RIO Developer Essentials Guide for Academia
RT PC code example
Send messages through a network stream channel
Send command and status messages through a low-latency lossless network-based data communication channel between the RT target and PC host system.
Use cases
- Exchange command and status messages with the PC host
- Intermittent data transfers that require minimum latency (delay)
- Lossless data transfer, i.e., when every data point matters
Features
- Ideal for exchanging messages between two endpoints, one on the RT target and the other on the PC host
- Transfers any LabVIEW data type – not necessary to first flatten the data to strings as required by standard UDP and TCP protocols as in the TCP client-server and UDP client-server code examples
Keep in mind
- Use a timeout value of zero for the stream writer to ensure that the message is immediately send through the channel, otherwise the network stream manager in an attempt for efficient use of the network connection may buffer (hold) several messages before sending in a batch
- Streams can be destroyed at either endpoint, i.e., it is not necessary for both VIs to do this
- Stopping a VI also destroys its endpoints – special care is required to ensure a VI does not attempt to read/write the network stream once the other VI has stopped
LabVIEW block diagram elements
Locate these elements with "Quick Drop" (press Ctrl+Space and start typing the name); click on an icon to see more sample code that uses that element:
Example code
- Connect your Academic RIO Device to your PC using USBLAN, Ethernet, or Wi-Fi. NOTE: Not all Academic RIO Devices have Ethernet and Wi-Fi connectivity options.
-
Download and unpack the
rt-pc_network-streams-message.zip (for use with NI myRIO 1900)
or the
NIELVISIII-rt-pc_network-streams-message.zip (for use with NI ELVIS III)
archive, and then double-click the ".lvproj" file to open the project. NOTE: This project was written for a NI myRIO 1900 or NI ELVIS III connected by USBLAN at IP address 172.22.11.2.
If you are using a different IP address or another Academic RIO Device (Example: NI myRIO 1950 or NI RIO Control Module) do the following:
- If using the NI myRIO 1950 or NI RIO Control Module start with the NI myRIO 1900 Archive.
- Different IP address: Right-click on the "NI myRIO 1900" Device, choose "Properties", and then enter the new IP address
- Different device:
- Right-click on the top of the project hierarchy, select "New Targets and Devices", keep the "Existing target or device" option, and then find and select your particular device
- Select all of the components under the "NI myRIO 1900" device: click the first one and then shift+click the last one
- Drag the selected components to the new device
- Right-click the "NI myRIO 1900" device and select "Remove from project"
- Run “RT Main”
- Review the available RT target IP addresses:
- “RT Main” creates one endpoint of the network stream as a reader, and then displays “messages” (integers) sent from the PC host as they arrive; the VI shuts down the endpoint when a negative integer is received
- Open “PC Main” and enter a suitable IP address (the USBLAN is default)
- Run “PC Main”
- “PC Main” creates the other endpoint of the network stream as a writer
- Enter a message of one or more integer values and then click “send”
- Change the message and click “send” again; right-click on the array control and choose “Insert Element Before” or “Delete Element” as needed
- Click “Stop” to send a -1 to shut down the RT VI
- Re-run “PC Main” without running “RT Main” and observe that the “RT connected?” indicator is off – this indicates that the reader endpoint of the network stream does not yet exist
- Click “send” and observe that the button is stuck down – this indicates that the PC host writer is blocking data flow until the network stream is established; run “RT Main” to clear the blockage
Expected results
https://youtu.be/N-bKjYHvXyg (2:20)
Developer walk-through
https://youtu.be/_axWmK2z5TQ (5:29)
Outline
- Review overall structure:
- PC Main
- Assemble URL of RT-side network stream reader endpoint
- Create network stream writer endpoint on PC side
- Indicate when endpoint on RT side is connected
- Send one or more integers as “messages” when “send” button is clicked
- Send -1 when “Stop” button is pressed to signal RT to stop
- Wait for RT endpoint to be destroyed and then stop
- RT Main
- Display available IP addresses
- Create network stream reader endpoint
- Read and display the integer “messages” received from PC
- Stop when -1 is received and destroy the endpoint
- PC Main
- “PC Main” details
- Network stream URL is of the form “//IPaddress/endpoint” where “IPaddress” designates the IP address in dot-decimal notation of the RT target and “endpoint” designates the name of the RT-side reader endpoint, e.g., “RT-reader”
- Boolean “F” constant initializes the front panel indicator “RT connected?”
- “Create Network Stream Writer Endpoint” function creates the PC-side endpoint with the name “PC-writer” and with buffer size and data type; use default timeout of -1ms to block until the RT creates its corresponding endpoint
- Property node confirms RT is connected and updates “RT connected?” indicator using a local variable
- Event structure:
- Write integer “messages” to the network stream when “send” button is clicked
- Important: Use 0 timeout to ensure that messages are transmitted immediately
- Send -1 when “Stop” button is pressed to signal RT to stop
- Property node in while loop polls network stream status to wait until the RT-side endpoint is destroyed
- “RT Main” details
- “String To IP” with “Multiple Output” option returns array of all available IP addresses of the RT target
- For-loop with “IP To String” with “dot notation” enabled converts IP addresses to dot-decimal notation; “PC Main” must use one of these addresses
- “Invoke Method” initializes the “messages received” indicator
- “Create Network Stream Reader Endpoint” function creates the RT-side endpoint with the name “RT-reader” and with buffer size and data type; use default timeout of -1ms to block until the PC creates its corresponding endpoint
- Read and display integer “messages” from the network stream as they arrive
- Reading a negative value for the message stops the main loop and destroys the RT-side endpoint
- Locate the functions subpalette
- Network streams
For more information
- Lossless Communication with Network Streams: Components, Architecture, and Performance (http://www.ni.com/white-paper/12267/en/)
An introduction to network streams, features, and performance.
- Using the Right Networking Protocol (http://www.ni.com/white-paper/12079/en)
Discusses the three most-common communication models -- command or message-based, process data, and streaming/buffered -- and the available networking protocols that are best suited to each model: TCP, UDP, network-published shared variables, network streams, and web services.