GEM SEMI standard (Generic Model for Communications and Control of Manufacturing Equipment) describes how a semi-conductor equipment shall communicate with a MES (Manufacturing Execution System), usually called ‘Host’ in GEM standard.
Equipment signals to host state changes such as cassette placement on a load port or process start. Those events are called Collection Event. They are usually associated with equipment internal variables: Status Variable or Data Variable. Status Variable are always valid whereas Data Variable validity is ensured only with associated Collection Events.
Events can be synchronous or asynchronous. Synchronous events are associated with state machines evolution representing equipment state from the Host point of view (Process State Machine as an exemple). Asynchronous events are not linked to state machine transitions.
Equipment shall also signal to the Host alarms that occur.
Online/Local mode is based on those two features (event sending and alarms). In this mode equipment is under operator control using Equipment Controller GUI. However equipment informs Host about its state changes.
In Online/Remote mode, equipment receives Remote Commands from Host to start a process or a job. In this mode operator interface is disabled. Equipment continues to send to Host its state changes that let Host synchronizes with equipment state to execute a complete automation scenario.
GEM also defined additional services such as Process Program (or Recipe) upload/download or regular sending of process data from equipment to the Host.
SEMI E30 GEM can be acquired on SEMI website.
SEMATECH organization also provide a detailed document on this standard:
Generic Equipment Model (GEM), Specification Manual: The GEM Specification as Viewed from the Host
Agileo Automation SECS/GEM and PV2 library implements SEMI GEM standards: