SECS/GEM Host and GEM Equipment Communication
In a factory SECS GEM implementation there are two parties, the host and equipment. The equipment runs GEM interface software on one of its computers that must implement and comply with the SEMI standards. The manufacturer (factory) runs GEM host software that establishes communication with the equipment's GEM interface. A host is also called a station controller or line manager. Often the host software is part of the factory's Manufacturing Execution System (MES). A host system can communicate with one or multiple equipment GEM interfaces at the same time.
The host communicates directly with each equipment using either the SEMI E4 SECS-I standard (RS-232 based serial communication) or SEMI E37.1 HSMS-SS standard (TCP/IP based network communication). Certainly the HSMS-SS standard is more appropriate and convenient for today's modern factories and is therefore used almost exclusively in modern factories.
A host does not have to comply with the SECS/GEM standard since the standards only set equipment expectations; however in order to make use of the GEM interface a host must implement the host-side of the communication. The SECS/GEM standards set clear equipment behavior expectations for each possible host message.
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SECS/GEM Feature Summary
The SECS/GEM standard's key features are described in the following paragraphs. Minimal GEM compliance requires only a small set of these features to be supported since many of the features described are optional, additional capabilities. Many of the features have state models to clearly define states, sub-states and transitions between states. The state models make GEM interface implementations consistent and predictable.
The SECS GEM standard defines a number of SECS-II messages scenarios; an ordered sequence of SECS-II message transactions. The SECS-II message scenarios establish an implementation guideline so that the equipment manufacturer can anticipate how the host might use the SECS/GEM standard.
The GEM SECS standard defines how an equipment and host initially establish communication. It also defines how communication is re-established when communication is broken. An on-line identification method verifies the equipment's hardware and software identity. Terminal service features allow the host operator and equipment operator to exchange text manually typed at a console.
The GEM/SECS standard outlines a control state model to define the level of cooperation between the host and equipment operator. GEM Equipment provide three basic levels of host control which determine the host's ability to control and monitor the equipment. The equipment operator sets the level of host control.
Remote control capabilities permit the host to send GEM-defined commands like "START", "STOP", "PAUSE", "RESUME", and "ABORT" to control the equipment's processing. The equipment can define additional custom commands. Each command can have one or more arguments with data to clarify the command.
Equipment constant features allow the host to set and retrieve equipment constant values which govern the equipment's behavior. GEM requires a small set of equipment constants to configure the GEM state machines. An equipment can define additional equipment constants to allow the host to configure any aspects of the equipment behavior.
Collection events and alarms allow the host to monitor the interesting equipment operation. Equipment collection events notify the host of significant normal and abnormal activity. Equipment alarms notify the host when potentially dangerous activity is detected and cleared. The host determines which collection events and alarms are setup for notification. The equipment sends SECS-II messages to the host only for the events and alarms that are enabled for notification. This minimizes communication traffic on the network. Certain events are required by the SECS GEM standard, but the equipment is expected to define additional events to allow the host to monitor the equipment-specific activities.
SECS/GEM defines six methods of gathering data. The host can gather data from the equipment, but an equipment cannot gather data from the host.
- A set of status variable values can be requested at any time.
- A set of equipment constant values can be requested at any time.
- A report containing status variable, data variable, and equipment constant values can be requested at any time.
- A host can define reports and attach them to collection events so that the report data is transmitted along with the collection event in the same SECS-II message. This feature enables data to be sent to the host as the values become available thereby reducing the host's obligation to poll information. This event report data collection also enables the host to gather the data related to each event.
- The host can define traces so that the equipment periodically transmits the specified status variable values at a set interval. This feature enables the host to poll the equipment status without having to ask the data at each interval.
- The host can configure limits monitoring so that the equipment notifies the host whenever a specified variable value transitions across a host-defined limit boundary. This feature eliminates the need for the host to poll critical values is situations where the host is only concerned when the value becomes too high or low. Multiple limit boundaries can be defined.
Process Program (Recipe) Management
A process program "is the set of instructions, settings, and parameters under control of the equipment that determine the processing environment seen by the manufactured object" (SEMI E30, 188.8.131.52). Process program management features include the following:
- Host can download a process program to the equipment for storage on the equipment.
- Host can query a process program from the equipment for storage on the host.
- Host can delete a process program on the equipment.
- Host can request a list of available process programs.
- Equipment operator can send a process program to the host.
- Equipment operator can request a process program from the host.
- Host can select a process program for execution using a PP-SELECT remote command
- Equipment will notify the host when a process program is created, edited, deleted, or selected by the equipment operator.
Spooling capabilities provide the means for the equipment to queue information intended for the host during communication failure. When communication is restored, the host can purge or request the queued data. The host can configure which information is queued, how a full queue is handled, the queue size, and how queued information is recovered. The host can also switch spooling features on or off.
The SECS/GEM standard requires that each equipment provide a GEM interface manual. It must include a GEM compliance statement, complete SECS-II message documentation, complete GEM state model documentation, and a description of all equipment variables, alarms, collection events, equipment constants, and remote commands.
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