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will see a phantomclosureat y2lxg as it scansthe key matrix. To suppress these piantom key closures, diodes have been added in series with the RESOLU- noN BANDWTDTH,MtN RF ATTEN,SpAN/DV,and gerta]l other keys in column6 and 7 of the k'eymatrix. In addition,an €rror detectionalgorithm is usedin the CPU to eliminateadditionalptraniomkey closuresthat mightoccur.

_ scanntng the FREoUENCycontrot coder. The FREQUENCycontrolcontainsa pairof phototransistors that outpula gray codethroughUt024EiandU1024Cto P27.and p26 (pins37 and 36; ot the CpU. This gray code signifiesthe directionthe controlis turned. Ou-rinia scan cycle,the GpU looks at th€ statusof the FRE- QUENCYcontrol code and if it detEctsa change,the CPUperformsa shiftand exclusive,ORoperationwhich derivesthe correctcode to outputouer ihe instrument bus to the masterprocessorto tell it whichdirectionto tunethe centerfrequency.

Outputtingthe Correct Code. The remainingtwo bits.out of port 2 (pZ4 and p25) drive the appropriate hardwareand initiatean SRe on the instrumentbus. When th€ SER REe line is pulled down, the master microprocessorwill serviceeitherthe keyboardor th€ fre-quencycoder. The front panetCpU (Uir0gg)initiates a SRQ by puttingdown p24 or p2S. A tow out ot pZ4 (pin 35) wilt initiate a keyboard SRe. The master microprocessorwill now servicethe requestby reading the keyboarddata in outputbufferU4$b. A low out ol P25 (pin 36) initiatesa FREQUENCycontrotSRe and causes the master microprocessorto service the request by readingthe frequencycode in the output buffer.

Alow out ot p24 is invertedby U2020Cso it clocks the flip-flopUg013B. The resultint low on the e(bar) output puils the sER REQ tine down. (Referto thg instrumentbus POLL sequencedescribedunder the master microprocessordescription for the service request sequence.)The master microprocessornow raises both the POLL tine and ABZ. This is gated throughu4014Aas a low to DBoon the instrumentbus. The mastermicroprocessorreadsthe bus and sees a low on DB0. This indicatesthat a keyboardinterrupt

has occurredand it must read the new kevboardcode. The master proeessorfirst clearsthe interruptby pul- ling AB7 and then the POLLtinetow. DBOnow goes high. The mastermicroprocessornow writes a 0 to DBo, the same as it read,and raisesthe pOLL line. This clocksU3013Aand resetsUg0138whichremoves the SRQ(bar).The instrumentprocessornow readsthe data in the output buffer,U4Og0,at addressF4. The front panel CPU now recognizesthat its output buffer has beenreadandit resetsp24to a 1. lt is now ready

Theory of Operation- 4g4[l4g4Ap Service,Vol. 1

for anothercycle.

Asimilarprocessoccurs when p25 (pin 36) of the CPUis p_u!l9dlow by a FREQUENCycoderinteirupt. A !9y on P25 is propagatedthrough UZ02OB,U20t3A, U20138,and U4014C;ontythis time DB3 is invotvedin the pofl. U2020Aand U40148 decodea low on AB7 and highon POLLtineto ctockU20t3Aand3013A.

soltware. The algorithmthat the cpu follows con_ sists of a main scan routine,which is an endlesstoop, and four subroutinesthat can be called. One su6_ routineruns the on-chiptimer that is usedto debounce the keys, Anothersubroutinereadsthe frequencyknob coder and derives th€ proper code to output io tne masterprocessor. The third subroutinereadsthe key_ board and stores the address of all keys that weie closed. The fourth subroutinelooks at the keycode from the key addressesthat were stored,and outputs the key codes andlor frequencycode for the master processor. There are also a number of checks and tests that haveto be done in eachroutinein additionto the obvioustasks.

Main Scan Routine. There are two types of scan; the first is madeafter a reset.the secondtype consists of the followingscans;the keyboard,frequencycoder, and the output data. Duringthe first scan,data in the GPUis initialized.The CpU reservespart of its RAMto store and rememberall key and frequencyknob coder settings. Duringall scans,the CpU readsthe frequency code and each row of keys on the keyboard. lt com- pareswhat it read to that storedin RAM and if thereis

adifference,the CPU calls the appropriatesubroutine for either the keyboardor the frequencycoder knob. After a completescan, the CPU checksto see if new information needs to be output to the instrumentpro- cessor, lf it does the CPU calls up the outputsubrou- tine.

Priorto the first scan,after reset.the CpU puts all 1's (highs) into its keyboard memory. This correspondsto op€n keys. On the ftrst scan, the CpU will note five apparentclosuresdue to the TIME/DIV, MtN RF ATTEN, SPAN/DV, RESOLUTION BANDWIDTH, and REFERENCELEVEL selectors. Theseclosuresare notedand outputto the masterpro- cessor. Becausethe masterprocessormemoryknows the positionof eachselectorto closea key,the proces- sor calls these the power-upsettings. When a front panelknob changespositionthe masterprocessorcan determinewhichdirectionthe knobchangedandwhatit must do to respondto the change. A completescan, withoutdetectingany key closurestakesabout800us,

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