Keyboard technology

Thereseveral typeskeyboard, usually differentiated byswitch technology employedtheir operation. Since thereso many switches needed (usually about 80-110)becausehavebe highly reliable, this usually defineskeyboard. The choiceswitch technology affects key response (the positive feedback thatkey has been pressed)travel (the distance neededpushkeyreliably entercharacter). Newer models use hybridsvarious technologiesachieve greater cost savings.

Dome switch keyboard

Dome switch keyboardskind ofhybridmembranemechanical keyboards. They bring two circuit board traces together underplastic "dome" or bubble. The top ofbubblecoatedsome conductive substance. Whenkeypressed,collapsesdome, which shorts outtwo circuit tracescompletesconnectionentercharacter. The pattern onPC boardoften gold-plated.

This iscommon switch technology usedmass market keyboards today. It's considered very quiet, but purists tendfind"mushy" becausecollapsing dome doesn't provide as much positive response ashard closing switch. Thesealsogood choiceoffice or consumer environments becausesharecertain degreeliquid resistancetheir membrane ancestors.

Capacitive keyboard

In this typekeyboard, pressingkey changescapacitance ofpattern printed onPC board. Usually this permitspulse or pulse trainbe sensed. Unlike "dome switch" keyboards,pattern will be covered bythin, insulating film. Capactive keyboardsinexpensive,resist wear, water, foreign objectsdirt. TheycommonPC keyboards.

Mechanical switch keyboard

Mechanical switch keyboards use real switches, one under each key. Depending onconstruction ofswitch, these keyboards have varying responsestravels. The best tendapproximateold IBM Selectric electric typewriter keyboard, which was consideredbenchmarkgood performancemany years. Because ofexpenseplacingswitch at each key position, these have fallen outfavourrecent years, despite their excellent response. Most IBM keyboards up intoPS/2 line werethis type,consideredbe amongbestbreed, although some complained thatwere "clackity."

Hall effect keyboard

Hall effect keyboards use magnets"Hall Effect" sensors insteadan actual switch. Whenkeydepressed,movesmagnet, whichdetected bysolid-state Hall-Effect sensor. These keyboardsextremely reliable, ableaccept millionskeystrokes before failing. Theyusedultra-high reliability applications,locations like nuclear powerplants or aircraft cockpits. Theyalso sometimes usedindustrial environments. These keyboards can be easily made totally waterproof. They also resist large amountsdustcontaminants. Becausemagnetsensorrequiredeach key, as well as custom control electronics, theyvery expensive.

Membrane keyboard

Membrane keyboardsusually flat. You would most probably see them on appliances like microwave ovens or photocopiers. A common design consiststhree layers. The top layer (andoneuser touches) haslabels printed on its frontconductive stripes printed onback. Underisspacer layer, which holdsfrontback layer apart so thatdon't normally make electrical contact. The back layer has conductive stripes printed perpendicularlythose offront layer.

When placed together,stripes formgrid. Whenuser pushes down atparticular position, his finger pushesfront layer down throughspacer layerclosecircuit at one ofintersections ofgrid. This indicates tocomputer or keyboard control processor thatparticular button has been pressed.

Membrane keyboards don't generally have much of"feel", so many machines which use them issuebeep or flashlight whenkeypressed. Theyoften usedharsh environments where water or leak proofingdesirable. Although used inearly days ofpersonal computer (onZX80, ZX81Atari 400),have been supplanted bymore tactile domemechanical switch keyboards.

Other partsPC keyboard

The modern PC keyboardmore than justswitch technology, however. It also includescontrol processorindicator lightsprovide feedback touser about what statekeyboardin. Depending onsophistication ofcontroller's programming,keyboard may also offer other special features.

The processorusuallysingle chip 8048 variant. The keyboard switch matrixwiredits inputsit processesincoming keystrokessendsresults downserial cable (the keyboard cord) toreceiver inmain computer box. It also controlsillumination of"caps lock", "num lock""scroll lock" lights.

To tell ifcomputercrashed hard, press"caps lock" key. The keyboard sendskey code toBIOS code running inmain computer, Ifmain computeroperating,commandslightturn on. Allother indicator lights work insimilar way. The BIOS also tracksshift, altcontrol state ofkeyboard.

When you presskeyboard key, you may think that you do so incontinuoussmooth motion, making firm contact,then releasingkey. Howeverkey actually "bounces" against its contacts several times beforesettles into firm contact. When released,bounces some more untilreverts touncontacted state. Ifcomputer was watchingeach pulse,would see many keystrokeswhat you thought was just one.

To resolve this problem,processoryour keyboard (or computer) "debounces" your keystrokes, by aggregating them across timeproduce one "confirmed" keystroke that (usually) correspondswhat you think issolid contact. It could be argued thatdome switch technology outlined above owes its popularity toability ofprocessoraccurately debouncekeystrokes. Early membrane keyboards limited typing speed becausehaddo significant debouncing. Anyone who ever tried word processing onZX81 will recall this.