There are no corresponding states in the initialization VIs. The initialization VIs runs on the execution host and can be used to configure the engine VI which runs on the execution target. Initialization VI runs one time, when the custom device is first added to the system definition. Main runs every time the operator click on the custom device in the system explorer. The states inside the driver VI specify when the state executes with respect to the rest of the NIVS Engine.
Take the Inline Hardware Interface template for example.
The inline hardware interface template is similar to state machine architecture. Some developers will recognize it as an action-engine. See NI Discussion Forums » LabVIEW » Community Nugget 4/08/2007 Action Engines for a discussion on action engines. The Primary Control Loop (PCL) specifies the case to execute. An uninitialized Feedback Node is used for iterative data transfer. There are five cases defined by the Operation enumerated control.
- Initialize
- Start
- Read Data from Hardware
- Write Data to Hardware
- Close
This custom device runs in-line with the PCL, which calls each case at a specific time with respect to the other components in the NI VeriStand engine. The PCL will not proceed until the custom device case has completed.
Initialize
The Initialize case executes before the PCL starts. In this case, you can read the device configuration information from properties using the reference to the device. Initialize data and buffers used internally in the device. The framework compiles the list of Data References for the custom device Inputs and Outputs in advance using Custom Device API » Driver Functions » Get Custom Device Channel List and Custom Device API.lvlib » Templates » RT Driver VIs » Inline » Inline Driver Utilities » Channel Data References » Get Channel Data Reference.vi. Since the PCL hasn’t started yet, you can't read or write channel values in the Initialize case.
Start
The Start case executes after Initialization and before the PCL starts running. There’s no difference between what code you can place in the Initialize and Start states. Since the PCL hasn’t started yet, you can't read or write channel values in the Start case.
Read Data from HW
The Read Data from HW case executes at the beginning of the PCL, before other components (such as Stimulus, Faults, Alarms, Procedures, et cetera) execute. The template contains a Flat Sequence frame named Read Hardware Channels. You can replace the code inside the frame with the API calls necessary to obtain data from a hardware API. Do not call Get or Set Channel Value by Data Reference outside the inline driver VI. Doing so could cause system instability or errors.
Write Data to HW
The Write Data to HW case executes at the end of the PCL, after the other components (such as Stimulus, Faults, Alarms, Procedures, et cetera) have executed. The case contains a Flat Sequence frame named Write Input Data to Hardware Channels. You can replace the code inside the frame with the API calls necessary to send data to a hardware device.
Close
The Close case executes after the PCL has finished executing. It's good practice to close references and release resources in this state. Since the PCL has terminated, you cannot read or write channel values in this case.
Steve K
