Learning Electronic Circuit from scratch (1): RTL (Resistor Transistor Logic) Inverter, NOR , OR

RTL stands for Resistor-Transistor Logic, which is different from the TTL (Transistor-Transistor Logic). Even it's almost replaced by TTL，the simplicity of it can still help us understand basic ideas.

The input is the Base, and the output is the Collector. When the input is high, the transistor turns fully on and grounds the output signal. When the input is grounded (logic 0), the transistor is off and the output signal is allowed to rise to +5 volts.

The transistor does invert the logic sense of the signal, but it also ensures that the output voltage will always be a valid logic level under all circumstances. So, RTL circuits can be cascaded indefinitely. 

1. Inverter

2. NOR

It may surprise the ordinary people that NOR instead of OR is the most basic gate from the hardware's point of view.

3 OR

OR gate can be implemented by combine NOR and NOT.

Here, the input of NOT gate is the output of the NOR gate. We can see that every output can be assured to have a valid voltage.

4 A bad OR gate

The problem here is the Output can NOT be connected to A or B to implement a latch.