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74F283 sometimes gives wrong output on digital probes

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Hello,

 

I'm having weird problems with the probes showing the wrong output. I'm trying to design a CPU, and this was happening on a more complicated design, and now even on a simpler one.

 

I am running:

Multisim 13.0 Education Edition

Application version: 13.0.0 (13.0.632), Database version: 13.0.a

Build date: Monday, July 13, 2013, 7:42:19PM

 

I'll be sure to include a copy of my simple circuit that i've reduced from a multipage 16-bit ALU of 74F181's and 74F182 to a four bit adder using a single 4-bit adder with fast carry, 74F283.

 

 

The setup:

I have 9 bits going to inputs. They are made of SPDT switches, each wired to VCC and Ground, selectively.

The first four "bits" or switches (keys Q,W,E,R) control A3..A0,

The second four switches (keys A,S,D,F) control B3..B0,

The spacebar controls Carry-In.

 

I also have 3 sets XOR's (74F86) switching each input and output selectively.

Key 1 flips the A input bits,

Key 2 flips the B input bits,

Key 3 flips the output bits.

 

The four Sum bits and the Carry-Out bit are each wire to their own Digital Logic Probe (PROBE_DIG_GREEN).

 

 

The Problem:

 

The output lights are sometime wrong, but the only simple way I know how to replicate the bug is as follows:

 

Load the file. and run the simulation.

Press Q to flip the MSB of the A input.

Look at the Probe Lights. They are probably showing an inverted output.

Press 3 to invert the output.

See the same lights.

Press 3 again to clear (un-invert) the output.

The output is corrected as it should have been from the beginning.

Alternatively press 3 and space to show crazy lights.

 

You can compare this to enabling the LSB of the A input (key R) instead of the MSB (key Q), and see how it's supposed to work.

 

Why doesn't this work? Am I doing something wrong? Is it bug with the software? Is there an update?

 

Please Help!!

 

 - Richard Collins

   Electrical and Computing Engineering Student at SUNY-IT

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Bumping for replies.

 

Still need help!! 😕

 

 - Rich

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Solution
Accepted by topic author rpc0618

Hi Rich,

 

I've replaced your DPST switch with an Interactive Digital Constant, it's easiest to see the digital input than the DPST switch.  You can find this part in the in the "Sources" group under the "Digital Sources" family.  Your schematic was missing a digital ground, it most cases this affect the circuits but once in a while it makes a different, the modified file is attached.

 

Also attached is an Excel file with the conditions you described and the output I got, I don't see a problem with the circuit logic. Please let me know if I am missing something.

 

Tien P.

National Instruments
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Tien,


Wow! This is awesome! This gives me great hope! Thanks so much for helping me with this!

 

So where is the new digital ground?  I'm checking out your new version, and I don't see any power or ground, or even a place to hook it up. I figured i only needed them because i was using switches, and assumed the 7400-series chips came automatically hooked up to gnd and vcc.

 

Thanks again,

 

 - Rich

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OHHHHhhh,

 

Nevermind. I see it.

 

I wasn't able to find a digital ground by searching "digital", "GND" or "Ground" before, but i have to manually pick it out of the Sources --> Power Sources --> DGND .

 

I replaced that in the original, and it works now!

 

Also, I love your replacement of the switches with "Interactive Digital Constant" much better!!!

 

Thanks again.

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