Queue-based state machines excel at implementing system control, data measurement and processing, and other tasks to respond to inputs from the surrounding physical system and user interface. Learn about three popular design patterns: queued state machine (QSM), queued message handler (QMH), and event-driven producer-consumer loops.
State machines perform system control, data processing, and any task that involves executing a sequence of activities in response to inputs from the surrounding physical system, the user interface, and other processes within the system. Popular state machine design patterns use queues to maintain the system state and to communicate with other state machines operating in parallel within the system.
Queued State Machine (QSM)
The Queued State Machine (QSM) is a popular design pattern that is flexible and versatile, easy to maintain, and computationally efficient. The QSM contains three primary elements contained within a while-loop structure:
“Guard Clause” VI – wait for a state (a string) to become available in the state queue and then dequeue it (remove it from the queue); when the input error cluster status is true, insert the inbound error cluster value into the error
handler queue and return “Error” as the state
“Kernel” VI – implements the actions to be performed for each state such as calculations, counting, decoding, peripheral I/O, data measurements, and network transactions; this VI can also insert states into its state queue
Feedback node – stores values that must persist from one state to another
Queued Message Handler (QMH)
The Queued Message Handler (QMH) implements a complete system by partitioning the system into well-defined tasks, with each task implemented by a QSM operating independently and in parallel with all of the other tasks. QSMs communicate with each other by sending messages directly to their queues. The QMH contains application-specific tasks as well as these standard tasks:
System Manager – Maintains the high-level operating mode of the overall system and issues commands to the task loops (QSMs); the task loops issue alert messages to the system manager and can also directly message other task loops to reduce the complexity of the system manager task
Command Parser – Accepts network connection requests from the human-machine interface (HMI) running on the PC host, parses the received commands, and inserts messages into the System Manager queue
Error Handler – All loops place error messages into the error handler queue and continue to run; when an error message is received the error handler issues a “Shutdown” message to all task loops and displays the message; other behavior can be designed, such as logging the error message to a file and continue running, if possible (normally the RT application should continue running while gracefully handling error conditions)
Event-driven producer-consumer state machine
The event-driven producer-consumer state machine creates a responsive user interface for the human-machine interface (HMI) running on the desktop PC. Based on two loops operating in parallel, the “producer” loop event structure responds immediately to user interactions such as button clicks and mouse movements to send commands through a queue to the “consumer” loop which performs the required tasks. Separating the state machine into two loops allows the user interface to remain responsive should a consumer task require an unusual amount of time or must wait for a shared resource to become available.
Queue mechanism to send messages or data between two or more parallel process loops