RT/host inter-target communication

Summary

• Inter-target communication refers to data exchanges between a target (FPGA or RT) and a host computer (RT or desktop PC):
  • FPGA/RT communication: RT serves as host to the FPGA target
  • RT/PC communication: PC serves as host to the RT target
• Four methods are available for the RT target:
  • Front panel communication
  • Network-published shared variables ( tagTransfer only the current (most recent) data value between two process loops or between two targets )
  • Network streams ( streamingTransfer every data value through a first-in first-out (FIFO) queue , messageTransfer every value of intermittent data with low latency. , buffered)
  • Standard protocols ( TCPTransmission Control Protocol, a connection-based means to send and receive data packets with guaranteed reliability , UDPUser Datagram Protocol, a connectionless protocol based on sending data packets known as datagrams, a word play on “data” and “telegrams” , HTTPHyperText Transport Protocol, the basis of the “world wide web” , and WebDAVWeb Distributed Authoring and Versioning, an Internet standard mechanism for editing and managing files between remote systems. )

Front panel communication

• Use this method when developing and debugging a VI on the RT target
• Host computer must have LabVIEW installed
• The PC host and the RT target execute the same VI on two systems: PC runs the front panel, and RT runs the block diagram
• Data flows between the PC host and RT target through the Academic RIO Device network connection (either USBLANUSB-based local area network (LAN) established between the Academic RIO and the PC host computer. or wireless) in a way that is completely transparent to the developer

Network-published shared variables

• Use a network-published shared variable (NPSV) just as you would a global variable; the only difference is that a NPSV has network scope, i.e., is accessible by multiple network-connected devices
• Especially useful when you need multiple host PCs to monitor a single RT target
• Because this is tag-based (latest value) communication, rapid changes of a variable in one process can be missed in the other process
• Ensure that control signals remain active long enough for the other process to detect them, or devise a hand-shaking scheme
• Suitable for small, frequent data transfers where relatively low throughput rate is acceptable; for example, the host could:
  • adjust RT configuration parameters
  • read RT status or a single data measurement
  • initiate an RT process
• Relatively low setup overhead per call, but relatively high CPU usage per call

Code examples

• Network-published shared variable (NPSV)
• Programmatically access a network-published shared variable (NPSV)

Procedures

• Create a network-published shared variable (NPSV)
• Bind a PC VI front-panel indicator to an NPSV

Network streams

• Use a network stream when every data point must be transferred
• Use cases include:
  • Streaming (message) data communication with host
  • Transferring data to the host for logging to a file
  • Sending data to host computer for memory-intensive processing and analysis
• Consists of a writer and reader endpoint
• Lossless, unidirectional, one-to-one communication channel
• Supports any LabVIEW data type, but most efficient with numeric scalars, Booleans, and 1-D arrays of same

Code examples

• Send messages through a ‘network stream’ channel with low latency (minimal delay)
• Stream high-speed data through a ‘network stream’ channel for efficient block transfers

Standard Internet protocols

• Use standard protocols for between LabVIEW applications connected by a network
• Especially useful to communicate with third-party IoTInternet of Things, i.e., networking in the context of machine-to-machine interactions; ubiquitous communication ability of embedded systems. appliances

Code examples

• Send messages as UDP datagrams with a client-server model
• Send messages through a TCP/IP connection with a client-server model
• Host an HTTP-based Web service on the Academic RIO Device and call that Web service from any network-capable device

Procedures

• Mount the Academic RIO Device file system as a drive on the PC host with the WebDAVWeb Distributed Authoring and Versioning, an Internet standard mechanism for editing and managing files between remote systems. protocol and then interact with the Academic RIO Device application by reading and writing files, for example, to control and monitor a datalogger

For more information

1. Using the LabVIEW Shared Variable (http://www.ni.com/white-paper/4679/en)
   Introduces the network-published shared variables and discusses features and performance.
   
2. Understanding Shared Variable Technology (http://zone.ni.com/reference/en-XX/help/371361M-01/lvconcepts/ni_psp)
   Describes the Shared Variable Engine (SVE) and NI Publish-Subscribe Protocol (NI-PSP) as the foundation of network-published shared variables.
   
3. 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.
   
4. 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.