Definition of a TTL (Transistor-Transistor Logic) Compatible Signal

Updated Apr 11, 2019

Reported In


  • Multifunction I/O Device
  • C Series Multifunction I/O Module
  • C Series Differential Digital I/O Module


  • NI-DAQmx

Issue Details

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


A TTL signal must meet an output voltage and current specification, an input voltage and current specification, and a rise/fall time specification. For any given hardware, the signal must also have a minimum pulse width.


Note: All specifications use the following notation:

  • Output Low = OL
  • Output High = OH
  • Input Low = IL
  • Input High = IH

The output voltage and current specifications are as follows:
  • VOL = 0.4 V
  • VOH = 2.4 V
  • IOL = 16 mA
  • IOH = 400 µA 

The input voltage and current specification are as follows:
  • VIL = 0.8 V
  • VIH = 2.0 V
  • IIL = 1.6 mA
  • IIH = 40 µA 

The maximum rise/fall time is 50 ns between VIL and VIH. The minimum pulse width for:
  • NI-STC devices is 10 ns.
  • NI-TIO devices is 5 ns. 

Additional Information

The ratio between IOL and IIL as well as IOH and IIH is 10 due to the standard fanout of TTL logic.