I have a question about the safety (to the ICs) of wiring the output of one IC directly to the input of another IC.
simulate this circuit – Schematic created using CircuitLab
For instance, in the above simple circuit we wire the output of IC1 (\$Q_{1,2}\$) to the input of IC2 (\$I_{1,2}\$). My understanding is that the gates inside the IC act as a buffer such that if the gate is active it acts as a voltage source at \$V_{CC}\$ volts, however, in this case, the connection between the output of one IC and the input of the other has approximately \$0\,\Omega\$ resistance and therefore will drastically exceed the current rating of the ICs.
Is this actually a safe way to wire up ICs or is a current-limiting resistor required, and in each case, why is this the case?
Best Answer
CMOS devices normally have a high input impedance. For the 74xx and similar logic series this information is often provided in a family guide instead of being repeated in each datasheet. For example the Philips 74HC/HCT/HCU family guide shows on page 6 the following that indicates a maximum input current of 1uA for the 74HCXXX series:
So while your wire will have a very small resistance in reality very little current will flow into the input pin, so it's a safe and common thing to do.