Control modules and equipment modules can use the command-driven algorithm. A command-driven algorithm uses SFC-based composite blocks (COMMAND blocks). A command-driven algorithm is an appropriate control strategy when there are several pieces of equipment that need to be coordinated (sequencing and timing relationships must be managed) and there are pre-defined operating conditions (COMMANDS) that must be supported.
To create a command-driven module, the user creates a new module, specifies that it uses a command-driven algorithm, and specifies a named set (the named set states are the command names for the module). The system creates SFC-based composite blocks for each of the command names. Named sets used for command-driven modules can contain as many as 120 named states.
The following table shows an example of how the command names correlate to the composite blocks for the application example in Equipment module example.
Command name |
State value |
COMMAND block |
|---|---|---|
totalize_accurate |
0 |
COMMAND_00000 |
totalize_fast |
1 |
COMMAND_00001 |
reset |
2 |
COMMAND_00002 |
undefined |
255 |
none |
The user is required to write SFC logic for each of the COMMAND composite blocks in order to define the behavior of the module. The named set used for the module must include a state with a value of 255. This value indicates an undefined state. When you create a command-driven module, the system also creates a composite block called MONITOR that is a function block algorithm.. The user can configure logic inside the MONITOR composite block defining error conditions and appropriate actions. The contents of the MONITOR composite block execute continuously
The command names are the valid values for the module's A_COMMAND parameter. For example, if the value of A_COMMAND is totalize_accurate, the module executes the SFC logic in COMMAND_00000.