Addendum to Week 22 External Devices


Four types of connectivity – Din, Mini-Din, USB, wireless

Straight Keyboard vs. Curved

Ergonomic – designed for human comfort – also to reduce lawsuits – Carpel Tunnel Syndrome – repetitive motion injuries

Proper way to type – hands and elbows level – fingers pointed down – feet below chair

Types of contacts

Foil – creates no noise

Metal – makes distinctive click when depressed – can be subject to corrosion

Connection types

DIN and mini-DIN


How they work.

Keyboards contain a controller chip called an 8042. This chip acts very much a mini, specialized CPU. It scans the keys for depression. When a key is depressed, the 8042 chip raised the IRQ line to the CPU seeking attention. Upon approval from the CPU that the bus is clear, the 8042 chip sends the data through the data bus and eventually to the CPU. The keyboard synchronizes its action by using the CPU clock.

The 8042 is A reduced Instruction Set device in that it contains all the lines of instructions necessary to communicate with the CPU. The 8042 chip is discovered during BIOS and communication rules are established during boot up.

Some keyboard bios allows keyboards to be password protected. Signs of password protection are all three green lights flashing after boot. Removal is done through bios – remove battery if necessary.

Maintenance – Keyboard are relatively cheap so the entire unit is considered to be a field replaceable unit. However, the inevitable spill or dropped liquid onto the keyboard can be cleaned by disassembly, cleaning by soaking or rag, and then thorough drying before reassembly and reinstallation. Biggest problem usually is broken keys and simplest solution to that (because of broken small pieces of plastic or springs missing) is to replace entire unit.

Mice and other pointing devices

Mice and other devices shown here are both input devices and control devices. They control the position of the curser but can control the software (opening menus, etc)

They connect to the computer either through the 9 pin serial port, 6 pin mini-Din port, an USB port, or wirelessly.

The standard mouse as shown here has three button type (or two buttons and a rotary wheel) with a track ball located under the mouse. This track ball rotates of all four axis and makes contact with the surface turning the internal axis shafts reporting to the CPU the location of the curser. This location finding and reporting are accomplished using two chopper wheels with LED beams passing through the slits in the chopper wheels. This information is then translated into X and Y coordinates.

Some mice are optical in that the send a beam of laser light to the surface under the mouse and read the reflecting light for positional information. As with keyboards, mice are usually a field replacement item but a little cleaning and maintenance will help to extend their lifespan.


Maintenance for mice include removal of the ball unit under the mouse and cleaning accumulated lint, hair, and fibers from the axis shafts. The exterior of the mouse can be cleaned with a soft detergent and dust free rag but immersion of the unit is not recommended unless complete drying time is allotted.

NOTE: Mouse can be installed into mini-DIN port for the keyboard and vice versa. Pay attention when installing or when someone reports a malfunctioning mouse and/or keyboard.

Instructions for the operation of the mouse are contained in either the standard BIOS or specialized software packages for those other devices.

Other devices include

Touch Screens – require specialized drivers and screens – contact is made by depression of a wire in the screen. Cleaning of a touch screen should be avoided unless you follow exactly the manufacturer’s recommendations – you may destroy the screen by cleaning it.

Track balls – work just like a mouse except that it doesn’t move around – a ball moves.

Joysticks – Gamers

Graphics pods or tablets – used a lot in engineering applications

All pointing devices are usually too expensive to repair so they are usually field replaceable items.


Monitors are an output device. It reports only what is sent to it by the CPU. Monitors require specialized drivers and video cards depending on the manufacturer of the device.

However, Windows has a basic driver that it uses when the card is first initialized. This basic VGA style plug and play monitor uses the driver display.inf or vga.sys as its drivers. This naturally reduces the effects you can use, etc. A symptom of the wrong driver is that you only can use 24 or 256 colors, can only use one screen size 600x480, and have no other characteristics.

Monitors are of two types – CRT and LCD

CRT stands for Cathode Ray Tube and is similar to a TV

LCD stands for Liquid Crystal Display (sometimes called plasma)

CRTs use electron guns to shoot a stream of electrons onto a phosphorous coated glass screen. Phosphorous glows when excited by this electron stream. Phosphorous is of three colors – red, green, and blue. The guns (also one for each color) scan left to right (when facing the screen) top to bottom. There are two types of scanning – non-interleaved and interleaved. Interleaved scanning skips a line on every pass then catches the missed lines on the second pass. Interleaved saves power consumption but at a loss of screen clarity. The three guns are supposed to line up and only excite the appropriate color of phosphorous pixel. When the guns are not lined up correctly, little multicolored ghosts of the original image will surround the image

The refresh rate is the rate at which the phosphorous has to be re-excited before it fades – in the United States that refresh rate varies from 43-120 Hz but the normal is 60 Hz.

The dot pitch of a monitor is the distance from the center of one pixel to the center of another one. The closer that distance, the sharper the image. Runs from .17 inch to .28 inch. The lower the number, the clearer the image.

Pixels – rectangular grouping of dots. #of pixels is called resolution. A 4 to 3 aspect ratio means that resolution has 640 x 480 pixels (that is a 4-3 ratio). I like the size 1024x768 - others may not.

The size of a monitor is the measure of the diagonal – salesman is why explain.

Viewable area – the actual size of the screen that contains data. Diagonal minus black edge.

Shadow mesh is a small black metal screen installed inside the glass of a CRT to help in the bleed over of phosphorous excitement to adjacent pixels. Result is a much sharper image on the screen.

Multimedia monitors do not seem to work as well as single purpose monitors for some reason. I think the presence of two powerful magnets near the side of the monitor may but just may skew the image a little – but I don’t know.

Typical controls are found on the buttons somewhere on the CRT. These buttons have an effect on the viewable area of the screen, they may increase decrease shift left or right, or shift up and down. They may correct bending or beveling of the image – draw on screen the controls typically found.

LCD – Liquid Crystal Display

Advantage is smaller footprint on the desktop

Disadvantages – not as clear but they are getting better – no controls – may be off-center

Made up of three components

Backlight – 1 or 2 fluorescent bulbs provide the brightness

Brightness is measured in nits and is brightest in center and trails to edge

Display – made up of liquid filled crystals and transistor matrix

Electronic control circuits to control the firing of the matrixed transistors.

Connectors for digital monitors

CRT’s typically use 15 pin VGA Analog signal

LCDs use DVI interface – use digital signal – uses new kind of memory chips

Expansion Cards

PCI – most older computers use PCI cards – not as fast or graphic intensive

AGP – Accelerated Graphics Port – two versions AGP 2.0 and AGP 3.0

Contains own CPU and memory on card – special connector to motherboard

Original card was the VGA – video graphics adapter (isa or eisa)

Replaced by Super VGA

AGP same speed as system bus – connects directly to memory – 32 bit data path shares memory with CPU

AGP 2.0 transfer rate 4X clock rate (264 MB/sec) has no slot on rear of card

AGP 3.0 transfer rate 8X clock rate (2.16 GB/sec) has slot on rear of card

Amount of memory onboard card is important to graphic intensive applications such as games.

Types of video memory

Vram – dual ported – input and output at same time

SGRam – synchronous graphics ram

Wram – Window panning memory

3-d Ram 3-d graphics

Direct RDram – video streaming

DDR and DDR2 available as new types

Technician notes – upgrade on original card may not be and usually isn’t possible. Must match current memory when replacing.

Texture and Lighting

Texture is created by using small pictures tiled again and again. Can be darkened or lightened to create the desired texture.

Drivers – uninstall old driver before installing new driver – system will default to windows basic driver – do through Device Manager