Campbell Scientific CR1000KD CR1000 Measurement and Control System - Page 135

Section 7. Installation Note Measurement tasks have priority over other tasks such as processing and communication to allow accurate timing needed within most measurement instructions. Care must be taken when initializing variables when multiple sequences are used in a program. If any sequence relies on something (variable, port, etc.) that is initialized in another sequence, there must be a handshaking scheme placed in the CRBasic program to make sure that the initializing sequence has completed before the dependent task can proceed. This can be done with a simple variable or even a delay, but understand that the CR1000 operating system will not do this handshaking between independent tasks. A similar concern is the reuse of the same variable in multiple tasks. Without some sort of messaging between the two tasks placed into the CRBasic program, unpredictable results are likely to occur. The SemaphoreGet() and SemaphoreRelease() instruction pair provide a tool to prevent unwanted access of an object (variable, COM port, etc.) by another task while the object is in use. Consult CRBasic Editor Help for information on using SemaphoreGet() and SemaphoreRelease(). 7.7.3.7 Execution Timing Timing of program execution is regulated by timing instructions listed in the following table. Table 17. Program Timing Instructions Instructions Scan() / NextScan SlowSequence / EndSequence General Guidelines Use in most programs. Begins / ends the main scan. Use when measurements or processing must run at slower frequencies than that of the main program. Syntax Form BeginProg Scan() '. '. '. NextScan EndProg BeginProg Scan() '. '. '. NextScan SlowSequence Scan() '. '. '. NextScan EndSequence EndProg 135