Software Solutions for Factory Connectivity and Advanced Motion Control

An Introduction to the GEM Standard

The Generic Model For Communications And Control Of Manufacturing Equipment (GEM) standard is maintained and published by the non-profit organization Semiconductor Equipment and Materials International (SEMI). It is designated as SEMI standard E30. GEM intends "to produce economic benefits for both device manufacturers and equipment suppliers..." by defining "... a common set of equipment behavior and communications capabilities that provide the functionality and flexibility to support the manufacturing automation programs of semiconductor device manufacturers" [SEMI E30, 1.3]. GEM is a standard implementation of the SECS-II standard, SEMI E5. The GEM standard is applicable to any manufacturing equipment. Many equipment in semiconductory, surface mount technology, and electronics assembly industries provide a GEM interface so that the factory host may interface to the machine for monitoring and/or controlling purposes. The standard is general enough to apply to any manufacturing equipment.

All GEM compliant manufacturing equipment share a consistent interface and certain consistent behavior. All may communicate with a GEM capable host using either TCP/IP (using the HSMS standard, SEMI E37) or RS-232 based protocol (using the SECS-I standard, SEMI E4). Often both protocols are supported. Each piece of equipment may be monitored and controlled using a common set of line management tools defined by GEM. When an equipment has a GEM interface, it takes just minutes (or even seconds) for factory GEM host software to establish communication and begin monitoring the machine's activity. This means that equipment manufacturers may spend more time and money improving the machine's quality by providing a common interface to all factories. It means that factories may spend more time and money improving production and processes, rather than setting up communication to the machines.

GEM Architecture

Typical GEM implementations incorporate an architecture where a single host computer acts a line management system for one or multiple manufacturing equipment. The host communicates directly with each equipment using either the SECS-I standard (RS-232 based serial communication) or HSMS-SS standard (TCP/IP based network communication). Certainly the HSMS-SS standard is more appropriate and convenient for today's modern factories.

Once low-level communication is established, then the host and equipment may exchange SECS-II messages. A SECS-II message is identified by a stream number (0-255) and a function number (0-255). An odd-numbered function is a primary SECS-II message. A consecutive, even-numbered function is a secondary message--the reply to the corresponding primary message. Unless the reply bit is clear, a primary message should always be responded to with a secondary message. For most SECS-II messages, a seconday reply message is required. For example, if the host sends an S1,F1 (stream 1, function 1) message to request 'Are you there?', then equipment will send a reply S2,F2 message to indicate 'I am here'. The SECS-II standard defines a large set of standard SECS-II messages, primary messages and the corresponding secondary messages. Some SECS-II messages may be sent by only the host. Some may be sent only by an equipment. Other messages may be sent by either the host or equipment.

SECS-II messages are sent as structured binary data. It is a very effective means to package information across a network without wasting bandwidth. When using the SECS-I standard, RS-232 serial communication, the message size is limited to about 8 MB. When using the HSMS standard, TCP/IP network communication, the message size is limited to about 16 MB.

The GEM standard defines a number of SECS-II messages scenarios; an ordered sequence of SECS-II messages. The SECS-II message scenarios establish an implementation guideline so that the equipment manufacturer can anticipate how the host might use the GEM standard.

Features

The GEM standard's key features are described in the following paragraphs. Minimal GEM compliance only requires a small set of these features to be supported. Many of the described features are optional.

Communication

The GEM 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.

Control

The GEM standard outlines a control state model to define the level of cooperation between the host and equipment. GEM Equipment provide three basic levels of host control which determine the host's ability to control and monitor the equipment.

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 may support custom commands. A command may have arguments with data.

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. A machine may define additional equipment constants to provide refined control of the equipment's behavior.

Operation Notification

Collection events and alarms allow the host to keep track of an equipment's operations. 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 to minimize and control communication traffic.

Data Gathering

GEM defines six methods of gathering data. The host may gather data from the equipment, but not vice-versa.