# Eplain how that works

Discussion in 'Homework Help' started by TylerDurden13, Jan 6, 2015.

1. ### TylerDurden13 Thread Starter New Member

Dec 15, 2014
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0
Hi i have a homework to school and i need to help , beacause i dont know how that circuits works

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2. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,906
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Work it from top left to right.

First, the input not made by a switch (can't read the letter very top left) will control IC30A and IC31A to light a red or green LED. That works separate from all else.

The top left has 8 inputs, each input is inverted and all 1 signals flow down.

8 of these 16 signals are routed to 16 NAND gates starting with IC1 and going down. Each gate will capture one input state which causes the gate output to go low.

The 16 NAND outputs are directed to more NAND gates at IC20A and below. The NAND output goes high when any of the inputs go low, or any of the state is met. These drives the segments of the display turning a segment ON when they go low.

The letters a-g on the seven segment are standard designation (Google them).

You now should be able make a table where you write down the states selected by IC1-IC19, then which segments are lit by what states.

Hopefully a pattern will emerge you can see. If not, come on back and post your work.

That is what is does. WHY it does what it does is an open question.

3. ### WBahn Moderator

Mar 31, 2012
20,081
5,670
Could you reduce the size of your image? I haven't been able to open it successfully, but I saw a snippet of it momentarily and it looks like it is huge. Open it in something like Paint and just resize it to be, say, 400 pixels wide. That will probably reduce it by a factor of 50 or more.

4. ### djsfantasi AAC Fanatic!

Apr 11, 2010
3,486
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I second that. Trying to open it hung my system.... I was afraid it was infected.

5. ### WBahn Moderator

Mar 31, 2012
20,081
5,670
Okay, I downloaded it and was able to open it. 12000 pixels wide! But I can see that it has so much in it that 400 pixels won't do it. I scaled it down to 1600 pixels which is probably about as far as is reasonable to go and attached the result.

This almost looks like a Boolean monotone function -- or a set of them, actually -- but I doubt that is the intent.

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6. ### WBahn Moderator

Mar 31, 2012
20,081
5,670
A slightly different explanation from ErnieM's (but should be equivalent) is to start from the right hand side. Note that in order for a particular segment to light up, the control signal to it must go LO. So that means that for most of the NAND gates in the right-hand column they are logically AND gates followed by an inverter in order to match the logic to the needs of the display.

Let's focus on the top NAND gate in the right-hand column, IC20A, which is driving segment A. In order for segment 'a' to light, all four of the inputs to IC20A have to be HI.

So now let's consider what each one of those inputs represents. If we look at the top NAND gate in the left-hand column, we see that it has eight inputs and that each input is tied to either the inverted or the non-inverted version of one of the eight input signals. As a NAND gate, it is HI except when all of it's inputs are HI. Thus there is just one choice of input signals that will result in its output going LO. This is called a minterm. Each of the 16 NAND gates in the left-hand column thus picks off one combination of inputs that will prevent a segment from lighting up (provided that signal is applied to the input of the corresponding NAND gate in the right-hand column). Thus there are four combinations of inputs that will prevent segment 'a' from lighting while all other combinations will allow it to light.

Note that segments 'b' and 'c' are slightly different in that the inverters following the NAND gates make it so that instead of the selected minterms causing the segment not to light, any of those minterms will cause that segment TO light up.

This almost looks like some kind of game where people have to try to find a combination of switch settings that either cause all of the segments to light up or all of the segments to be dark, with perhaps the input at the far left being a selection between two different games? Just a wild guess.

7. ### TylerDurden13 Thread Starter New Member

Dec 15, 2014
3
0
Thank you very much ...!