You can achieve this function if you connect two external power supplies, one is +6V, one is -6V. The circuit diagram is shown in figure 1.
Figure 1. Circuit Diagram
I have tested this function according to the circuit diagram：
- Use the SMU PXI-4130 to generate +6V and -6V power supplies;
- Plug the NI 9477 in a compact DAQ chassis；
- Use the LabVIEW example of Digital-continuous output.vi to generate a PWM signal in the DO terminal, for example figure 2;
Figure 2. LabVIEW Example
- Use an oscilloscope to view the PWM signal generated at DO，the result is shown in figure 3;
Figure 3. Output Waveform
You can change the voltage level by the power supplies.
Additional Information
Power supplies are not have to be generated by SMU, you can configure them as your wish. Besides, it is not specifically for "6V", this method can be applied to any voltage generated by power supply(considering the hardware spec).