The PCs keyboard.docx

The PCs keyboard.docx

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ThePCskeyboard

The PC's keyboard.

WhywouldyouwanttointerfacetheKeyboard?

TheIBMkeyboardcanbeacheapalternativetoakeyboardonaMicroprocessordevelopmentsystem.Ormaybeyouwantaremoteterminal,justcoupleitwithaLCDModule.

MaybeyouhaveaRS-232BarcodeScannerorotherinputdevices,whichyouwanttousewithexistingsoftwarewhichonlyallowsyoutokeyinnumbersorletters.YoucoulddesignyourselfalittleboxtoconvertRS-232intoaKeyboardTransmission,makingittransparenttothesoftware.

Aninterfacingexampleisgivenshowingthekeyboard'sprotocolsinaction.Thisinterfacingexampleusesa68HC705J1AMCUtodecodeanIBMATkeyboardandoutputtheASCIIequivalentofthekeypressedat9600BPS.

NotethatthispageonlydealswithATKeyboards.IfyouhaveanyXTkeyboards,youwishtointerface,considerplacingtheminamuseum.Wewillnotdealwiththistypeofkeyboardinthisdocument.XTKeyboardsuseadifferentprotocolcomparedtotheAT,thuscodecontainedonthispagewillbeincompatible.

PCKeyboardTheory

TheIBMkeyboardyoumostprobablyhavesittinginfrontofyou,sendsscancodestoyourcomputer.ThescancodestellyourKeyboardBios,whatkeysyouhavepressedorreleased.Takeforexamplethe'A'Key.The'A'keyhasascancodeof1C（hex）.Whenyoupressthe'A'key,yourkeyboardwillsend1Cdownit'sserialline.Ifyouarestillholdingitdown,forlongerthanit'stypematicdelay,another1Cwillbesent.Thiskeepsoccurringuntilanotherkeyhasbeenpressed,orifthe'A'keyhasbeenreleased.

Howeveryourkeyboardwillalsosendanothercodewhenthekeyhasbeenreleased.Taketheexampleofthe'A'keyagain,whenreleased,thekeyboardwillsendF0（hex）totellyouthatthekeywiththeproceedingscancodehasbeenreleased.Itwillthensend1C,soyouknowwhichkeyhasbeenreleased.

Yourkeyboardonlyhasonecodeforeachkey.Itdoesn'tcareittheshiftkeyhasbeenpressed.Itwillstillsendyouthesamecode.It'suptoyourkeyboardBIOStodeterminethisandtaketheappropriateaction.Yourkeyboarddoesn'tevenprocesstheNumLock,CapsLockandScrollLock.WhenyoupresstheCapsLockforexample,thekeyboardwillsendthescancodeforthecaplocks.ItisthenuptoyourkeyboardBIOStosendacodetothekeyboardtoturnontheCapslockLED.

Nowthere's101keysand8bitsmake256differentcombinations,thusyouonlyneedtosendonebyteperkey,right?

Nop.Unfortunatelyahandfulofthekeysfoundonyourkeyboardareextendedkeys,andthusrequiretwoscancode.ThesekeysareprecededbyaE0（hex）.Butitdoesn'tstopattwoscancodeseither.HowaboutE1,14,77,E1,F0,14,F0,77!

Nowthatcan'tbeavalidscancode?

Wrongagain.It'shappenstobesentwhenyoupressthePause/breakkey.Don'taskmewhytheyhavetomakeitsolong!

Maybetheywerehavingabaddayorsomething?

Whenanextendedkeyhasbeenreleased,itwouldbeexpectthatF0wouldbesenttotellyouthatakeyhasbeenreleased.ThenyouwouldexpectE0,tellingyouitwasanextendedkeyfollowedbythescancodeforthekeypressed.Howeverthisisnotthecase.E0issentfirst,followedbyF0,whenanextendedkeyhasbeenreleased.

KeyboardCommands

BesidesScancodes,commandscanalsobesenttoandfromthekeyboard.Thefollowingsectiondetailsthefunctionofthesecommands.Bynomeansisthisacompletelist.Theseareonlysomeofthemorecommoncommands.

HostCommands

ThesecommandsaresentbytheHosttotheKeyboard.Themostcommoncommandwouldbethesetting/resettingoftheStatusIndicators（i.e.theNumlock,CapsLock&ScrollLockLEDs）.Themorecommonandusefulcommandsareshownbelow.

ED

SetStatusLED's-ThiscommandcanbeusedtoturnonandofftheNumLock,CapsLock&ScrollLockLED's.AfterSendingED,keyboardwillreplywithACK（FA）andwaitforanotherbytewhichdeterminestheirStatus.Bit0controlstheScrollLock,Bit1theNumLockandBit2theCapslock.Bits3to7areignored.

EE

Echo-UponsendingaEchocommandtotheKeyboard,thekeyboardshouldreplywithaEcho（EE）

F0

SetScanCodeSet.UponSendingF0,keyboardwillreplywithACK（FA）andwaitforanotherbyte,01-03whichdeterminestheScanCodeUsed.Sending00asthesecondbytewillreturntheScanCodeSetcurrentlyinUse

F3

SetTypematicRepeatRate.KeyboardwillAcknowledgecommandwithFAandwaitforsecondbyte,whichdeterminestheTypematicRepeatRate.

F4

KeyboardEnable-Clearsthekeyboardsoutputbuffer,enablesKeyboardScanningandreturnsanAcknowledgment.

F5

KeyboardDisable-Resetsthekeyboard,disablesKeyboardScanningandreturnsanAcknowledgment.

FE

Resend-Uponreceiptoftheresendcommandthekeyboardwillre-transmitthelastbytesent.

FF

Reset-ResetstheKeyboard.

Command