Can I Use 3.3V TTL to Trigger a 3.3V CMOS Device?

Updated Mar 10, 2020

Issue Details

  • I need to send a trigger signal to my 3.3V CMOS device. Can I use a 3.3V TTL signal to do this?
  • Are 3.3V TTL and 3.3V CMOS logic levels compatible?
  • I need a 3.3V CMOS signal but only have NI hardware that outputs 3.3V TTL. How can I make this work?

Solution

Generally, 3.3V TTL signals will have a suitable voltage cross-over with 3.3V CMOS and therefore, the TTL signal can be used to trigger the CMOS device.

The logic level thresholds for 3.3V CMOS are a known standard. For a 3.3V CMOS device to acknowledge a logic high or low, the required voltages are as follows:
  • Logic low - input must be between 0V and 0.8V.
  • Logic high - input must be between 2V and 3.3V.
To be certain of whether your TTL output is compatible with your CMOS device, refer to your device's specifications sheet.
 

Additional Information

The TTL voltage ranges can be found in NI's hardware datasheet. For example, see the below image that highlights the TTL input and output voltage thresholds:


Note: This image shows the TTL voltage levels for the NI-9402 C Series Module.

If your device uses 5V TTL, these conditions may not apply because the voltage level thresholds will differ from the picture above.