I've been having a problem trying to build a breadboard circuit.
The problem being, that some IC high outputs doesn't reach a high enough voltage, staying around ~2V. That is not sufficient for a clear high signal, and indeed the logic gates that receive such a signal end up not working correctly.
Here is a pic of my wiring (in the pic there are some disconnected input pins, but the problem persists even when they are all connected):
The IC in question is the 74LS173, a 4-bit tri-state register. Link for its datasheet.
I feel like I must be missing something pretty simple, but I can't for the life of me find out what is it.
In any case, thanks in advance for the patience and for taking the time to help!
Best Answer
You appear to have some LEDs connected to your outputs. These will limit the voltage to 2(ish) volts if driven direct from the outputs. TTL outputs are not designed to drive LEDs to ground and have a valid logic high level. Their weak drive current is however convenient for driving LEDs like this.
TTL is much better at sinking current than sourcing. If you want to drive a LED, you should connect it to the +ve rail, in series with a current-limiting resistor. Unfortunately, that means it will be 'on' for a low output. This is a small price to pay for driving the LED properly, and delivering valid logic states as well.