Editing Resistor–transistor logic

{{Short description|Class of digital circuits}}
'''Resistor–transistor logic''' ('''RTL'''), sometimes also known as '''transistor–resistor logic''' ('''TRL'''), is a class of [[digital circuit]]s built using [[resistor]]s as the input network and [[bipolar junction transistor]]s (BJTs) as switching devices. RTL is the earliest class of transistorized digital logic circuit; it was succeeded by [[diode–transistor logic]] (DTL) and [[transistor–transistor logic]] (TTL). 

RTL circuits were first constructed with [[discrete component]]s, but in 1961 it became the first digital [[logic family]] to be produced as a [[monolithic integrated circuit]]. RTL integrated circuits were used in the [[Apollo Guidance Computer]], whose design began in 1961 and which first flew in 1966.<ref>{{cite book |chapter=2. Computers On Board The Apollo Spacecraft §2.5 The Apollo guidance computer: Hardware |chapter-url=https://history.nasa.gov/computers/Ch2-5.html |title=Computers in Spaceflight: The NASA Experience |publisher=NASA History Division |date=1987 |url=https://history.nasa.gov/computers/contents.html}}</ref>

== Implementation ==

=== RTL inverter ===

A bipolar [[Transistor#Transistor as a switch|transistor switch]] is the simplest RTL gate ([[Inverter (logic gate)|inverter]] or NOT gate) implementing [[logical negation]].<ref>[http://www.play-hookey.com/digital_electronics/rtl_gates.html Resistor-Transistor Logic] {{Webarchive|url=https://web.archive.org/web/20181002181106/http://www.play-hookey.com/digital_electronics/rtl_gates.html |date=2018-10-02 }} explains the basic RTL gates and gives some useful calculations</ref> It consists of a [[Common emitter|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 very small 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]]).

=== One-transistor RTL NOR gate ===
[[File:RTL NPN NOR Gate.svg|right|thumb|200px|Schematic of a one-transistor RTL NOR gate.]]

With two or more base resistors (R<sub>3</sub> and R<sub>4</sub>) instead of one, the inverter becomes a two-input RTL [[NOR gate]] (see the figure on the right). The logical operation [[Logical disjunction|OR]] is performed by applying consecutively the two arithmetic operations [[addition]] and [[Comparison (mathematics)|comparison]] (the input resistor network acts as a parallel ''voltage summer'' with equally weighted inputs and the following 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"), the transistor is cut-off. The [[pull-down resistor]] R<sub>1</sub> biases the transistor to the appropriate on-off threshold.  The output is inverted since the collector-emitter voltage of transistor Q<sub>1</sub> is taken as output, and is high when the inputs are low. Thus, the analog resistive network and the analog transistor stage perform the logic function NOR.<ref>{{cite book |author=IBM |id=Form 223-6889|year=1960 |title=Transistor Component Circuits |publisher=IBM |url=http://ibm-1401.info/Form223-6889-TransistorComponentCircuits.pdf |series=Customer Engineering Manual of Instruction |accessdate=2010-01-04 |quote=The logical function is performed by the input resistor network and the invert function is accomplished by the common emitter transistor configuration... }}</ref>

=== Multi-transistor RTL NOR gate ===
[[File:RTL 3-Input NOR Gate.svg|right|thumb|300px|Schematic of a multi-transistor RTL NOR gate, as used in the [[Apollo Guidance Computer]] integrated circuits.<ref name=AGC2005011>[[Apollo Guidance Computer]] schematics, [http://klabs.org/history/ech/agc_schematics/logic/5011-1.jpg Dwg. No. 2005011].</ref>]]
[[File:Agc nor2.jpg|right|thumb|300px|Photograph of the dual 3-input NOR gate chip used to build the [[Apollo Guidance Computer]]. Connections (clockwise from top center) ground, inputs (3), output, power (V<sub>cc</sub>), output, inputs (3). The six transistors (two groups of three) are in the center. The thin wires from the terminals to the transistors are resistors. ]]
[[File:Agc flatp.jpg|thumb|right|[[Flatpack (electronics)|Flatpack]] RTL NOR gate integrated circuits in the [[Apollo guidance computer]] ]]

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 [[Direct-coupled transistor logic|DCTL]], [[Emitter-coupled logic|ECL]], some [[Transistor-transistor logic|TTL]] (7450, 7460), [[NMOS logic|NMOS]] and [[CMOS]] gates.

