The computer encasement is made from a heavy metal and has two major functions:

The case comes in two major configurations: the desktop model and the tower (Figure 7-4). The desktop model is generally positioned in a horizontal encasement, whereas a tower model is in a vertical encasement. As the name implies, most desktop models are placed on the desk underneath the monitor. The tower model is generally placed underneath the desk within arm’s reach of the operator. The biggest disadvantages of the desktop model are (1) the space it takes up on the desk and (2) having less room for expansion and upgrades because of the smaller encasement. The tower model consistently provides adequate room for expansion of components, and it is easily placed out of the way and off the work surface.

The motherboard (Figure 7-5) is the largest circuitry board inside the computer, and it contains many important small components to make the computer function properly. This chapter covers only a few of the motherboard’s components in detail: the CPU, basic input/output system (BIOS), bus, memory, ports, and complementary metal oxide semiconductor (CMOS).

The CPU.

Many people refer to the personal computer’s (PC) encasement as the CPU. This is incorrect. The central processing unit (CPU), or microprocessor, is a small chip found on the motherboard (Figure 7-6). The microprocessor is the brain of the computer. It consists of a series of transistors that are arranged to manipulate data received from the software.

Microprocessors come in many different sizes and speeds and are manufactured by two major companies, Intel and Advanced Micro Devices (AMD). The CPU’s basic tasks are to read data from storage, manipulate the data, and then move the data back to storage or send it to external devices, such as monitors or printers.

The microprocessor is named after its manufacturer and the speed at which it manipulates data. The first microprocessor to be placed in a computer was made in 1979 by Intel and was called the 8088.

It had a clock speed of a mere 4.77 MHz. The more modern Pentium 4 microprocessor has speeds upward of 3.2 to 3.8 GHz. To put these speeds in perspective, the 8088 needed about 12 cycles to complete one basic instruction, and the modern Pentium processor can complete one instruction per cycle.

Memory.

The memory in the computer is used to store information currently being processed within the CPU (Figure 7-7). This memory is also known as random access memory (RAM). The RAM is short-term storage for open programs. The microprocessor has a small amount of memory within itself but not enough to tackle the large amounts of data being generated by high-level programs. The RAM will take the data from the CPU so that the CPU can handle the processing needs of the programs that are running. The RAM is only temporary; once the computer has been turned off, the RAM is wiped clean. With today’s complex programs and graphics, computers require more memory to function at an acceptable level. There are many different types of RAM available: DRAM, EDO RAM, VRAM, SRAM, SDRAM, SIMM, DIMM, and ECO. Most modern PCs have an SDRAM-DDR, specifically DDR3 SDRAM (double data rate type three synchronous dynamic random access memory). Memory is measured in bytes and can be found in configurations such as 4 gigabytes (GB), 8 GB, 12 GB, and so on. In 1975 the Altair 8800 came with 0.25 kilobytes (KB) of memory at a cost of $103; at that price, 1 gigabyte would cost approximately $432 million. With more modern pricing, one can purchase 4 GB of DDR3 for approximately $25, which equates to $6.25 per gigabyte. These figures are given for perspective purposes and become quickly outdated, so please research current memory capacities and prices for up-to-date information.

Ports.

The computer’s ports are a collection of connectors sticking out of the back of the PC that link adapter cards, drives, printers, scanners, keyboards, mice, and other peripherals that may be used. There are many different types of ports, such as parallel, serial, USB, integrated drive electronics (IDE), and small computer system interface (SCSI). We discuss each of these types and how they may be used within a system.

A parallel port is a 25-pin connector (Figure 7-8). The parallel port was synonymous with a printer port because it was most often used for that purpose prior to the widespread use of the USB connection. A parallel port could send 8 bits of data through the connection, whereas a serial port could send only 1 bit of data down a single wire. A serial port can be universally used for many of the components plugged into the computer, such as a mouse, which does not require the speed of a parallel port. Most serial ports are of the 9-pin variety, but some can have up to 25-pin connectors.

A USB connection is the most common wired connection used between devices today (Figure 7-9). The advantage of a USB port is that multiple devices may be connected into one port. In older computers there were only ports for the keyboard and the mouse, one parallel port for a printer, and one serial port for a modem. By using USB ports the user can connect up to 127 devices to one single USB port. Most computers have more than one USB port available, so the possible connections are many.

IDE ports can be found on the motherboard and connect the hard drive, floppy drive, and CD-ROM drive to the board. A series of ribbon cables runs throughout the computer to connect the IDE devices to the IDE port on the motherboard. The fifth type of port is the SCSI port. It is the fastest and most versatile way for a PC to communicate with its peripherals. A single SCSI controller can manage up to seven devices through a daisy chain connection. The most common SCSI devices are hard drives, CD-ROM drives, scanners, and printers.

The network interface card (NIC) can come either as an expansion card (Figure 7-10) plugged into a slot or as part of the PC motherboard circuitry. The network card will have an RJ-45 adapter jack (Figure 7-11) at the rear of the PC for the acceptance of a twisted-pair wire with RJ-45 connector (Figure 7-12). This network card will enable the PC to connect to other PCs that are on the same network. Detailed information about networks is discussed in the next chapter.

