Resistor–transistor logic
Resistor–transistor logic (RTL), sometimes also known as transistor–resistor logic (TRL), is a class of
RTL circuits were first constructed with
Implementation
RTL inverter
A bipolar
One-transistor RTL NOR gate
With two or more base resistors (R3 and R4) instead of one, the inverter becomes a two-input RTL
Multi-transistor RTL NOR gate
The limitations of the one-transistor RTL NOR gate are overcome by the multi-transistor RTL implementation. It consists of a set of parallel-connected transistor switches driven by the logic inputs (see the figure on the right). In this configuration, the inputs are completely separated and the number of inputs is limited only by the small leakage current of the cut-off transistors at output logical "1". The same idea was used later for building
gates.Transistor bias
To ensure stability and predictable output of the bipolar transistors their base-inputs (Vb or base-terminal voltage) is biased.
Advantages
The primary advantage of RTL technology was that it used a minimum number of transistors. In circuits using discrete components, before integrated circuits, 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 in 1963 at Sylvania Electric Products), since diodes and transistors were no more expensive than resistors in the IC.[5]
Limitations
The disadvantage of RTL is its high power dissipation when the transistor is switched on, by current flowing in the collector and base resistors. This requires that more current be supplied to and heat be removed from RTL circuits. In contrast, TTL circuits with "
Another limitation of RTL is its limited fan-in: 3 inputs being the limit for many circuit designs, before it completely loses usable noise immunity.[citation needed] It has a low noise margin. 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.[6]
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.[6]
Speeding up RTL
Various companies applied the following speed-up methods to discrete RTL.
Transistor switching speed has increased steadily from the first transistorized computers through the present. The GE Transistor Manual (7th ed., p. 181, or 3rd ed., p. 97 or intermediate editions) recommends gaining speed by using higher-frequency transistors, or capacitors, or a diode from base to collector (parallel negative feedback) to prevent saturation.[7]
Placing a capacitor in parallel with each input resistor decreases the time needed for a driving stage to forward-bias a driven stage's base-emitter junction. Engineers and technicians use "RCTL" (resistor-capacitor-transistor logic) to designate gates equipped with "speed-up capacitors". The Lincoln Laboratory TX-0 computer's circuits included some RCTL.[8] However, methods involving capacitors were unsuitable for integrated circuits.[citation needed]
Using a high collector supply voltage and diode clamping decreased collector-base and wiring capacitance charging time. This arrangement required diode clamping the collector to the design logic level. This method was also applied to discrete DTL (diode–transistor logic).[9]
Another method that was familiar in
See also
References
- ^ "2. Computers On Board The Apollo Spacecraft §2.5 The Apollo guidance computer: Hardware". Computers in Spaceflight: The NASA Experience. NASA History Division. 1987.
- ^ Resistor-Transistor Logic Archived 2018-10-02 at the Wayback Machine explains the basic RTL gates and gives some useful calculations
- ^ IBM (1960). Transistor Component Circuits (PDF). Customer Engineering Manual of Instruction. IBM. Form 223-6889. Retrieved 2010-01-04.
The logical function is performed by the input resistor network and the invert function is accomplished by the common emitter transistor configuration...
- ^ Apollo Guidance Computer schematics, Dwg. No. 2005011.
- ISBN 978-0-8018-8773-4.
- ^ a b
Donald E. Lancaster (1969). RTL cookbook. Bobbs-Merrill Co. (or Howard W Sams). ISBN 0-672-20715-X.
- ^ a b Cleary, J. F., ed. (1958–1964). GE Transistor Manual (3rd–7th ed.). General Electric, Semiconductor Products Department, Syracuse, NY.
- ^ Fadiman, J. R. (1956). TX0 Computer Circuitry (PDF). MIT Lincoln Laboratory. Retrieved 2011-09-09.
- ^ The Digital Logic Handbook Flip Chip Modules. Digital Equipment Corporation. 1967. 1750·3/67. Retrieved 2008-03-08 – via Bitsavers.
Further reading
- RTL Cookbook; 1st Ed; ISBN 978-0672207150. (3ed archive)