Channel wire

Use cases

• Communicate data, control, and status between two or more parallel process loops contained within the same VI (“VI-scoped”)
• Make the latest value of a variable available to other process loops (tag)
• Establish point-to-point lossless communication (stream)
• Stop multiple parallel loops from a single control

Features

• A channel wireVisual representation of asynchronous data flow between parallel loops (new feature in LabVIEW 2016) directly links endpoints contained in separate process loops
• Greatly simplifies wiring effort, reduces complexity
• Simplest method for stopping parallel loops
• Makes dataflow visually evident, i.e., can see a wire spanning process loop boundaries
• Provides a variety of data communication schemes:
  • Stream: buffered channel between two endpoints, operates like a queue or FIFO (first-in first-out) buffer; options for lossless, lossy, single-element, and real-time streams
  • Tag: single-value lossy buffer that allows multiple writers and readers
  • Messenger: many-to-many communications channel
• “Real Time Stream” channel wire links deterministic (timed) process loops to nondeterministic loops without inducing jitter

Keep in mind

• Requires LabVIEW 2016 or later versions
• Consider local variables, global variables, or any of the other RT inter-process communication techniques if channel wires are not available in your LabVIEW development environment

LabVIEW block diagram elements

Channel wire endpoint writers and readers are subVIs created “on the fly” instead of existing in subpalettes. Simply right-click on an output terminal or wire and select “Create | Channel Writer” (similar to the way you create an indicator); right-click on a channel writer output or channel wire and select “Create | Channel Reader”.

Expect to see a noticeable delay as the subVI is constructed.

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_channel-wire.zip (for use with NI myRIO 1900) or the NIELVISIII-rt_channel-wire.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 the “RT Main” VI:
  • The “fast counter” in Process Loop #1 increments once each loop cycle
  • The Boolean “reset” generated by Process Loop #2 resets the fast counter
  • The “slow counter” in Process Loop #2 increments each time the fast counter reaches 10 counts; this condition also enables the “reset” signal
  • The “slowest counter” in Process Loop #3 increments every two seconds
• Stop the VI by clicking the “Stop” button.

Expected results

Developer walk-through

For more information

1. Stopping Parallel While Loops in LabVIEW with One Stop Button (http://digital.ni.com/public.nsf/allkb/267704CDE91156D186256F6D00711AAE)
   Stop multiple process loops with channel wires, local variables, property nodes, notifiers, and queues.
   
2. Channel Wires (http://www.ni.com/white-paper/53423/en/)
   From the overview: "The channel wire, a new, feature in LabVIEW 2016, simplifies the programming involved in data transfer between parallel loops, such as the common producer/consumer pattern. This white paper provides an introduction to channel wires and highlights specific use cases".
   
3. Real Time Stream (Channel Wires) (https://zone.ni.com/reference/en-XX/help/371361N-01/glang/channel_rt_stream/)
   Describes how to select the "real time stream" channel wire elements.