HP Car 3490A manual +il5. DCSWTTCHNGLoGrC, +II7 . Referencesupplies

Model3490A SectionIV

Count 32 line from the Timing Counter. This line goes HIGH at the overloadpoint, 120 7oof runge.The output from this AND gatethen goesHIGH to produce Transfer

andZero Detect commands.

4-110. NegativeInput. If the 34904 input is negative, the Polarity StorageQ output to the next AND gatewill be HIGH. Since Transfer Enable is already HIGH, when Analog Zerc Detecr goesHIGH at the end of run-down, Transfer andZero Detect commandsare given.

+111. Positive Input. WhenJhe 3490A input is posi- tive, the Polarity Storage Q output to the other three-input AND gate is positive. Again, TransferEnable is HIGH, so when Analog Zero Detect goesLOW at end of rundown, this signal is inverted and applied to the third input of the AND gate, resulting in Transfer and ZercDetect commands.

4-112. False Transfer. One input of a two-input AND gate is connected to f 5 V, enabling the other input to control the output. When False Transfer goes LOW during Logic Test operation, Transfer and Zero Detect commandsare issued.

4113.OvedoadStorage.

+114. When a Zero Detect command is issued,the Overload Storage D flipflop is LOW, indicating the measurementis less than l2OVo of range, the flip-flop

EXTERNALREFERENCE

A M P L I F I E R

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I N P U T

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R A T I OO P T I O NIS I N S T A L L E O .

output will be LOW. If Time Count 32 is HIGH, indicating a measurementof greater thm 120% of range,the flip-flop output will be HIGH. T[ris output goesto the Data Output circuits(Option 021), andto the Display.A HIGH Overloadsigralis invertedin the Displaycircuitsto turn on the Overloadannunciator.

+il5. DCSWTTCHNGLoGrC.

4-116. The DC Switching Logc uses range, function, and polarity information, together with the three main Time Bits to control all inputs to the DC Amplifier during both run-up and run-down. This includes input protection on the two lower dc Ranges,input attenua- tion on the two higher dc ranges,and the output ofthe AC Converter in ac measurements.The DC Amplifier gain is also selected by the DC Switching Logic. In addition, this circuit also enablesthe Ohms Converter during resistancemeasurements,and controls the reed

relay at the input to the AC Converter for ac measure- ments. A ,Read Only Memory and three dual-input

NAND gatesmake up the DC Switching Logic.

+II7 . REFERENCESUPPLIES .

ll.l 18. VoltageMea$rements.

4-119. Figure 4-13 is a simplified diagram of the Reference and Ratio circuits. An accurate and stable + l0 V Referencefrom a voltageregulatedpower supply

REFERENCEPOLARITYLOGIC

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INPUTPOLARITY

R A T I OP O L A R I T Y

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3 4 9 0 - 8 - 2 9 3 0

Figurez1-13.SimplifiedDiagram,Referenceand Ratio Circuits.

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