Electronic – Why do TTL pulse generators generate an asymmetrical square wave (unlike CMOS)

For the same logic family, 74LS in this case, tie an output directly to an input. It doesn't matter what the logic function is, output ties directly to input. For the 74LS family, one output can drive 20 inputs.

You are thinking of unused inputs, and you're in for trouble. Yes, you tie an input low by connecting it to ground. For a high, though, you must not let it float. The easiest way is to use an unused gate, such as a 74LS00 or 74LS04, whose input was tied low. Another way is to use a pullup resistor. 10K to +5 will work just fine. If you let the input float, a 74LS gate will usually work, but every once in a while it will act like an antenna, and change state at apparently random intervals.

While 74LS will usually be OK when floating, the same is not true of CMOS logic. ALL unused CMOS inputs MUST be tied either to ground or power. And I'm not speaking hypothetically here - trust me.

Like others have already said (I'll just be a little more elaborate), unused CMOS input pins must never be unconnected, because they tend to float towards the dangerous region which is in the middle between VDD and GND. The input pin invariably is connected to another complementary MOS pair's gates, and the process parameters are often optimized for performance so that both the high side and the low side will start conducting a little earlier than in the exact middle point. So in this "middle gray area", both the high side and the low side FETs will conduct some electricity, which results in current consumption, or in some cases, even oscillation if there's a positive feedback path to be found somewhere.

The easiest case and electrically the most stable case is to connect all unused CMOS inputs to ground. But in microcontrollers this can be a little dangerous, because software may make use pins as inputs or outputs. A software update might then make an output from a pin which has previously been unused. In this case, the safest option is to use separate pull-down resistors for each pin. If that is too costly due to the number of resistors and PCB space needed, you can also connect a group of adjacent pins together and pull them low with a single pull-down resistor. In that case, the extra current consumption caused by the surprise software change is usually not that big a problem.

In your specific case of using a hex inverter, there is yet another possibility, which is often used. You can connect the unused inverter inputs and outputs together with some inverter, which is used in the system: connect several inverters in parallel. This is often done to increase the drive capability and thus speed of the inverter, especially when driving large MOSFET gate loads.