Tektronix 494AP O o o o o a o o, Troubleshootingon Instrumentbus, Yr iffiI, Y7 offii

Maintenance- 494A/494ApServtceVol. 1

Instrument Bus Test

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lf the microcomputer performs the power-up self- test, but fails to properly control the instrumeht,the instrument bus interface may be faulty. Select the instrumentbus test by setting the option switch as shown in Table 6-10.The microcomputercontinuously writes to the instrumentbus in a repetitivemannertso the instrumentdoesnot operatenormally.

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The pattern on the instrument bus toggles DATA VALID and POLL and exercisesthe addressand data lines. The addresslines changewhen DATAVALIDls

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low and the data lines change when DATA VALID ls high.However,if an assemblyon the bus is requesting service because of the way it powered up, DBo-DB4 may continueto changeafter DATAVALIDgoes low. In this case, an assemblyor assembliesmay respondto the high stat€ of POLL and the changingstateof AB7 and attemptto report status.

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-rloFlgurc.6-37. Chlp relectr Y0, Yl, Y5, and y7 In retadonto

FlgureG-38. Chlp relects Y2, y4, and y6 in relationtilll6.

Clocks and Control Llnes - The 680g clock input line shouldbe a squarewave with a periodof approxi- mately0.293ps. The c 2 outputon pin 37 shoutdhave

aperiodof approximately1.17;rs. VMA,RESET,NMt, andR/W shouldbe high. TFd instrumentbus powerup.

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The patternfor the upper addressand data lines is shown in Figure6€9. From addressor data line 7 to linE0, each line changesat twic€ the rate of the previ- ous lin€, resultingIn 128 cycleson the LSB lines. ThE initial pulse on the upper four data lines is not part of the +2 pattgrn and is not repeated on the lower four data lines. lt is possibleto discoveropen or shorted lines by comparingthe patterns to those in Figure6€9, checkingthat they +t. Look for linesthat stay high or low, change together or at wrong times in the pattern, or go to indeterminatelogic levels(1V to 2 V).

TROUBLESHOOTINGON THE

INSTRUMENTBUS

lnstrument Bus Data Transfers

There are two commands and queries provided to aid troubleshootingof circuit functionscontrolledby the instrumentbus. Thesecircuitsget datafromthe micro- computeror respondwith data for the microcomputer. The ADDR commandand ADDR query set and retum the instrumentbus address for the DATA command. The DATA command and DATA query set and return data on the instrumentbus.

Becauseth€ DATA commandchangesthe status of internal hardware,its use may prevent normal spectrum Analyzer opera- tion. Incorrect,settingsof some hardware couldcauseinstrumentdamage.

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