# TTL input resistor

Discussion in 'Homework Help' started by selfbg, Dec 6, 2010.

1. ### selfbg Thread Starter Member

Jun 11, 2010
23
0
I have this homework that i have to create custom logic with TTL ICs. At the input I have 3 DPST switches (1 Pole for Vcc and 1 for GND). Also I have '279 (RS latch) to remove the peaks from the mechanical contact (sorry I dont know the english word for that ). My question is what shoud be the value of the current limiting resistor at the input of the TTL ICs. I know the Iih and Iil. Do I have to use 2 different resistors for 1 and 0 ?

2. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
The Iih and Iil characteristics on the datasheet refer on the passive quantities of the IC. That is, the IC will draw Iih current when you give it HIGH input and Iil current when you give it LOW input. Your power supply must be able to deliver those current if you want your IC to work correctly.

It doesn't mean that you must prevent the current from going any higher. The IC's internal resistance does that. What you must do is to prevent the input voltage from going any higher that the rated figure.

3. ### beenthere Retired Moderator

Apr 20, 2004
15,815
290
For TTL, the inputs do not need limiting resistors at the inputs, only correct voltage levels. For your switches, circuit ground is good to logic 0. Using a 4.7K pullup resistor will give any TTL input a good logic 1 level - assuming the voltage is the same as the TTL chip's Vcc.

It is a good idea to use a resistor when applying an input voltage to TTL. The input can fail in such a way as to create a fairly hard short to the power supply.

The use of the