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Chapter 11
Function Sequence Tables
11.1 About this chapter
This chapter discusses the following topics:
• FST overview
• FSTs and Model 3711 memory
• FST Editor
• Developing an FST
• Monitoring and tracing an FST
• The FST command library
• FST examples
11.2 FST overview
A Function Sequence Table (FST) is a user-defined program that resides in the Model 3711. The
Model 3711 supports up to two FSTs.
An FST contains a set of functions. Each function defines an action to be taken when a specified
condition exists. FSTs can be written specifically for applications that require special control features,
such as logic sequencing capability. For example, you can use an FST for emergency shutdown
control when a parameter exceeds a low or high limit.
FSTs are programmed and configured using the FST Editor and the FST command library. The FST
command library provides mathematical and logical operations, database access operations, historical
commands, testing, and branching operations, and control-related operations. Each FST may consist
of as many functions as can fit into the memory reserved for the FSTs. Reserved memory is
pre-determined:
• Maximum size of a single FST: 3000 bytes
• Maximum number of lines per FST: 300
Note: To determine the size of an existing FST, view the Code Size parameter in the Advanced panel
of the FST Registers window (see Step 3, Figure 11-4).
Functions are normally executed in the order in which they are defined in the FST; however, the
sequence may be altered by specific functions within the FST itself. By default, the FST executes 20
functions per execution period (one second). This setting can be changed in the Device Information
dialog box (see Section 3.4). For example, an FST with 30 functions will execute 20 in the first
execution period and 10 in the next. Once started, an FST runs continuously; as soon as it reaches the
end of its instruction set, it starts again at the first function when the next execution cycle begins.
As the FST executes, two memory locations store intermediate results:
• The Results Register (RR) stores a floating-point value called the Signal Value Analog (SVA).
• The Compare Flag (CF) stores a discrete value called the Signal Value Discrete (SVD).