Model3490A

7.77.LOGICGATESYMBOLS.

7 -78. L humberof logic gateintegratedcircuitsareused in the 3490A. Most of theseare TTL circuits,in which the HIGH logic level is 2+ 2.4 V and LOW is (r 0'5V. The normal symbol used to indicate a 2-input NAND Gateis shownbelow, accompaniedby the truth table.

A

Truth Table

 

A

B

C

C

L

L

H

 

H

L

H

 

L

H

H

NAND GATE

H

H

L

As the truth table indicates, both inputs must be HIGH (true) to obtain a LOW output. The purposeofthe circle (invert symbol) at the gate output might then be assumedto indicate that the signalis LOW true at that point. The truth statement for this NAND Gate may be restatedto read,"A LOW signalat eitherinput resultsin a HIGH output," and the sametruth table applies.In keeping with this statement and the use of the circle to indicate a LOW true sigtal, the following "OR" function symbol may be used.

In some cases,a NAND Gate circuit may be used to invert a logic signal.This may be done by connectingthe samesigral to both inputs. If this is done, the following symbol may be used, indicating that the circuit merely inverts the sienal.

If the input sigral is LOW true, the circle may be placed on the input sideof the symbol.

SectionVII

The NAND Gate may also be used as an inverter by connectingone input permanentlyto HIGH (+ 5 V). In this case,the circuit may be shown asfollows.

+ 5 V

A

Note that the sametruth table applies,and that if one input is held HIGH, the output is the inverse of the other input.

7-76. Another gate circuit used is the Exclusive OR Gate,normally drawn:

 

Truth Table

 

A

B

C

 

L

L

L

 

H

L

H

 

L

H

H

ExclusiveOR Gate

H

H

L

Note from the truth table that the output is HIGH if one, and only one, input is HIGH. Whenboth inputs are either HIGH or LOW, the output is LOW. If one input is held LOW (connected to ground, for example) at all times,the output follows the other input. Whenusedin this manner,this symbolmay be used.

If one input is held HIGH (+ 5 V), an inverter symbol

-, may bg used,becausethe output becomesthe inverseof the input.

+5 V

7-9