Electronic – 1,2V on input pin of a 74LS273


We're building Ben Eater's CPU. We have voltage on the bus of ~1.4V.

We isolated a 74LS273 and wired it like so:


simulate this circuit – Schematic created using CircuitLab

At VM, we're measuring 1.2 V even with a 10k pulldown resistor.

Why is that and how do I get it closer to 0V? I need the voltage below ~0.3V so that other components as the 74LS157 will properly detect it as a digital LOW.

Of course I can replace the pulldown resistor by 0 Ohm (we tried and it worked), but then the input becomes completely unusable.

This is in a less professional environment. We have ESD protection, but I cannot completely ensure it has always been used. Is it an indicator that the 74LS273 is broken?

Best Answer

In Bens CPU the input of the 74LS273 is connected directly to the BUS. The BUS is pulled down to ground using a 10K Resistor. As you can measure the 10k resistor is not enough to pull it to ground.

There are several other components connected to the BUS. One of the components is a Tri-State-Buffer, the 74LS245. When the output of the 74LS245 is activated, a signal is put onto the BUS. If the output of the 74LS245 is LOW, it will pull the input of the 74LS273 to ground and the 74LS273 will detect it as LOW.

There is a voltage at the input of the 74LS273 because no other component is connected to it which acts output (eg. no 74LS245 has its outputs enabled).

Try to put the signal on the BUS using a 74LS245, then the issue is gone.