- Data Variable (DV)
- Status Variable (SV)
- Equipment Constant (EC)
Data Variable (DV) – data items that can be gathered when an equipment event occurs. This data is only guaranteed to be valid in the context of the event. For example, the GEM interface may provide an event called PPChanged (triggered when a recipe changes). The interface may also provide a data variable called changed recipe. The value of this DV is only valid in the context of the PPChanged event. Polling the value at a different time may have invalid or unexpected data.
Status Variable (SV) – data items that contain information about the equipment. This data is guaranteed to be valid at any time. For example, the equipment may have a temperature sensor in a process module. The GEM interface may provide a ModuleTemperature status variable. The host can request the value of this SV at any time and expect the value to be accurate.
Equipment Constant (EC) – data items that contain equipment settings. Equipment Constants determine how equipment will behave. For example, a GEM interface may have an equipment constant called MaxSimultaneousTraces which specifies the maximum number of traces that can be requested simultaneously from the host. The value of equipment constants is always guaranteed to be valid and up to date.