Notebook computers use PCMCIA cards of two types for expansion: 18-bit PC Cards based on the ISA bus and 32-bit CardBus cards based on the PCI bus. No matter which standard a given card follows, however, it must follow exactly the same physical specifications as all PCMCIA cards. Every card will slide into every slot—well, almost. The standard allows for three thickness variations, but one of which is almost never found, and nearly all slots will accommodate the other two sizes. In other words, physically, PC Cards and CardBus cards are identical, and both use exactly the same connector.

Under the PCMCIA standard, PC Cards and CardBus cards come in three sizes, differing only in thickness. The basic unit of measurement—the size of the typical card slot—is based on the medium-thickness card, designated Type II. Measuring 54 by 85 millimeters (2.126 by 3.37 inches) and 5 mm (about three-sixteenths of a inch) thick.

PC Cards and CardBus cards physically follow the form factor of earlier memory cards (including the IC Card) standardized by JEIDA (the Japan Electronic Industry Development Association). The first release of the PCMCIA specification paired this single-size card with a Fujitsu-style 68-pin connector. Under the current PCMCIA specification, version 8.0 (and all versions since the 2.1 specification), this form factor is designated as the Type I PC Card.

The thinness of the Type I card proved an unacceptable limitation. Even without allowing for the PC Card packaging, some solid-state devices are themselves thicker than 3.3 mm. Most important among these “fat” devices are the EPROMs used for nonvolatile storage. (Most computers use EPROMs to store their system BIOS, for example.) Unlike ordinary, thin ROMs, EPROMs can be reprogrammed, but this requires a transparent window to admit the ultraviolet radiation used to erase the programming of the chip. The windowed packaging makes most EPROMs themselves 3.3 mm or thicker.

Fujitsu faced this problem when developing the firmware to be encoded on memory cards and therefore developed a somewhat thicker card that could be plugged into the same sockets as could standard memory cards. Modem and other peripheral makers found the Fujitsu fat card more suited to their purposes. To accommodate them, PCMCIA 2.0 standardized an alternative: the Type II PC Card. Essentially based on the old Fujitsu developmental EPROM form factor, Type II PC Cards are 5 mm thick but otherwise conform to the same dimensions as Type I cards.

The PCMCIA standard puts the extra thickness in a planar bulge, called the substrate area, in the middle of the card. This thicker area measures 48 mm wide and 75 mm long. Three millimeters along each side of the Type II card are kept to the thinness of the Type I standard so that the same card guides can be used for either card type. Similarly, the front 10 mm of a Type II card maintain the 3.3 mm thickness of the Type I standard so that the same connector can be used for either card type. Naturally, the actual card slot for a Type II card must be wide enough to accommodate the maximum thickness of the card.

In September 1992, PCMCIA approved a third (Type III) form factor for PC Cards. These still-thicker cards expand the bulge of Type II from 5 mm to 10.5 mm and are designed to accommodate miniaturized hard disks and similar mechanical components. As with Type II cards, Type III PC Cards remain thin at the edges to fit standard card guides and standard connectors. Although a number of hard disk drives appeared in this format, few are used today in notebook computers. However, high-capacity MP3 players have embraced the Type III card design.

In practical terms, a Type I card comes closest to being a truly flat, credit-card style card. Type II cards have small bulges at the top and bottom to accommodate circuitry. Type III cards have thick lumps to hold a disk drive. Figure 30.9 illustrates the apparent differences between the three card types.

Note that many notebook computers lay out their endowment of two PCMCIA slots with one over the other and without a metal separator. This design allows the two slots to hold two individual Type II or a single Type III card. Nearly all current PCMCIA slots are wide enough to accommodate Type II cards. The only place you’re likely to encounter a slot that will accept only Type I cards is equipment manufactured before 1992.

Under the current PCMCIA standard, both Type I and Type II cards can be implemented in extended form. That is, their depth can be increased by an additional 50 mm (to 135 mm) to hold additional componentry—for example, the antennae of WiFi network adapters. Such extended cards project about two inches more from standard PCMCIA slots.

To ensure that all cards easily and securely mate with their connectors, the PCMCIA standard requires that card guides be at least 40 mm long and that the PCMCIA card connector must engage and guide the connector pins for 10 mm before the connector bottoms out.

