USB DAQ Physical Channels
Dev1 in physical channel names is the default device name for USB
Series DAQ devices. You can change these names in MAX.
Analog Input
The NI USB-9211, NI USB-9211A, NI USB-9215, NI USB-9215A, NI USB-9219, NI USB-9229, NI USB-9233, and NI USB-9239 have four analog input physical
channels named Dev1/ai0 to Dev1/ai3. The NI USB-9201 and
NI USB-9221 have eight analog input physical channels named Dev1/ai0
to Dev1/ai7.
Strain and Wheatstone Bridge Measurements
The NI USB-9237 supports only the AI Strain Gage and AI Custom Voltage With Excitation channel types.
When using the NI USB-9219 or NI USB-9237 with an AI Custom Voltage With Excitation channel, you must set the AI.Excit.UseForScaling attribute/property to true. This attribute/property causes the channel to return ratiometric data: Vin/Vex. The NI USB-9219 and NI USB-9237 modules perform this division in hardware.
For the NI USB-9219, NI-DAQmx requires the AI.Excit.Val attribute/property to be set to 2.5 V for AI Strain Gage and AI Custom Voltage With Excitation channel types and to 500 µA for resistance and RTD measurements. The actual excitation voltage or current output by the NI USB-9219 varies with the sensor resistance or the load being measured.
With the NI USB-9219 and NI USB-9237, the AI.Bridge.InitialVoltage attribute/property refers to a voltage, not a ratio, so it should be set to the ratio Vin/Vex returned by the device multiplied by Vex.
The NI USB-9219 does not have quarter bridge completion circuitry, which affects AI Strain Gage Quarter Bridge I channels and AI Custom Voltage With Excitation Quarter Bridge channels (but not AI Strain Gage Quarter Bridge II channels). With these channels, the NI USB-9219 performs a 2-wire resistance measurement on the active gage element, then NI-DAQmx uses software scaling to convert the resistance measurement into a bridge ratio. For these channels, the polynomial coefficients specified by the AI.DevScalingCoeff attribute/property convert unscaled data into Ohms, not into V/V. Likewise, the AI.Rng.High/AI.Rng.Low attributes/properties should be specified in units of Ohms, not V/V.
When the NI USB-9219 is in quarter bridge mode, you need to use the AI.Bridge.NomResistance attribute/property to control whether the channel uses the 120 Ω range or the 350 Ω range.
Analog Output
The NI USB-9263 has four analog output physical channels named Dev1/ao0 to Dev1/ao3.
When using the the NI USB-9263, you can run only one type of timing at a time. You can have one software-timed task per channel or one hardware-timed task running on a device at one time, but you cannot have a combination of timing on that device. For instance, you can run up to four software-timed tasks on an the NI USB-9263 concurrently, but running one hardware-timed task with one software-timed task generates an error. Additionally, the NI USB-9263 can run only one hardware-timed analog output task per device at a given time.