Sequence Step Delta Time

When enabled, sequence step delta time enforces a fixed time dt between the start and end of steps in a simple or advanced sequence. This level of determinism allows you to, for example, create periodic voltage waveforms with supported NI-DCPower instruments.

Note The terms sequence step delta time and timed output mode are equivalent.

The following figure illustrates using sequence step delta time to create two steps of a periodic voltage waveform for two iterations, where:

  • Sequence step delta time is the same for each step;
  • The voltage level and source delay alternate with each step; and
  • The Measure When property is set to Automatically After Source Complete
Figure 1. Sequence Step Delta Time Source Model


When an NI-DCPower instrument uses sequence step delta time, the source unit operates according to the following steps:

  1. Upon receiving a Start trigger, the source unit applies the first voltage level in the sequence.
  2. Once the specified source delay elapses, the source unit generates a Source Complete event.
  3. Because the Measure When property is set to Automatically After Source Complete, the following occur:
    1. The measure unit takes a measurement immediately upon receiving the Source Complete event.
    2. The measure unit generates a Measure Complete event.
  4. Upon receiving the Measure Complete event, the source unit prepares for the next step of the sequence by waiting for the remainder of the time dt you specify with the Sequence Step dt property to elapse.
  5. Once the sequence step delta time elapses, the source unit exports the Source trigger, and the next step of the sequence begins.
    Note The source unit exports the Sequence Advance trigger on the first step of any subsequent sequence iterations.
  6. The sequence iterates until completion.
    Note The sequence step dt does not apply to the final step of the final iteration of a sequence.

You can enable sequence step delta time with the Sequence Step dt Enabled property and specify the dt itself with the Sequence Step dt property.

The PXIe-4139 supports configurable step duration for sequences via the Sequence Step Delta Time property for voltage levels and limits <|42.4 V| only.

Note Sequence step delta time is supported with only DC voltage outputs; it is not supported with pulse voltage and pulse current outputs.

Programming Sequence Step Delta Time

Use sequence step delta time to control the duration of each step of a sequence when the step timing is important.

Configure the following properties to apply sequence step delta time to a sequence:
  1. Set the Source Mode property to Sequence.
    Note Sequence step delta time does not apply in Single Point source mode.
  2. Set the Source Trigger Type property to None.
    This is the default value.
  3. Set the Sequence Advance Trigger Type property to None.
    This is the default value.
  4. Set the Output Function property to DC Voltage or DC Current.
    Sequence step delta time is not compatible with pulsed outputs.
  5. Enable sequence step delta time by setting the Sequence Step dt Enabled property to True.
  6. Set the desired step duration dt with the Sequence Step dt property.

Troubleshooting Sequence Step Delta Time Timing Issues

It is possible to set values for other properties that conflict with the value for specify for sequence step delta time. In general, if a sequence step cannot be completed within the dt you specify, NI-DCPower returns an error.

The following topics list situations that can lead to timing conflicts between sequence step delta time and other properties and attributes.

Source Delay, Measurement Time Exceed Sequence Step Delta Time

The total Source Delay, measurement time, and programming setup time for a given sequence step must be less than the dt you specify.

Advanced Sequencing and Sequence Step Delta Time

The additional programming time when using both an advanced sequence and sequence step delta time may cause the total time for a step to exceed the dt you specify.

The likelihood that an error related to advanced sequence programming time increases with the number of properties you configure for each step in the advanced sequence.

Range Changes and Sequence Step Delta Time

If a sequence step includes a range change, the time it takes to complete the step may exceed the sequence step dt you specify due to the additional time involved in configuring the source unit for the new range.

The following table describes the behavior of NI-DCPower when a range change occurs in a sequence while sequence step delta time is in use.

Table 14. Effect of Ranges Changes on Sequence Step Delta Time
Range Change Location Effect of Range Change
step[0]
  • The setpoint of the step may be generated for an amount of time that differs from the configured dt.
  • NI-DCPower does not generate an error if the time required to change ranges causes the channel to exceed your dt.
step[i]
  • The setpoints of step[i - 1] and step[i] may be generated for an amount of time that differs from the configured dt.
  • NI-DCPower generates an error if the time required to change ranges causes the channel to exceed your dt.

