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For dynamic generation operations, the data appears at the input of the three-level driver after the Pattern Generation Timing and Control module applies the selected data position and data delay to the data. The three-level driver converts the data signal to the programmable voltage levels (Generation Voltage High, Generation Voltage Low, or VTT) before sending the data signal to the DDC connector on the NI 6555/6556 front panel.
You can set the programmable voltage driver to 50 Ω termination or high-impedance (default) generation with the Tristate Mode property or the NIHSDIO_ATTR_DATA_TRISTATE_MODE attribute. The channel is set automatically to tristate by the Initial and Idle States or you can programmatically set it with the niHSDIO Tristate Channels VI or the niHSDIO_TristateChannels function.
The following image shows the driver circuitry on the NI 6555/6556.