What Is the Definition of a TTL (Transistor-Transistor Logic) Compatible Signal

Updated Mar 15, 2022

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What is the definition of a TTL (Transistor-Transistor Logic) compatible signal


A TTL signal must comply with the following specifications for the voltage and current output as well as for the voltage and current input, in addition it must comply with the pulse rise and fall time specification (rise/fall time). Additionally, they must have a minimum pulse width.

The specifications use the following notation:

  • Output Low = OL
  • Output High = OH
  • Input Low = IL
  • Input High = IH
1. Specifications for current and voltage output are as follows:
  • V OL = 0.4V
  • V OH = 2.4V
  • I OL = 16mA
  • I OH = 400µA
2. Specifications for current and voltage input are as follows
  • V IL = 0.8V
  • V IH = 2.0V
  • I IL = 1.6mA
  • I IH = 40µA
3. The maximum rise and fall time must be 50ns between high and low voltage.
4. The minimum width for different devices:
  • Devices with NI-STC are 10ns.
  • Devices with NI-TIO are 5ns.