The following figure illustrates how the dt you specify interacts with a sequence that comprises two steps and two iterations and that may involve range changes, where:

  • Sequence step delta time is the same for each step;
  • The Measure When property is set to Automatically After Source Complete; and
  • The shaded sections represent the time the channel waits for the dt you specify to elapse.
Figure 1. Sequence Step Delta Time in NI-DCPower Sequences


Note the following:

  • Source generation time increases if a voltage or current range change is required, which can delay generation of the new setpoint. The channel changes ranges if the voltage or current ranges you specify for the step differ from the previously configured ranges.
  • The sequence step dt does not apply to the final step of the final iteration of a sequence.

Avoiding Range Changes in the First Step for Sequence Step Delta Time

The time involved in executing a range change in the first step of a sequence can cause the channel to exceed the dt you specify for that step.

You can place your channel in the desired voltage or current range before using Sequence Step Delta Time to ensure the channel generates the first voltage or current level in a sequence for the configured dt.

Complete the following steps to avoid exceeding your dt due to range changes in the first step.

  1. Set the Source Mode property to Single Point.
  2. Set the Output Function property to the desired output function, DC Voltage or DC Current.
  3. Set the Source Trigger Type and Sequence Advance Trigger Type properties to None.
  4. Configure the voltage or current level, level range, limit, and limit range as appropriate for your output function.
  5. Call the Initiate With Channels function.
  6. Call the Wait For Event With Channels function and pass the Source Complete Event to the event input.
  7. Call the Abort With Channels function.
  8. Continue configuring the instrument.
    1. Set the Source Mode property to Sequence.
    2. Set the Sequence Step dt Enabled property to True.
    3. Set the Sequence Step dt property to the desired dt.
    4. Create your sequence.
    • Create a simple sequence with the Set Sequence function.
    • Create an advanced sequence with the Create Advanced Sequence With Channels function and add steps to it.
When you initiate your sequence, any range changes will have occurred during the single point you generated.

Troubleshooting Sequence Step Delta Time Event Interactions

Certain NI-DCPower events have unique interactions with sequence step delta time. Applications that rely on these events and make use of sequence step delta time may encounter certain issues.

The following events have unique interactions with sequence step delta time:

Event Behavior
Sequence Engine Done For the final step of the final iteration of a sequence using sequence step delta time, the source unit does not wait for a dt period to elapse before emitting the Sequence Engine Done event. Therefore, this event for this step is emitted early with respect to dt.
Sequence Iteration Complete When sequence step delta time is enabled, the source unit does not emit the Sequence Iteration Complete event.

These event interactions may cause issues with applications that make use of sequence step delta time if you are using the event as an input to the following functions:

  • Configure Trigger With Channels
  • Export Signal With Channels
  • Wait For Event With Channels

Recovering from Sequence Step Delta Time Errors

To recover from a sequence step delta time error, choose one of the following error recovery methods:
  • Call the Reset With Channels function.
  • Call the Disable function.
  • Call the Reset Device function or reset with MAX.

Sequence Step Delta Time Timing Error Conditions

The dt value you set to enforce a sequence step delta time can conflict with the actual time it takes the hardware to generate a sequence step.

NI-DCPower returns an error due to timing conflicts between the value of the Sequence Step dt property and actual step length, as derived from the value of other properties, based on the relationships in the following table.

Table 15. Sequence Step Delta Time Timing Error Conditions
Setting(s) Errors When
  • Sequence Step dt Enabled is True
 Sequence Step dt – Minimum Sequence Step dt < Source Delay
  • Sequence Step dt Enabled is True;
  • Measure When is set to Automatically After Source Complete; and
  • Measure Record Length = 1
 Sequence Step dt – Minimum Sequence Step dt < Source Delay + Aperture Time + Measure Complete Event Delay
  • Sequence Step dt Enabled is True;
  • Measure When is set to Automatically After Source Complete; and
  • Measure Record Length > 1; and
  • DC Noise Rejection is set to Normal
 Sequence Step dt – Minimum Sequence Step dt < Source Delay + Aperture Time + ((Measure Record Length – 1) x Measure Record dt) + Measure Complete Event Delay
Measure Record dt = Aperture Time
  • Sequence Step dt Enabled is True;
  • Measure When is set to Automatically After Source Complete; and
  • Measure Record Length > 1; and
  • DC Noise Rejection is set to Second-Order
 Minimum Sequence Step dt – Minimum Sequence Step dt < Source Delay + Aperture Time + ((Measure Record Length – 1) x Measure Record dt) + Measure Complete Event Delay
Measure Record dt = Aperture Time ÷ 2