Campbell Scientific CR6 CR6 Measurement and Control System - Page 154

Section 7. Installation Pipeline scheduling requires that the program be written such that measurements are executed every scan. Because multiple tasks are taking place at the same time, the sequence in which the instructions are executed may not be in the order in which they appear in the program. Therefore, conditional measurements are not allowed in pipeline mode. Because of the precise execution of measurement instructions, processing in the current scan (including update of public variables and data storage) is delayed until all measurements are complete. Some processing, such as transferring variables to control instructions, like PortSet() and ExciteV(), may not be completed until the next scan. When a condition is true for a task to start, it is put in a queue. Because all tasks are given the same priority, the task is put at the back of the queue. Every 1 ms (or faster if a new task is triggered) the task currently running is paused and put at the back of the queue, and the next task in the queue begins running. In this way, all tasks are given equal processing time by the CR6. All tasks are given the same general priority. However, when a conflict arises between tasks, program execution adheres to the priority schedule in table Pipeline Mode Task Priorities (p. 154). Table 25. Pipeline Mode Task Priorities 1. Measurements in main program 2. Background calibration 3. Measurements in slow sequences 4. Processing tasks 7.8.3.6.2 Sequential Mode Sequential mode executes instructions in the sequence in which they are written in the program. Sequential mode may be slower than pipeline mode since it executes only one line of code at a time. After a measurement is made, the result is converted to a value determined by processing arguments that are included in the measurement command, and then program execution proceeds to the next instruction. This line-by-line execution allows writing conditional measurements into the program. Note The exact time at which measurements are made in sequential mode may vary if other measurements or processing are made conditionally, if there is heavy communication activity, or if other interrupts, such as accessing a Campbell Scientific mass storage device or memory card, occur. When running in sequential mode, the datalogger uses a queuing system for processing tasks similar to the one used in pipeline mode. The main difference when running a program in sequential mode is that there is no pre-scheduling of measurements; instead, all instructions are executed in the programmed order. A priority scheme is used to avoid conflicting use of measurement hardware. The main scan has the highest priority and prevents other sequences from using measurement hardware until the main scan, including processing, is complete. Other tasks, such as processing from other sequences and communications, can occur while the main sequence is running. Once the main scan has finished, other sequences have access to measurement hardware with the order of priority being 154