The power supply (Figure 7-13) delivers all electricity to the PC and contains a fan to help keep the inside of the computer cool. It contains a transformer that converts the wall outlet alternating current (AC) to direct current (DC) in the voltages appropriate for each powered device. All components, from the motherboard to the hard drive, get their power directly from the main supply through different colored wires that end in plastic shielded connectors. The power supplies deliver ±12 V, ±5 V, and in some machines +3.3 V. Power supplies are rated in watts. Most power supplies deliver between 150 and 300 W, but some computers require a 400-W power supply. The power supply is designed to take the brunt of the force if the computer ever receives a power surge. In such a case, the power supply is easily replaced.

The hard drive is the main repository for programs and documents on a PC. The hard drive is made up of many hard, thin magnetic platters that are stacked one on top of the other with only enough space for a read-write head to glide over the surface of the disks (Figure 7-14). The disks are spun at a fast speed by a small motor, and the read/write head glides to the area that houses the particular information needed and reads or writes as asked.

The early disks had a storage capacity of 10 MB and could be accessed in approximately 80 ms. In 1980 a 26-MG drive was priced at $5,000. The more modern disks can hold upward of 3 terabytes (TB) with an access speed of 4 ms at a price of $160. In 1980, this 3 TB of storage would have cost approximately $19.7 million. As storage capacity has skyrocketed, the price per gigabyte of storage has drastically decreased. The drives may be faster than ever, but they are still the slowest part of the PC because they are both mechanical and electrical. These figures were given for perspective purposes and become quickly outdated, so please research current hard drive capacities and prices.

A compact disk (CD) is a thin injection-molded polycarbonate plastic disk (Figure 7-15). The disk is impressed from a mold to form microscopic bumps that indicate either a 1 or a 0 to the computer. Over the bumps is a reflective layer of aluminum, and over that is a clear protective coat of acrylic. A CD can hold up to 74 minutes of music or approximately 780 MB of data.

A digital versatile disk (DVD) holds up to seven times more than the CD, which equates to about 9.4 (single-sided) to 17 GB (double-sided) of data. A DVD has multiple layers of polycarbonate plastic. Aluminum is used behind the inner layers, and gold is used behind the outer layers. The gold is semireflective so that it allows the laser to penetrate through to the inner layers of plastic.

There are three main types of CD/DVD drives available in today’s market: the ROM (read-only memory), the R (write once–read many), and the RW (read and write many times). CD-ROM drives were placed into early computers. Few computers today can be bought with a simple ROM drive installed. Most modern computers have either a CD-RW or a CD/DVD-RW. With an R or RW drive, information that needs to be saved, transported, or archived can be “burned” (information written on a disk). The information is burned onto the disk, starting in the center and spiraling out to the edge of the disk. The laser burns a tiny depression (pit) into the disk to represent the data being saved. A burned disk will be a series of pits and lands, or areas that were not burned by the laser. Two-sided DVDs can be burned on both sides to double the capacity of the disk.

CRT monitors were used in the first generation of PCs, and this type of monitor continues to be popular (Figure 7-16). The CRT consists of a cathode and anode within a vacuum tube. The CRT works much like an x-ray tube, in that the cathode boils off a cloud of electrons and then a potential difference is placed on the tube. A stream of electrons is sent across to the monitor’s anode, which is a sheet of glass coated with a phosphor layer. The electrons strike the phosphor on the glass, causing the glass to emit a color that is determined by the intensity of the interaction and area with which the electrons interacted.

The electrons interact with either a red, green, or blue dot to form the color and image that is being sent from the video card signal. The electron beam starts in the upper left corner and scans across the glass from side to side and top to bottom, and once it reaches the bottom, it starts again at the top left. On average, most monitors have 350 lines to be scanned. Earlier we discussed the refresh rate being 60 to 75 Hz. This equates to 350 lines being scanned 60 to 75 times per second.

An LCD monitor produces images by shining or reflecting light through a layer of liquid crystal and a series of color filters (Figure 7-17). An LCD has two pieces of polarized glass with a liquid crystal material between the two. Light is allowed through the first layer of glass, and when a current is applied to the liquid crystal, it aligns and allows light in varying intensities through to the next layer of glass through color filters to form the colors and images seen on the display. These monitors are particularly effective in brightly lit rooms. One drawback, however, is that LCD monitors are best viewed straight from the monitor surface because the image fades as the viewing angle moves away. Research is ongoing to minimize viewing issues.

Most consumers want a monitor that can provide the highest resolution for the best price. Table 7-1 outlines the advantages and disadvantages of the two major types of monitors. Historically, most radiology departments used the CRT because of its superior resolution, but LCDs have now taken over health care because they are slimmer and lighter. The LCD monitor has improved many of its early shortcomings to overtake the CRT in medical imaging.