery charging from the USB connection. Too many of these can cause a power drain on the system and cause the devices to not work correctly. Another thing to watch out for with USB is to note that the cable must be less than 5 meters long.

When installing a USB device, it is important to remember to install the driver before connecting the device. The reason for this is that Microsoft operating systems come with generic built-in drivers and when you plug in a USB device, Windows will typically install one of these if it can't find the official driver. The Microsoft drivers may or may not work, but more importantly, they may be missing features that your product supports. That is why it is always a good idea to install the driver that came with the product, or download and updated one from the manufacturer's web site.

Firewire:
Firewire was developed by Apple and is also known as IEEE 1394. Firewire is another high speed connection type for input/output devices that has most of the same features as USB. In fact, Firewire came out before USB and was much faster. Firewire did not become the standard that many thought it would be due to licensing fees implemented by Apple. Firewire is still around, but is not nearly as common as USB despite the fact that theoretically and practically, it is much faster.

Firewire has 2 speeds: 1394a which runs at 400 Mbps and 1394b which runs at 800 Mbps. It can support 63 devices which can use a hub, or be daisy chained (like SCSI devices). The maximum cable length between 2 devices is 4.5 meters.

Firewire has 2 different connector types, both of which can be found on a PC. One is a 6-pin connector that provides power to input devices and the other is a 4-pin connector that does not. Like USB devices, it is generally a good practice to install the driver before connecting the device


Keyboards:
Keyboards are one of the oldest and most common input devices. They connect to the motherboard at the back of the PC (USB keyboards can connect in front if there front-side USB ports). Older keyboards used a 6-pin mini DIN connector knows as PS/2, however, newer keyboards are typically USB or wireless. Most keyboards are of the 101/102 key variety.

Most keyboards simply plug in and work. Some advanced keyboards may need to have a driver installed for advanced features. Keyboard features can be configured in the keyboard control panel applet.

A PS/2 keyboard should never be attached or unplugged while the computer is on as they are not hot-swappable like USB keyboards. If a wireless keyboard start behaving erratically, it most likely needs new batteries.

Mice:
The other most common input device is the mouse which has evolved significantly over the last several years. Older mice also used a PS/2 connection as described above. The mice and keyboards were color coded to help prevent them from being plugged into the wrong port. If you plugged the mouse into the keyboard port and the keyboard into the mouse port, neither would work. Normally, you would get a 302 keyboard error during the boot process. Like keyboards, PS/2 mice have all but been replaced by USB and wireless varieties.

In addition to the connection type, the mouse itself has changed in recent years. Older mice used a ball and rollers which often got dirty and had to be regularly cleaned. Newer mice, called optical mice, use LEDs or lasers to track the movements of the mouse. Optical mice rarely require any maintenance or cleaning. In addition to this improvement, many mice now come with a scroll wheel between the 2 buttons that allows you to scroll pages on the screen up and down.

A mouse can be configured in the Mouse control panel applet. This allows you to adjust such settings as double-click speed, acceleration, right-hand/left-hand orientation, and other settings.

Digital Cameras:
Digital cameras have been slowly replacing film cameras for many applications as prices have come down over the years. They offer the advantage of being able to view a picture right after it is taken, massive storage, and of course, the ability to connect to a PC. Almost all digital cameras connect to PCs via a USB connection - typically USB mini-B. Once connected, pictures can be saved to the hard drive, emailed to people, and printed. Most cameras come with some sort of software for managing pictures. Windows XP offers a wizard that will help you download the pictures from your camera. Many cameras will also show up in My Computer and you can open the camera like you would your hard drive and browse the contents. Some computers and printers have card readers that you can insert the memory card directly in to.

The quality of picture a digital camera can take is measured in megapixels - higher being better. It is important to note that the higher the quality, the bigger the file size and the less pictures you can fit on a memory card.

Web Cameras:
Web cameras, or webcams, are mainly used for web conferencing (and porn, but that won't be on the exam) and typically connect via USB. Applications such as MSN Messenger and Skype allow you to video conference with another person with a webcam for free. Not all webcams are equal and vary widely on quality. Quality is basically determined by 2 things: resolution and frame rate. Like digital cameras, resolution is measured in megapixels. There are webcams with very high resolution, however, this creates large video sizes that are difficult to transmit over the internet. The frame rate is the number of "pictures" your camera takes per second. The higher the frame rate, the smoother the video will appear on the other end.

Most webcams come with drivers that should be installed before connecting the camera. Once installed, you can usually access the camera's properties in the control panel where you can modify audio settings, video settings, and access special features such as facial recognition if offered.

Specialty Devices:
There are many less common input devices, some proprietary or custom built, that can be connected to a PC. Below is a brief discussion of a few of them.

Biometrics are playing an increasing role in computer security and biometric devices are becoming more common on PCs. These devices provide access security by requiring verification of a person's identity. There are many different kinds including fingerprint scanners, retina scanners, and voice recognition software just to name a few. These devices can be built into the PC, keyboard, or other device, and they can also be external devices usually connected to a USB port. After installing such a device, you will likely need to do an initial scan of whatever biometric information the device measures so that it knows what to look for in the future.

Touch screens are computer monitors that are technically input and output devices. You input commands by touching the screen with your finger or a stylus. These are most commonly found in PDA format or in store/information kiosks. Touch screens allow one to get rid of the keyboard and mouse. In addition to the regular monitor connection, there will usually be a separate USB or PS/2 connection for the mouse portion of the monitor. This will typically require driver installation.
 

Bar code readers are used to read standardized Universal Product Code (UPC) bar codes that are commonly found on most products you will find in any major store. Bar codes readers can scan product information to the PC to update inventory databases. Modern readers use a USB, PS/2, or wireless connection and typically interface with specialized software.