DT Logic

Diode Transistor Logic (DTL) and the related Resistor Transistor Logic (RTL) were popular for constructing Transistorised Computers in the 1950's and early 1960's, for example the Apollo Guidance Computer.

Here is a very short summary of Diode Transistor Logic.

Diodes conduct electricity in one direction (the direction of the arrow) and not in the other.   In practice you've got to exceed a minimum voltage (about 0.6v for a signal diode) before they start to conduct.
Diodes can be used to construct AND and OR gates.   Wikipedia has a good full article on Diode Logic, here we use examples from hyperphysics.phy-astr.gsu.edu


AND gates are used to decide whether to pay attention to an input.   The data is always present on A, but it only appears on the output if the data select, B, is also high.

OR gates combine many results into one.   For example, each of the functions of the ALU can be selected using AND gates so that only one is active, they are then all combined with a multiple input OR gate.

NPN transistors are current amplifiers.   A small current into the base allows a much larger (typically two orders of magnitude larger) current to flow between the emitter and collector.   They are commonly wired up like this:

This is the basis of a NOR gate, if no input is high (nothing supplies current) then the output is high, but if any input is high (something supplies current) then the output is low.