The layout of a PC Card or CardBus card is essentially symmetrical, meaning that it could inadvertently be inserted upside down. The PCMCIA design allows for such cases of brain fade by eliminating the risk of damage. Although the cards do not work while inverted, neither they nor the computers into which they are plugged will suffer damage.

Because the size and placement of labels on the cards is part of the standard, when you are familiar with the layout of one PC Card, you will know the proper orientation of them all, and CardBus cards as well. Moreover, other physical aspects of the cards—the position of the write-protect switch (if any) and battery (if needed)—are standardized as well. The PCMCIA standard also recommends that the batteries in all cards be oriented in the same direction (positive terminal up).

In addition to the physical measures that facilitate getting the cards into their sockets, two pins—one on each side of the connector—allow the computer host to determine whether the card is properly seated. If the signal (ground) from one is present and the other is not, the system knows that the card is skewed or otherwise improperly inserted in the connector.

Connector

All types of PCMCIA cards use the same 68-pin connector, whose contacts are arranged in two parallel rows of 34 pins. The lines are spaced at 1.27 mm (0.050 inch) intervals between rows and between adjacent pins in the same row. Male pins on the card engage a single molded socket on the host.

To ensure proper powering up of the card, the pins are arranged so that the power and ground connections are longer (3.6 mm) than the signal leads (3.2 mm). Because of their greater length, therefore, power leads engage first so that potentially damaging signals are not applied to unpowered circuits. The two pins (36 and 67) that signal that the card has been inserted all the way are shorter (2.6 mm) than the signal leads.

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Besides heat, all electrical circuits radiate something else—electromagnetic fields. Every flow of electrical energy sets up an electromagnetic field that radiates away. Radio and television stations push kilowatts of energy through their antennae so that this energy (accompanied by programming in the form of modulation) radiates over the countryside, eventually to be hauled in by a radio or television set for your enjoyment or disgruntlement.

The electrical circuits inside all computers work the same way but on a smaller scale. The circuit board traces act as antennae and radiate electromagnetic energy whenever the computer is turned on. When the thinking gets intense, so does the radiation.

You can’t see, hear, feel, taste, or smell this radiation, just as you can’t detect the emissions from a radio station (at least not without a radio), so you would think there would be no reason for concern about the radiation from your computer. But even invisible signals can be dangerous, and their very invisibility makes them more worrisome—you may never know if they are there or not. The case of your computer is your primary (often only) line of defense against radiation from its electronic circuitry.

The problems of radiation are twofold: the radiation interfering with other, more desirable signals in the air, and the radiation affecting your health.

Radio Frequency Interference

The signals radiated by a computer typically fall in the microwatt range, perhaps a billion times weaker than those emitted by a broadcasting station. You would think that the broadcast signals would easily overwhelm the inadvertent emissions from your computer. But the strength of signals falls off dramatically with distance from the source. They follow the inverse-square law; therefore, a signal from a source a thousand times farther away would be a million times weaker. Radio and television stations are typically miles away, so the emissions from a computer can easily overwhelm nearby broadcast signals, turning transmissions into gibberish.

The radiation from the computer circuitry occurs at a wide variety of frequencies, including not only the range occupied by your favorite radio and television stations but also aviation navigation systems, emergency radio services, and even the eavesdropping equipment some initialed government agency may have buried in your walls. Unchecked, these untamed radiations from within your computer can compete with broadcast signals, not only for the ears of your radio but those of your neighbors. These radio-like signals emitted by the computer generate what is termed radio frequency interference (RFI), so called because they interfere with other signals in the radio spectrum.

The government agency charged with the chore of managing interference—the Federal Communications Commission—has set strict standards on the radio waves that personal computers can emit. These standards are complex, and ensuring your computer is actually in compliance would require expensive test equipment. The law does not require that you check—the burden is on the manufacturer. Moreover, at their hearts, the FCC standards simply enforce a good-neighbor policy. They require that the RFI from computers be so weak that it won’t bother your neighbors, although it may garble radio signals in your own home or office.

The FCC sets two standards: Class A and Class B. Computer equipment must be verified to meet the FCC Class A standard to be legally sold for business use. Computers must be certified to conform with the more stringent FCC Class B standard to be sold for home use.

