o o o o

o

o o o o

a a o o

o

o o o a o

,o

o o o o

to o

I

o a o

I

o o a a

I

o o o a o o

Tabte7-23

ROMBank SelectionData

Bank DO cE0-7

ROM

0

0

0

454U3060

1

1

0

454U3060

2

o

1

456U1010

3

1

1

456U1010

4

0

2

456Ur020

5

1

2

456U1020

6

0

3

456U1025

7

1

3

456U1025

I

0

4

456U1035

9

1

4

As6Ul035

1 0

0

5

456U301

1 1

1

5

A56U301

1 2

0

6

456U3020

1 3

1

6

456U3020

1 4

0

7

A56U3030

1 5

1

7

456U3030

GPIB Swltches. At address 7C00, buffer u2O4E writes the rear-panelGplB switch data onto the data

bus.combinationdecoupleseach

.A.resistor-capacitor

switch line to minimize noise and unwantedpulses picked up on the long circuit board lines to th6 rear panel.

GPIA. Generat purpose InterfaceAdapter (GplA) U2050 translates microprocessorcommandson the microcomputerbus into appropriatecodEsand protocol for the GPIB bus. lt also decodesdatafrom the GplB for the microcomputerbus._lnterruptsare generatedby pullingdown on the GptB SRe line. fne Cnf CLK tini providesthe clock reference. This lC is accessedat address7A00.

The GPIB Interface board (A30A57)connectsthe r€ar-panelIEEE 488 PORT (GptB connector)to the G.PIBboard,.throughthe GptB ExtenderUoarO(nSOet;. The interface board contains two octal transceiveri, Ul011 and U1012,that transferGptBdatabetweenthe rear-panelconnectorand the GplA circuit.

Accessories lnterface (Diagram44)

The Accessorieslnterfaceboard(A90A76)provides access to the instrument bus, and the external MARKER/VIDEOinput and controt tine. The MARKERIVIDEOinput is througha coaxialconnector. The other lines are availabls through the ACCES_ SORIESconnector.

Theoryof Operatlon- 4g4ful494ApServlce,Vol. 1

Iq displayan Externatsignalthat is appliedto the MARKEF/VDEOinput,puttthe EXTvtDEo SEtect tine (pin1 of the ACCESSORTESconnector)tow.

The instrumentbus is bufferedand broughtout to the rear panel with the lines named to indacatetheir relation to the internat bus: ADV for DATA VALID. APOLLfor POLL.etc.

Two lines, tNT CONT and DATA BUS ENABLE. defineth€ instrumentbus/externaldevicelnterface.An externalcontrollergains control by pullingthe INTER- NAL CONTROLline tow. This disabtes th€ internal microcomputer'sinstrumentbus buffers and sets th€ data direction of buffers U201Sand U2033. In this state,the externalcontrollersends addressesand the DATA VALID and pOLL signals to the instrurnent.lt also allows th€ instrumentcircuits to s€nd a service request(SR)signalto the externalcontroller.For inter- nalcontrol,the buffersreversedirection.

Data buffer U2038transfersdata to and from the external instrumentbus. Data direction dependson whethercontrolis internalor externaland on what the address is. The buffer senses the most significant addressbit, AB7, so that when in externalcontrol.the upperaddresses(AB7high)senddatato the instrument and th€ lower half of the addresses(AB7 low) receive data from the instrument.For internalcontrol,thEdata directionreverses.

The DATABUS ENABLEline is assertedlow by an externaldeviceto €nablethe data buffer. As long as this line i$ unasserted,the data bufferis set to its high impedancestate and the data dirEctioninout has no effecton its output.

Front Panel (Diagram 45)

The Front Panel board (A38) acts as an interface betweenthe user and the instrument. These circuits translateoperatoractionson front-panelcontrols,into data for the microcomputerto read and implement.lt outputs data showingcurrent operatingmodesto the uservia LED's(lightemittingdiodes)andcrt readout.

Outputof data is providedby five shift registersthat drive LED'Sto light various front-panelpush buttons and indicatorsto show the instrumentoperatingmode. Operatorinput information,via push buttonsor rotary switches,is read by the front-panetCpU.The CpUthen outputs the data to the master microprocessorfor action. Th€ front-panelCPUscansall pushbuttonsand rotaryselectorson the keyboardmatrix plus the coder for the FREQUENCYknob looking for changesin the keyboardcodes or frequencycoder. lt then translates these changes for the master microprocessorfor

7-97

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Tektronix 494AP service manual A a O a o o