SectionIV

Model3490A

 

O U R I N G R U N . D O W NO N L Y

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Figure4-7. SimplifiedDiagram,OhmsConverter.

466. 0hmmeterReference.

proper combination of signalsto the Scan Decoder.

+67 . An

emitter follower, whose base is referenced

Threescansignals,A, B, and C, are appliedto the Data

Counter to releasethe stored BCD count information

through

a voltage divider to a 5.6 V zener diode.

one digit at a time, beginningwith the leastsignificant

providesa stablereferenceof approximately- I V across

digit, Nl. Thesescansignalsare also used in the Data

the reference resistor Rrs6 (Figure 4-7). This reference

Output and Remotecircuits.

voltage, applied to Rpsg,determinesthe amount of

 

 

current supplied to R1 by the current source, as

4-75. ScanDecoder.

explainedin Paragraph442. A resistancemeasuremenr,

 

 

then, is the ratio of the voltageacrossR* to the voltage

4-16. The four signaloutputs from the ScanGenerator

across R1s;, both voltages being determined by the

are fed into a 4-to-10 line decoder.The decoderthen

current resultingfrom the referencevoltage.

appliesan enablepulseto eachdisplayunit at the same

468. InputProtection.

time that unit

receivesthe BCD count information from

 

 

 

 

the Data Counter. Scanningis continuous,with all six

4-69. Protection againstexcessive

display units being scannedin approximately 1.6 ms,

 

 

voltagewhich may be

beginningwith the leastsignificantdigit, Nl.

accidentallyapplied to the front panel Q Signaltermi-

 

 

nals is provided by a gas dischargedevice acrossth:ä

 

 

terminals and a resistancein series with the hieh

 

 

terminal. Voltagesin excessof 250 Vrms may deströy

4-78. The function signals,from either the front panel

the I kfl seriesresistor.

switch or remotecontrol, aregatedsothat the symbol is

 

 

 

+70. 0ISPLAYASSEMBLY(Figure

disabledduring ac and ohms measurementsWhen. the

 

 

7-2?).

symbol is enabled,the minus sign is on continuously,

 

 

 

471. DisplayUnits.

and the vertical bar is turned on when the readingis

positive. The sample rate indicator, contained in the

 

 

 

4-72. Each of the six numericaldisplayunits containsa

polarity unit, is on when Main Timing Bit A is HIGH.

 

 

4 x 7 dot matrix of light-emittingdiodes(LED$ to form

4-79.DecimalsandAnnunciators.

the digits. In addition, each unit contains a decoding

 

 

circuit to

light the proper LEDs, and a latchingcircuit,

4{0. Range information from the Range Counter is

so that the displaycanbe changedonly duringan enable

gatedto turn on the correctdecimalfor eachrange.The

signal.Eachunit alsocontainsa decimalpoint to the left

Overload annunciator is turned on by the Overload

of the

number. The polarity unit also contains the

signalstored in the Logic. The Remote annunciatoris

samplerate indicator. Each annunciatorat the right side

driven by a circuit on the RemoteAssembly.The other

of the displayis a singleLED.

annunciators(optional) are turned on by gating the

 

 

 

 

 

 

Function signalsEach.annunciator

473. ScanGenerator.

 

is a singleLED.

 

 

+74. A relaxationoscillatoroperatingat approximately

4 . 81 .L0GrCCRCUTS .

 

 

l0 kHz providesthe clock signalfor the ScanGenerator.

4-82.Clock.

 

The oscillatoroutput drivesa 4-bit synchronouscounter.

 

 

The counter outputs are gatedand fed backto resetthe

4-83. Figure 4-8 is a block diagramof the Clock and

counter after every I 2th input cycle, thus providingthe

Counter circuits. The basictiming for the 3490A Ilgic

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