# Resistor in TTL Circuit

#### drnate

Mar 12, 2018
1
Hello! My parents gave me the Computer Logic Lab by NSI as a Christmas Present while I was growing up. I had fun learning a lot about electronics. Now it's time for me to pass it on the my son. A couple of things have bothered me through the years, and I'd like to try to complete my understanding:

- In the circuit diagram attached (similiar to others in the workbook), why do I need a 2.2K resistor is connected to the output of the the 7400 IC. There are some circuits that do not call for that resistor.
- Why does the attached circuit also need a 1K resistor attached to the input of the IC?

The circuit is powered by three 1.5 volt batteries in series, giving a total of 4.5 volts.

#### davenn

Moderator
Sep 5, 2009
14,077
- In the circuit diagram attached (similiar to others in the workbook), why do I need a 2.2K resistor is connected to the output of the the 7400 IC. There are some circuits that do not call for that resistor.

The 2k2 resistor in the input to the second gate is pulling that input high .... called a pull up resistor

and likewise the 2 x 1k resistors on each of the inputs of the first gate are pulling the gates low till the switches A and or B are activated ... called pull down resistors

this is particularly important for the two input pins 1 and 2, they cannot be left floating (not connected to anything) else false triggering of the gates can occur due to
the pickup of noise. That would happen if the 1k resistors were not there and the switches A and B were open

Dave

#### AnalogKid

Jun 10, 2015
2,670
Are you sure the original IC for that Lab was a 7400? Two things ...

1. A 7400 quad NAND does not need a pull up resistor on any output. However, some other part that was RTL or DTL might.

2. A true 7400 gate needs a lower resistance to GND to guarantee a logic 0 at an input. The classic value is 500 ohms or less: 0.8 V / 1.6 mA. Again, for some other logic family this could be higher.

ak

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