Equipment-makers, rather than users, must pass FCC muster. You are responsible, however, for ensuring that your equipment does not interfere with your neighbors. If your computer does interfere, legally you have the responsibility for eliminating the problem. While you can sneak Class A equipment into your home, you have good reasons not to. The job of interference elimination is easier with Class B certified equipment because it starts off radiating lower signal levels, so Class B machines give you a head start. Moreover, meeting the Class B standards requires better overall construction, which helps ensure that you get a better case and a better computer.

Minimizing Interference

Most television interference takes one of two forms: noise and signal interference.

Noise interference appears on the screen as lines and dots that jump randomly about. The random appearance of noise reflects its origins. Noise arises from random pulses of electrical energy. The most common source for noise is electric motors. Every spark in the brushes of an electric motor radiates a broad spectrum of radio frequency signals that your television may receive along with its normal signals. Some computer peripherals may also generate such noise.

Signal interference usually appears as a pattern of some sort on your screen. For example, a series of tilted horizontal bars or noise-like snow on the screen that stays in a fixed pattern instead of jumping madly about. Signal interference is caused by regular, periodic electrical signals.

Television interference most commonly occurs when you rely on a “rabbit ear” antenna for your television reception. Such antennae pull signals from the air in the immediate vicinity of the television set, so if your computer is nearby, its signals are more likely to be received. Moving to cable or an external antenna relocates the point your TV picks up its signals to a distant location and will likely minimize or eliminate interference from a computer near the TV set.

You can minimize the interference your computer radiates and improve your television reception by taking several preventive measures.

The first step is to make sure the lid is on your computer’s case and that it and all expansion boards are firmly screwed into place. Fill all empty expansion slots with blank panels. Firmly affixing the screws is important because they ground the expansion boards or blank panels, which helps them shield your computer. This strategy also helps minimize the already small fire hazard your computer presents.

If the interference persists after you’ve screwed everything down in your computer, next check to see if you can locate where the interference leaks out of your computer. The most likely suspects are the various cables that trail out of your computer and link to peripherals such as your monitor, keyboard, and printer. Disconnect cabled peripherals one at a time and observe whether the disconnection reduces the interference.

Because it operates at the highest speed (and therefore the highest frequency), an external SCSI cable is most prone to radiating interference. All external SCSI cables should be shielded.

Your mouse is the most unlikely part of your computer to cause TV interference. The mouse operates at serial data rates, which are much too low to interfere even with VHF television.

If disconnecting a cable reduces onscreen TV interference, the next step is to get the offending signal out of the cable. The best way is to add a ferrite core around the cable. Many computer cables already have ferrite cores installed. They are the cylindrical lumps in the cable near one or the other connector. Install the ferrite core by putting it around the offending cable near where the cable leaves your computer. You can buy clamp-on ferrite cores from many electronic-parts stores.

Unplugging one cable—your computer’s power cable—should completely eliminate the interference radiated by your computer. After all, the computer won’t work without power and can’t generate or radiate anything. You can reduce the interference traveling on the power line by adding a noise filter between your computer’s plug and its power outlet. You can usually obtain noise filters from electronic-parts suppliers. Although a noise filter is not the same thing as a surge suppresser, most of the better surge suppressers also include noise filtering.

Health Concerns

Some radiation emitted by computers is of such low frequencies that it falls below the range used by any radio station. These very low frequency and extremely low frequency signals (often called VLF and ELF) are thought by some people to cause a variety of health problems.

Your computer’s case is the first line of defense against these signals. A metal case blocks low-frequency magnetic fields, which some epidemiological studies have hinted might be dangerous, and shields against the emission of electrical fields. Plastic cases are less effective. By themselves they offer no electrical or magnetic shielding. But plain plastic cases would also flunk the FCC tests. Most manufacturers coat plastic cases with a conductive paint to contain interference. However, these coatings are largely ineffective against magnetic fields. Most modern systems now use metal cases or internal metal shielding inside plastic cases to minimize radiation.

No matter the construction of your computer, you can minimize your exposure to radiation from its case by ensuring that it is properly and securely assembled. Minimizing interference means screwing in the retaining brackets of all the expansion boards inside your computer and keeping the lid tightly screwed into the chassis. Keeping a tight computer not only helps keep you safe, it also keeps your system safe and intact.

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