=== Transistor bias ===
{{Main|Bipolar transistor biasing}}
To ensure stability and predictable output of the bipolar transistors their base-inputs (V<sub>b</sub> 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.<ref>{{cite book | title = Electronics: The Life Story of a Technology | author = David L. Morton Jr. and Joseph Gabriel | publisher = JHU Press | year = 2007 | isbn = 978-0-8018-8773-4 | url = https://books.google.com/books?id=rABggQmp31MC&q=rtl+dtl+ttl+integrated+circuits&pg=PA84 }}</ref>

== 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 "[[Totem pole output|totem-pole]]" output stage minimize both of these requirements.

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|date=January 2008}} 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.<ref name=Lancaster>
{{cite book
|author=Donald E. Lancaster
|title=RTL cookbook
|year= 1969
|isbn=0-672-20715-X
|publisher=Bobbs-Merrill Co. (or Howard W Sams)
|url=http://www.alibris.com/booksearch?qsort=p&siteID=BMAI54k.xMY-6bJvSC2hb2j9kKSb5V.rLA&qtit=RTL+Cookbook}}
</ref>

A standard integrated circuit RTL NOR [[Logic gate|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.<ref name=Lancaster/>

== Speeding up RTL ==

Various companies have 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.&nbsp;181, or 3rd ed., p.&nbsp;97 or intermediate editions) recommends gaining speed by using higher-frequency transistors, or capacitors, or a diode from base to collector ([[Baker clamp|parallel negative feedback]]) to prevent saturation.<ref name=Cleary>
{{cite book
  | editor = Cleary, J. F.
  | title = ''GE Transistor Manual''
  | edition = 3rd–7th
  | publisher = General Electric, Semiconductor Products Department, Syracuse, NY
  | year = 1958–1964 }}
</ref>

{{anchor|RCTL}}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|TX-0 computer's]] circuits included some RCTL.<ref name=Fadiman>
{{cite book | last=Fadiman | first=J. R. | year=1956 | title=TX0 Computer Circuitry  | publisher = MIT Lincoln Laboratory | url=http://bitsavers.org/pdf/ibm/140x/1401_CE_Drws_1962.pdf | accessdate=2011-09-09 }}
</ref> However, methods involving capacitors were unsuitable for integrated circuits.{{Citation needed|date=April 2011}}

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]]).<ref name=DEC>
{{cite book | year=1967 | title=The Digital Logic Handbook Flip Chip Modules | publisher = Digital Equipment Corporation| url=http://www.bitsavers.org/pdf/dec/handbooks/ |id=1750·3/67 |via=Bitsavers | accessdate=2008-03-08 }}
</ref>

Another method that was familiar in [[discrete-device]] logic circuits used a diode and a resistor, a germanium and a silicon diode, or three diodes in a negative feedback arrangement. These diode networks known as various [[Baker clamp]]s reduced the voltage applied to the base as the collector approached saturation. Because the transistor went less deeply into saturation, the transistor accumulated fewer stored charge carriers. Therefore, less time was required to clear stored charge during transistor turn off.<ref name=Cleary/> A low-voltage diode arranged to prevent saturation of the transistor was applied to integrated logic families by using [[Schottky diode]]s, as in Schottky [[Transistor–transistor logic|TTL]].

==See also==
* [[NORBIT]]
* [[Transistor–transistor logic]]
* [[Diode–transistor logic]]
* [[Diode logic]]

== References ==
{{Reflist}}

==Further reading==
* ''RTL Cookbook''; 1st Ed; [[Don Lancaster]]; Sams; 240 pages; 1969; {{ISBN|978-0672207150}}. <small>[https://archive.org/details/RTL_Resistor-Transistor_Logic_Cookbook ''(3ed archive)'']</small>

{{Logic Families}}

{{DEFAULTSORT:Resistor-transistor logic}}
[[Category:Integrated circuits]]
[[Category:Logic families]]
[[Category:Apollo program hardware]]