Resistor–transistor logic (RTL) is a class of digital circuits built using resistors as the input network and bipolar junction transistors (BJTs) as switching devices. RTL is the earliest class of transistorized digital logic circuit used; other classes include diode–transistor logic (DTL) and transistor–transistor logic (TTL).
Schematic of basic two-input RTL NOR gate
Advantages:
The primary advantage of RTL technology was that it involved a minimum number of transistors, which was an important consideration before integrated circuit technology (that is, in circuits using discrete components), as transistors were the most expensive component to produce. Early IC logic production (such as Fairchild's in 1961) used the same approach briefly, but quickly transitioned to higher-performance circuits such as diode–transistor logic and then transistor–transistor logic (starting 1963 at Sylvania), since diodes and transistors were no more expensive than resistors in the IC.
Limitations:
The obvious disadvantage of RTL is its high power dissipation when the transistor is switched on (the power is dissipated mainly by the base resistors connected to logical "1" and by the collector resistor). This requires that more current be supplied to and heat be removed from RTL circuits. In contrast, TTL circuits with "totem-pole" output stage minimize both of these requirements. Lancaster says that integrated circuit RTL NOR gates (which have one transistor per input) may be constructed with "any reasonable number" of logic inputs, and gives an example of an 8-input NOR gate. A standard integrated circuit RTL NOR gate can drive up to 3 other similar gates. Alternatively, it has enough output to drive up to 2 standard integrated circuit RTL "buffers", each of which can drive up to 25 other standard RTL NOR gates.
