resistorâtransistor 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 diodeâtransistor logic (dtl) and transistorâtransistor logic (ttl). the concept had been used in early computers with electron tubes, and in rtl circuits constructed with discrete components, but in 1961 it became the first digital logic family to be produced as a monolithic integrated circuit. such integrated circuits were used in the us space program in 1962. == implementation == === rtl inverter === a bipolar transistor switch is the simplest rtl gate (inverter or not gate) implementing logical negation.resistor-transistor logic explains the basic rtl gates and gives some useful calculations it consists of a common-emitter stage with a base resistor connected between the base and the input voltage source. the role of the base resistor is to expand the negligible transistor input voltage range (about 0.7 v) to the logical "1" level (about 3.5 v) by converting the input voltage into current. its resistance is settled by a compromise: it is chosen low enough to saturate the transistor and high enough to obtain high input resistance. the role of the collector resistor is to convert the collector current into voltage; its resistance is chosen high enough to saturate the transistor and low enough to obtain low output resistance (high fan-out). right|thumb|200px|schematic of a one-transistor rtl nor gate. === one-transistor rtl nor gate === with two base resistors (r3 and r4) instead of one, the inverter becomes a two-input rtl nor gate (see the figure on the right). the logical operation or is performed by applying consecutively the two arithmetic operations addition and comparison (the input resistor network acts as a parallel voltage summer with equally weighted inputs and the next common-emitter transistor stage - as a voltage comparator with a threshold about 0.7 v). the equivalent resistance of all the resistors connected to logical "1" and the equivalent resistance of all the resistors connected to logical "0" form the two legs of a composed voltage divider driving the transistor. the base resistances and the number of the inputs are chosen (limited) so that only one logical "1" is sufficient to create base-emitter voltage exceeding the threshold and, as a result, saturating the transistor. if all the input voltages are low (logical "0"),
Apollo_program_hardware Integrated_circuits Logic_families