# 4 active low inputs to 7 active high outputs decoder

Discussion in 'Homework Help' started by 0Wafi0, May 15, 2015.

1. ### 0Wafi0 Thread Starter New Member

May 15, 2015
25
0
hello there!
This is m first post here. I'm Wafi and I'm a second year aerospace avionics major. I'm posting my first question about active low and active high inputs and I've struggling to understand this for a while now!
Any help or push in the right direction would be greatly appreciated.

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2. ### MrCarlos Active Member

Jan 2, 2010
400
134
Hello 0Wafi0

The first thing you should do is fill in the truth table in columns a to g.
For example, in the first row, from A to D contain 0’s. It means that no input is selected.

The next line says: 0001, means that the switch has enabled the input D. So that segments that will be active high Are: b c d e g.
And so on.

Does not look complete truth table but should be considered, when more than one entry is Active Low. In this case should light segments to form the letter "F".

3. ### 0Wafi0 Thread Starter New Member

May 15, 2015
25
0
awesome thanks...I knew about the truth table but I was confused about this active low inputs with active high outputs...It's easy just work through that...but I wanted to know how the active low input affects this whole thing with the active high output...

4. ### MrCarlos Active Member

Jan 2, 2010
400
134
Hello 0Wafi0

Ok. Normally in positive logic:
"Active Low" is a voltage level close to zero volts,
while:
"Active High" is a voltage close to the bias voltage (VCC).

So:
Active Low = 0
Active High = 1

Note that the truth table "A", "B", "C" and "D" column are represented in the other direction, it means that:
1 Yes, yes it is active.
0 No, it is not active.

Apr 5, 2008
15,809
2,390
Hello,

@MrCarlos : may I remind you of the fact that this is an ENGLISH ONLY forum.
Please translate the post into english.

Bertus

MrCarlos likes this.
6. ### WBahn Moderator

Mar 31, 2012
18,096
4,920
The normal convention is that 0 means LO (low voltage) and 1 means HI (high voltage). Active-LO means LO is TRUE (hence, "active-LO) so that 0 means TRUE and 1 means FALSE. In this case, inputs 0 0 0 0 means that ALL inputs are being asserted, while if the switch is open-circuited it means that the inputs are 1 1 1 1 (as a result of the pullup resistors).

7. ### 0Wafi0 Thread Starter New Member

May 15, 2015
25
0
awesome guys!!! thanks for the help!!! so basically in the truth table I would have to flip everythin for ABCD? correct?

8. ### MrCarlos Active Member

Jan 2, 2010
400
134
Hello 0Wafi0

Yes you are correct.

Change the zeros by ones, and ones with zeros

9. ### 0Wafi0 Thread Starter New Member

May 15, 2015
25
0
so after that i do everything else normally? like just follow the new flipped inputs?

10. ### MrCarlos Active Member

Jan 2, 2010
400
134
Hello 0Wafi0

Yes, of course

Note now that the inputs A, B, C and D are as mentioned in the phrase that we see in the image that you enclose in your post #1.

Now 0 represents the letter is active.

In that image there is a box to the right of the 7-Segment Display.
Under the "Sw Input" column are the letters when they are "Active Low" light segments named in the "Segs" column.

For example: for A, the segments should light are abcefg.
They will be "active high".

Remember:
When no input (ABCD) is activated, the display must be turned off.
When more than one input are active the display should show a F.

11. ### 0Wafi0 Thread Starter New Member

May 15, 2015
25
0
awesome thanks a lot!!!