Configuring the NI 940x Modules for Sinking or Sourcing Digital Outputs

Updated Jan 10, 2018

Reported In

Hardware

  • NI-9401
  • NI-9402
  • NI-9403

Issue Details

My 9401/9402/9403 module is listed as having bidirectional digital outputs. How do I configure it to be sinking or sourcing?

Solution

The 9401, 9402, and 9403 modules are designed to be bidirectional without requiring any hardware or software configuration.  This is done by taking advantage of the similarities between a sourcing output and an inverted sinking output.  

The figures below include a simplified circuit to demonstrate the functionality of the 940x devices as bidirectional sinking/sourcing outputs:
 
Figure 1: 940x Module Wired as a Sourcing Digital Output

When the load is wired up in a sourcing configuration, as shown in Figure 1, the 9401, 9402, and 9403 will push current through the load when the digital output is set to high.  When the output is low, the digital line is grounded.
 
Figure 2: 940x Module Wired as a Sinking Digital Output
 
When the load is wired up in a sinking configuration, as shown in Figure 2,  the 9401, 9402, and 9403 will pull current through the load when the digital output is set to low.  

Notice that when the digital output is set to high, the digital output is 5V rather than an open circuit, which prohibits current flow so long as the external power supply matches the internal supply.

Additional Information

For the 9402, the internal and external sources should be 3.3V instead of 5V in the figures above.

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