Digital (square) signal error

Question

Hello
I have a problem with MultiSIM 9.0.155.
I have built a circuit shown on this picture http://img237.imageshack.us/img237/9764/digitaltoa​nalogcircutzw1.png.It is supposed to convert a digital (4bit) signal into analog signal (by it's value) so that when the digital signal is 0000 the analog value is 0V, and when it's for ex. digital sig is 1111 the analog value is of 2V... (this circuit uses a two 8-LED bars to indicate which "switch is on", and there isnt an analog signal output yet...)
This circut should work fine, but: When I tried to simulate a 4bit value with a four function generators (with frequencyes set to 1Hz, 2Hz, 4Hz, and 8Hz) the result is something like this:
http://img528.imageshack.us/img528/4268/digitaltoa​nalogcircutpoig1.png. In this picture the 4 keys are unpressed, and even then i still have a signal like that on the picture...??
I replaced four function gens with 4 batteries as a source but still the  signal is the same??...-> an square alternating current with rising and falling edges...
Here is a closer picture with the batteries as a sourcehttp://img300.imageshack.us/img300/9616/digitaltoa​nalogsourcera4.png.
I don't know if something is not set properly (but it shouldn't get an square signal with the batteries as a source and the 4 keys unpressed), so if anyone sees what's the problem post an answer please.. 
I have tried to restart MultiSIM but still with the same results...

Answer 1

Hi there,
I'm not sure how you created this schematic, but somehow, the 4 nets (a.k.a wires) that you are measuring have the same name! When Multisim starts simulating, it treats all wires by the same name as a single electrically connected wire. That is why you saw the same signal on all 4 channels of your oscilloscope. There is a way to fix this:
Right click on one of the wires, lets say the one you labeled bit 0
Click properties
Change the net name to b0
Click OK
Multisim is going to tell you that you have other wires that have the same name but are not connected. Tell Multisim that you do not want to change the names of the other wires.
Now repeat this for the remaining 3 bits, but name them b1, b2 and b3
I bet there may be other wires in your design that have the same problem. Unfortunately, I think you will have to manually go through your design and find them all.
If you ever find out what you did to make this happen, please let us know. We are always interested to know how to make Multisim easier to use so that mistakes like these don't happen.
Hope that helps!
Message Edited by yyao on 02-05-2009 05:26 PM
Yi
Software Developer
National Instruments - Electronics Workbench Group