High Threshold Logic

File:HTL.jpg

Schematic of basic three-input HTL NAND gate.

High Threshold Logic (HTL) is a variant of DTL which is used in such environments where noise is very high.

Operation

The threshold values at the input to a 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 1V and 3V). HTL incorporates Zener diodes 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
• Spike Control
• High Noise Threshold Value

Disadvantage

• slow speed due to increased supply voltage resulting in use of high value resistors.

Usage

It is used extensively in industrial environments. e.g.

• Logic Controllers with heavy noise
• Heavy Process Machinery

See also

• Diode logic (DL)
• Emitter-coupled logic (ECL)
• Integrated injection logic (I2L)
• Resistor–transistor logic (RTL)
• Transistor–transistor logic (TTL)