I was doing a simple logic circuit in proteus when i realised that something was going wrong.

It looks like the NAND gate provides high voltage to its inputs from nowhere. No errors provided, but if i let the simulation running for a while, it always end with an unexpected close.

You can see that in the picture that i'm getting 4 volts from the inputs of a non-connected nand gate, and you can also see that if you connect the terminal to the ground(the ones on the top of the image), it seems to work as expected.

enter image description here Should the nand gate have +4V on their input terminals when they are not connected? I think that it will only should have high state on the output terminal if the inputs are not connected or connected to the ground.

I'm newbie, its never late to learn. Pls help :D

  • \$\begingroup\$ Left unconnected (we call it floating), input pins of some logic gate technologies (especially non-CMOS) will tend to be "high". Even if being "high" is what you want, it is not good practice to leave pins floating as they are subject to changing states with noise, etc. \$\endgroup\$
    – mike65535
    Commented Jul 23, 2019 at 13:54

1 Answer 1


The 74S37 is a TTL logic IC. If you leave the inputs floating, then they will usually be seen as a logic 1, or the input will 'float' in the unknown region, where it could be a 1 or a 0.

Proteus will use the red or blue logic indicators to show whether something is at a logic '1' (red) or logic '0' (blue). If you were to remove the multimeter, you will notice that the input logic indicators will go grey. This means they are undefined, although most TTL will tend to go high.

For a NAND gate, use pull-up resistors on the inputs to tie them high, then you can drive the input low with your control signal. More on using resistors on logic gates can be found HERE.

This is what I mean with proteus. You can see by leaving the input floating, they will go to an undefined state, which Proteus will mark as a grey logic indicator:

enter image description here

enter image description here

The voltmeter in Proteus has a modelled 10M(?) resistance which will allow the program to then register it as a logic 1, and the logic indicator will go red.

  • 1
    \$\begingroup\$ Thanks a lot, your answer solved my problem and helped a lot. \$\endgroup\$
    – Laiiser
    Commented Jul 24, 2019 at 7:01
  • \$\begingroup\$ No problem, glad to have helped \$\endgroup\$
    – MCG
    Commented Jul 24, 2019 at 7:33

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