Nash - Scientific Computing with PCs (523165), страница 33
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The new track does not completely obliterate the edgesof the old, so that subsequent attempts to read the disk on a 360K drive may fail. Occurrences of thistype of error can be minimized by writing low-capacity disks on 360K drives only. That is, we onlyread them on the high capacity drives. In practice, we have found quite low error rates, and that somediskettes seem more prone to the problem than others.11.4 Tapes, CDs and Other Storage MediaMost of the above comments about cleanliness and gentle handling apply equally to all forms of storage.Similarly, we want to maintain adequate labelling and documentation so that we do not mix up differentvolumes of data.
This is particularly important for tapes, since they are used mostly for backup purposes,so represent our "insurance" against disaster. We recommend developing a set of simple procedures tocarry out backups, following suggestions from the manufacturer of the tape drive and the developer ofthe backup software for handling tapes.
CD-ROMs are usually well-labelled by their producer. The onlyproblems we have noted are read-errors, probably caused by dust.Other storage media will generally have a proprietary physical form. However, there are some seeminglystandard disk drives that use nonstandard format disks of a regular physical size to achieve high capacity.Some of these drives may also be able to handle standard formats, but in any event, users should take careto distinguish the "special" disks from the regular ones.11.5 Power ConsiderationsThe quality of power supplied to electronic equipment is important in preventing damage to the parts orerrors in programs and data.
Alternating current supplied to households or offices has specified voltageand frequency. Circuits drawn from such mains supply are rated for specific maximum current (amperage)or power (wattage). In North America typical ratings are 115 Volts, 60 Hz (or cycles/second), 15 Amps,while in the UK and Europe we would see 240 Volts, 50 Hz, 13 Amps.The supplied electrical energy may, however, fail to meet specifications:•It may be below voltage, even to the extent of a "brownout";•It may be over-voltage, so lights burn very brightly or fail; or•The frequency may be unsteady (this usually precedes a blackout).Furthermore, the electrical grid serves many users and is a dynamic system. Just as one may experiencechanging water pressure when someone flushes a toilet, so power supplied to a given wall outlet mayfluctuate.
Such fluctuation may be due to high voltage DC pulses like lightning, temporary voltage dips11: HARDWARE AND OPERATING PRACTICE95due to heavy demand suddenly placed on the system such as a furnace motor switching on, or highfrequency transients that are superimposed on the AC power from equipment switching or frominteractions between devices connected to the system.The PC and peripherals are equipped to handle some of these conditions, but we may wish to add extraprotection with commercially available line filters, isolation transformers, or uninterruptable powersupplies. Our own practice has been to use inexpensive surge-protected power bars.
These offer onlymodest protection from the hazards above, but they cost little more than conventional power bars. We alsouse a telephone line protector on our modem line.When power difficulties are suspected, one has the choice of buying or renting expensive monitoringequipment or installing protective devices. No device used between the AC line and PC offers totalprotection. One choice is a battery powered machine. We have seen our own notebook run through ablackout without a hiccup (we were running it off the mains as we charged the battery).
The conventionalwisdom with Ni-Cad rechargeable batteries is to run equipment to complete discharge then recharge toavoid the "memory" of this type of battery.96Copyright © 1984, 1994 J C & M M NashSCIENTIFIC COMPUTING WITH PCsNash Information Services Inc., 1975 Bel Air Drive, Ottawa, ON K2C 0X1 CanadaFigure 11.3.1Disk holder made from single sheet paper.disks.Copy for:Dr. Dobb’s Journalb should be 3.5" for 5.25" disks; 2.5" for 3.5"Apart from the electricity coming from it, the wall outlet needs to be considered in other ways. Walloutlets should have three holes; a live (or hot) connection, a neutral and a ground or earth. Figure 11.5.1shows a typical North American configuration. Unfortunately, it is very easy to wire a socket incorrectly;it may still seem to work, but the ground may not be connected and/or the hot and neutral may bereversed. The user can verify the socket by means of commercial devices (e.g., Leviton Circuit Tester,Model 49662, Cable Electrical Products Inc., Providence, RI) or by carefully checking voltages.
Correctwiring implies: Live - Neutral 115 V AC; Live - Ground 115 V AC; Neutral - Ground 0 V AC. Circuitsshould have a correctly rated fuse or circuit breaker.Figure 11.5.1 North American 110-115 Volt AC wall outlet11: HARDWARE AND OPERATING PRACTICE97On the subject of fuses, users should be aware of the locations and ratings of fuses in equipment. Fusesare the first place to look when a failure occurs. Simple replacement of a fuse rarely solves a problem.Fuses blow for a reason, for example, a jam in a printer.
However, by knowing which fuse has failed, wemay quickly localize the trouble.11.6 Interference and Radio Frequency ShieldingThe two issues here are the interference with the PC and the emanation of radiative energy from the PC.It is difficult to identify cases where radiation from outside the PC has upset its operation. There havebeen concerns that the very low charge used to store data in modern memory chips could be altered byparticle radiation from the decay of radio isotopes in the chip substrate or packaging.
Cosmic rays arepresumably another menace. Man-made radiation, for example, over-limit CB radio transmissions or staticfrom unfiltered automotive engines, is a potential source of trouble because it can be clearly registeredon stereos and television receivers. However, well-documented cases of such interference with PCs appearto be lacking.The only clear cases of interference come from the modem — the link between the PC and the telephonenetwork. Here line noises from whatever cause are a common source of data transmission errors.We may divide emanations from the PC into three parts:•Ionizing radiation (ultra-violet or X-rays);•Radio frequency radiation;•Glare, noise or other stress inducing factors.From available evidence (Treurniet, 1982, and similar reports since), it does not appear that PCs orvideo-display terminals emit ionizing radiation in significant amounts.
This does not mean that long-termhuman health anxieties should be disparaged. However, it does appear that ionizing radiation is not ahazard of PC equipment.Radio frequency (RF) radiation from PCs and related equipment is another matter. This can be detectedwith an ordinary radio or television as audible or visible "noise" received at broadcast frequencies. SuchRF energy may have health implications for operators of PC equipment.
However, since all kinds of radiotransmitters, both public and private, are pumping a great deal of electromagnetic energy into ourenvironment, avoiding such energy requires a metal suit.Governments have made regulations concerning RF emanations from PCs. In particular, the United StatesFederal Communications Commission requires manufacturers to conform to strict tests and to provideshielding. Cables running between PC components may serve as an antenna to broadcast RF signals fromwithin a PC case.
Shielding cables is troublesome. Some military applications require no signal leakage,as enemy detectors could then fix the position of equipment quite easily. The TEMPEST shieldingtechnologies developed for such applications will come to the consumer product area when the cost ofcomplying with stricter RF emanation rules makes them worthwhile.Stress induced by glare and noise also receive attention. There are special anti-glare filters to place in frontof the screen.
Impact printers have acoustic covers to muffle the noise of the print head. Fans haveaerodynamic blades to keep them quiet. Such trends are complementary to ergonomically designedequipment intended to aid user productivity and acceptance of the new technology.11.7 Servicing and Spare PartsEvery PC will fail from time to time, so users must be prepared for "down time" and repairs. The user’sresponse to hardware failure will depend on his or her ability to repair things, on the type of use to which98Copyright © 1984, 1994 J C & M M NashNash Information Services Inc., 1975 Bel Air Drive, Ottawa, ON K2C 0X1 CanadaSCIENTIFIC COMPUTING WITH PCsCopy for:Dr.
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