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High-threshold logic (HTL), also known as ''low-speed logic'' (''LSL'') or ''high-level logic'' (''HLL''), is a variant of 
diode–transistor logic
Diode–transistor logic (DTL) is a class of digital circuits that is the direct ancestor of transistor–transistor logic. It is called so because the logic gating function (e.g., AND) is performed by a diode network and the amplifying functio ...
used in environments where noise is very high.
Operation
The threshold values at the input to alogic gate
A logic gate is an idealized or physical device implementing a Boolean function, a logical operation performed on one or more binary inputs that produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, ...
determine whether a particular input is interpreted as a logic 0 or a logic 1 (e.g. anything less than 1 V is a logic 0, and anything above 3 V is a logic 1; in this example, the threshold values are 1 V and 3 V). HTL incorporates Zener diode
A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" (inverted polarity) when a certain set reverse voltage, known as the ''Zener voltage'', is reached.
Zener diodes are manufactured with a great varie ...
s to create a large offset between logic 1 and logic 0 voltage levels. These devices usually ran off a 15 V power supply and were found in industrial control, where the high differential was intended to minimize the effect of noise.
Advantages
* Increased noise margin * High noise threshold valueDisadvantage
*Slow speed due to increased supply voltage resulting in use of high value resistors. *High power drawnUsage
It is used extensively in industrial environments. e.g. * Logic controllers with heavy noise * Heavy-process machinerySimilar circuits
The buffer in this device is exactly the same of what was used in RGB video output stages in TV circuits in the way that the upper NPN transistor quickly rises a cathode capacitance with a relatively high load resistor on the lower NPN transistor, while the lower NPN transistor controls the turning on of the output voltage. The principle of improving charge–discharge parasitic capacitances used here is the same as in high-threshold logic circuits.See also
*Diode logic
Diode logic (DL), or diode-resistor logic (DRL), is the construction of Boolean logic gates from diodes. Diode logic was used extensively in the construction of early computers, where semiconductor diodes could replace bulky and costly active v ...
(DL)
* Diode-transistor logic (DTL)
*Emitter-coupled logic
In electronics, emitter-coupled logic (ECL) is a high-speed integrated circuit bipolar transistor logic family. ECL uses an overdriven bipolar junction transistor (BJT) differential amplifier with single-ended input and limited emitter current to ...
(ECL)
*Integrated injection logic
Integrated injection logic (IIL, I2L, or I2L) is a class of digital circuits built with multiple collector bipolar junction transistors (BJT). When introduced it had speed comparable to TTL yet was almost as low power as CMOS, making it ideal fo ...
(I2L)
*Resistor–transistor logic Resistor–transistor logic (RTL) (sometimes also transistor–resistor logic (TRL)) 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 o ...
(RTL)
* Transistor–transistor logic (TTL)References
Logic families {{Logic Families