# Unchanging Display Value On Any Input

Discussion in 'Homework Help' started by bobon721, Aug 10, 2011.

1. ### bobon721 Thread Starter New Member

Feb 8, 2011
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0
I'm making a logic circuit that compares 2 digits. I'm still on the A=B part (A and B are my input digits. My problem is that be A=B or not, the seven-segment display displays only 0. I require assistance in troubleshooting this because I always end up in dead ends on my own. I have my schematic attached. Thanks.

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2. ### praondevou AAC Fanatic!

Jul 9, 2011
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I post the circuit as png.

Not everybody is using Multisim here.

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3. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Crack open your voltmeter, it should be all you need here. Start on the left, work your way to the right. Do the '147's outputs make sense when you change the switches? Do the '04's properly invert the signals? Are these signals making it into my '85? Is the '85's output correct?

I don't think the 7447 is correct unless you want the display to show what S2 is doing unless S1 == S2.

4. ### bobon721 Thread Starter New Member

Feb 8, 2011
18
0
This is actually just a part of my whole circuit. I have yet to construct the parts that display the octal equivalent of B-A when B>A and the binary equivalent of 10A+B when A>B. I can't proceed to them without finishing this first when it's supposed to be the easiest.
Just finished testing with a multitester. 5V would be a logical 1 while 0V would be a logical 0 right? I I got some serious voltage reductions at the inverter outputs resulting to around 1mV and negative voltages at the BCD to 7 segment display outputs. Is this normal?
I just attached the greater than and less than parts of the schematic. Now Multisim is saying there's something wrong with my subcircuit connections. I can't see anything wrong with them at all.
I'm actually curious about the difference of digital and analog grounds. When do I use the analog ground?

• ###### logics lab mt.zip
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Last edited: Aug 11, 2011
5. ### MrCarlos Active Member

Jan 2, 2010
400
136
Hello bobon721

I I've looked at your original circuit in Multisim-11 simulator.
All connections between devices are quite good.
However there are some opportunities for improvement, especially in the attributes of the devices.
For some reason, related to attributes, 74147 outputs do not change state even if you change the state on it's inputs.
Another opportunity for improvement is that the resistors connected between the DIP-Switch and Vcc. are too low, typically used 10K.
Take a look at the circuit -AAA Lab Logics MT (1).MS11- included in the ZIP attachment.
I hope this helps you to continue your project.

Greetings