The scan rate of control modules affects CPU loading. A newly created module has a default scan rate of 1 second. The library's module templates have default scan rates between 500 milliseconds and 5 seconds. Before increasing the scan rate of a module, consider process dynamics, the response times of the transmitters, and final control elements associated with the module. Faster module scan rates can consume CPU capacity without improving control performance.
Setting a reasonable scan rate requires an understanding of the process dynamics for analog control modules. When the controlled variable reacts slowly to changes in the manipulated variable, the module scan rate can be set slower than when the controlled variable reacts quickly. This means that flow and liquid pressure loops must have a faster scan rate (500 milliseconds or 1 second) than a temperature loop scan rate (5 or 10 seconds). The scan rate of a level loop depends on the throughput (versus the size) of the vessel but can often be 2 or 5 seconds. Although it is customary to overscan analog control modules slightly, running all loops faster than necessary quickly consumes the CPU.
Monitoring modules, which gather data to display in the operator environment, should have a maximum scan rate of 1 second, which is the fastest display update rate. These modules can often be executed at a 2 or 5 second rate, based on the speed of the transmitter to detect process parameters.
Modules that control discrete devices (for example, motors and valves) can usually be scanned at 1 second. At this rate, outputs are driven soon after the setpoint is changed. When process interlocks are involved, it is sometimes necessary to speed up the scan rate to 500 milliseconds. If the module needs to sense momentary push buttons in the field, set the scan rate to 500 milliseconds.