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"Tag" communication

LabVIEW RT block diagram snippet: Process Loop #1 writes an indicator, Process Loop #2 reads the indicator value with a local variable

Local variable (RT)
RT code

Use a local variable (front-panel indicator) to communicate between two parallel process loops contained within the same VI, and use a local variable to stop parallel loops with one "stop" button.

LabVIEW RT block diagrams: VI #1 writes a global variable, VI #2 reads the same global variable

Global variable (RT)
RT code

Use a global variable to communicate between two parallel process loops contained within different VIs under the same target, and use a global variable to stop parallel loops with one "stop" button.

LabVIEW RT block diagram snippet: Read a single-process shared variable in one process loop that was written by another process loop

Single-process shared variable (SPSV)
RT code

A single-process shared variable (SPSV) behaves like a global variable that links deterministic and non-deterministic process loops, effectively shielding the deterministic loop from elements that contribute jitter.

LabVIEW RT block diagram snippet: Functional global variable subVI with unitialized while-loop shift register and case structure

Functional global variable (FGV)
RT code

Use a functional global variable (FGV) to communicate between two parallel process loops contained within different VIs under the same target, and use a FGV to stop parallel loops with one "stop" button. The "functional" nature of the FGV means that you can create additional functionality beyond that of a basic global variable, e.g., counting and calculations that operate on the stored value.

LabVIEW RT block diagram snippet: Process Loop #1 writes a channel wire, Process Loop #2 reads the channel wire

Channel wire
RT code

Use a channel wire to communicate between two (or more) parallel process loops contained within the same VI, and use a channel wire to stop parallel loops with one "stop" button. Requires LabVIEW 2016 or later version.

LabVIEW FPGA block diagram snippet: Process Loop #1 writes an indicator, Process Loop #2 reads the indicator value with a local variable

Local variable (FPGA)
FPGA code

Use a local variable (front-panel indicator) to communicate between two parallel process loops contained within the same VI.

LabVIEW FPGA block diagrams: VI #1 writes a global variable, VI #2 reads the same global variable

Global variable (FPGA)
FPGA code

Use a global variable to communicate between two parallel process loops contained within different VIs under the same target.

LabVIEW FPGA block diagram snippet: RT VI operates the FPGA front panel

Programmatic front-panel communication with RT
FPGA code RT code

The RT VI operates (writes) the front-panel controls of the FPGA VI and reads its indicators.

Shared variables icons in LabVIEW project

Create a network-published shared variable (NPSV)
RT procedure PC procedure

The network-published shared variable (NPSV) behaves like a global variable to link targets through the network.

Front-panel indicator LED with green bound variable indicator

Bind a PC VI front-panel indicator to a network-published shared variable (NPSV)
RT procedure PC procedure

A quick and easy way for a PC HMI (human-machine interface) VI to observe the state of an RT-hosted network-published shared variable (NPSV) without creating any block diagram code; an alternative to programmatically accessing the NPSV value.

LabVIEW RT block diagram snippet: RT and PC read and write the same network-published shared variables

Network-published shared variable (NPSV)
RT code PC code

A network-published shared variable (NPSV) behaves like a global variable that links process loops residing in two or more network-connected targets.

LabVIEW RT block diagram snippet: RT opens a handle to a network-published shared variable and reads its value

Programmatically access a network-published shared variable (NPSV)
RT code PC code

Use the NPSV programmatic API (application programmer's interface) VIs as an alternative method to a shared variable node.

LabVIEW FPGA block diagram snippet: PC VI operates the FPGA front panel

Programmatic front-panel communication with PC
FPGA code PC code

The PC VI operates (writes) the front-panel controls of the FPGA VI and reads its indicators.

LabVIEW front panels of the "Home Security System" application: RT target with virtual user interface and PC host remote interface

System controller application example: Home Security System
RT code PC code

Example of a complete RT system controller based on the Queued Message Handler (QMH) design pattern with multiple parallel task loops implementing behaviors with queued state machines (QSMs), various inter-process communication techniques (queues and local variables), and inter-target communication techniques (network-published shared variables (NPSVs) and network streams). The PC host human-machine interface (HMI) can remotely connect to the system through the network, monitor the status of the security system, and control it remotely.