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So, Im wireding this Register

Edit: Everyone switch ON for good reading enter image description here

And the output is the green leds of each protoboard, 2bits for each protoboard, and with those bits, I have to do a "AND" or "OR" so, to do that, 1 bit of my switch is connected to a NOT, in order to send voltaje to the 7432 or 7408, depends on that bit... The AND is working perfectly, but the OR isn´t Wired

Green cable, input for 7432(Not working as expected) Yellow cable, input for 7408(Works fine) Blue cables, output of each result Large Yellow/Orage cable voltaje to each logic gate

So, on the 7432 I have a input of 0´s(from the green bits) and the output is 1, as far as I understand, this is sendind 0´s, so the output should be 0, but only if I connect ground directly outputs the correct bit.


Status input: 0-0 / 0-0 excpected output: 0-0 enter image description here

red led indicates which logic gate is on Full I add a NOT that is not connected, since this emulator do not recreate the logic gate, this NOT is the one who allow the "AND" or "OR" to turn on(Large Yello and Orange cables or the red LED)

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  • \$\begingroup\$ Your first figure has dark gray lines on a darker gray background, making it virtually unreadable. Could you convert that to a color scheme with higher contrast, preferably with a white background? \$\endgroup\$ Apr 26, 2021 at 21:04
  • \$\begingroup\$ @ElliotAlderson I cant change the backgroung, but I can turn on all, so you can see most of them \$\endgroup\$
    – Alfa Rojo
    Apr 26, 2021 at 21:12
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    \$\begingroup\$ Please draw a schematic diagram of your circuit - particularly how you are providing "0" and "1" inputs to the ICs. Are you using 7432, 74LS32, or something else - the letters in the middle of the IC number are important. \$\endgroup\$ Apr 26, 2021 at 22:00
  • \$\begingroup\$ There is a schematic editor built into site. Edit your question showing enough so we can help you. As in how Data, Storage and Clock are created. Add some decoupling caps. It appears you are sourcing current to LEDs. 74LS32 can source 0.4mA and sink 8mA, so include LEDs and resistors. This may be a factor. And AND-OR can be replaced by NAND-NAND (which has nothing to do with your problem). \$\endgroup\$ Apr 26, 2021 at 23:59

2 Answers 2

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It appears that you are using 74LS32, and other 74LS parts. With these bipolar TTL parts, you must connect inputs to Ground/Zero Volts to input a Logic 0. If the inputs are left unconnected, they will act as Logic 1.

A proper schematic diagram would make it much easier for us to diagnose your problem - I won't try to figure out your circuit from your breadboard photos and other strange drawings.

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So to summarize the question, the 74__32 (quad AND gate) is giving incorrect output: "logic low" input yet output is "logic high". Even without seeing a schematic, but based on the photos of the breadboards, I think I see the problem.

Each of the digital chips requires a local 0.1uF ceramic capacitor, within 5mm, to act as a power supply bypass. This is a commonly known construction technique that usually isn't stated in the datasheet, but is required for reliable operation.

Missing those 0.1uF bypass capacitors can cause a lot of weird problems on digital ICs. Whenever any of the internal transistors switch on or off, the current in both the supply wire and the ground return wire changes. This change in current, together with the inductance and resistance of long wires, causes the supply voltage to (very briefly) go out of specification.

Even if you don't intend to operate the logic at high speed, the long wires and the solderless breadboard have parasitic inductance, which can cause unwanted oscillation at high frequencies, and the only symptom is that the logic gates are giving wrong answers. It's not only high clock rates, but fast rising or falling edges that can trigger these problems. So even though leaving out the bypass capacitors sometimes works, it usually leads to the type of problem you are seeing here. Professional designs always include bypass capacitors to avoid this type of issue.

As an example of using bypass capacitors on a solderless breadboard, here's a photo of a minimal ATmega328 on solderless breadboard (this is equivalent to an Arduino Pro Mini or an Arduino UNO without the USB interface):

enter image description here

Somewhat edited from one of my earlier answers here

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  • \$\begingroup\$ Always learn something new, interesting your idea... And I try it, but doesnt work as you told me \$\endgroup\$
    – Alfa Rojo
    Apr 26, 2021 at 22:30

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