What is SECS/GEM?

SECS/GEM Communication Standards

SECS/GEM isn’t just one rule. Not even close. It’s more like a family of SEMI standards working together. Each one handles a different piece of the conversation between semiconductor equipment and factory systems.

Inside a semiconductor fab, machines constantly exchange information. Status updates. Process data. Alarms. Commands. It’s a lot happening at once. The SECS/GEM framework keeps that communication structured so equipment and host systems understand each other.

Different standards handle different layers of this communication stack. Transport. Message structure. Equipment behavior. Put them together and suddenly equipment from completely different vendors can still work together. Pretty impressive, actually.

Let’s walk through the core pieces.

SECS-I (SEMI E4)

SECS-I is one of the original communication methods used in semiconductor manufacturing. Old school, you could say. It defines how equipment and host systems communicate over serial connections, usually RS-232.

Back in the early days of semiconductor fabs, Ethernet networks were not common. Equipment connected directly to host computers using serial cables. SECS-I standardized how data should move across those connections.

The protocol defines several important things:

• Message block structure
• Transmission timing rules
• Handshake procedures
• Error detection mechanisms

Messages are transmitted in blocks, and each block includes header information, the actual data payload, and error checking bits. The host system acknowledges receipt before the next block is sent. Slow, but reliable.

Bandwidth is limited though. Serial communication isn’t exactly fast. But reliability mattered more than speed when the standard was first introduced.

Interestingly, some semiconductor fabs still operate legacy equipment that relies on SECS-I communication. Those tools might be decades old. Still producing wafers though. Still talking through serial ports.

In modern fabs, these older tools are often connected through SECS-I to HSMS gateways, allowing them to communicate with newer factory automation systems. Not elegant maybe, but it works.

HSMS (SEMI E37)

Then came HSMS. Things started to move faster.

HSMS stands for High-Speed SECS Message Services, defined in SEMI E37. Instead of serial connections, HSMS uses TCP/IP Ethernet networks for communication between equipment and host systems.

This was a big shift. Ethernet networks provided higher bandwidth, lower latency, and much better scalability.

HSMS was designed to overcome several limitations of SECS-I:

• Low communication speed
• Limited scalability
• Hardware connection constraints
• Lack of modern networking support

With HSMS, equipment can exchange messages with host systems over standard factory networks. Much faster. Much more flexible.

HSMS communication introduces the concept of session management. Equipment and host systems establish a connection session before exchanging messages.

The protocol defines connection states such as:

• NOT CONNECTED
• CONNECTED
• SELECTED

Once the connection reaches the SELECTED state, normal SECS message exchange begins.

HSMS also supports two communication roles.

• Active mode – equipment initiates the connection
• Passive mode – host initiates the connection

This flexibility allows factories to design different network architectures depending on their automation infrastructure.

Today, most modern semiconductor manufacturing equipment uses HSMS communication instead of SECS-I. Ethernet is simply easier to manage at scale. And faster too.

GEM (SEMI E30)

Transport protocols move messages around. But something still needs to define how equipment behaves during communication. That’s where GEM comes in.

GEM, or Generic Equipment Model, is defined in SEMI E30. It provides a standard equipment communication model that factory systems can interact with.

Think of GEM as the rules that define how equipment should behave once the connection exists.

One important part of GEM is the equipment state model. Equipment can operate in different communication states such as:

• OFFLINE
• ONLINE LOCAL
• ONLINE REMOTE

When equipment is ONLINE REMOTE, the factory host system can send commands and control operations. When it’s in ONLINE LOCAL, operators maintain direct control at the equipment interface.

GEM also defines event reporting. Equipment generates events whenever specific conditions occur, including:

• Process start
• Process completion
• Material load events
• Alarm conditions

Factory systems subscribe to these events and receive notifications as they happen.

Then there’s data collection. Equipment variables can be reported to host systems for monitoring and analysis.

GEM also standardizes alarm management and remote command execution. Host systems can send commands such as:

• Start process
• Stop process
• Select recipe
• Reset alarms

This enables centralized factory automation where systems coordinate equipment operations automatically.

SEMI E5 (SECS-II Message Structure)

Messages still need structure. Otherwise systems wouldn’t know how to interpret incoming data.

That’s the role of SEMI E5, which defines the SECS-II message format used for communication between equipment and host systems.

Every SECS message follows a structured format built around Stream and Function identifiers. Messages are labeled as SxFy.

• Stream (S) represents the message category
• Function (F) represents the specific operation

Examples include:

• S1F1 — Equipment status request
• S1F2 — Equipment status response
• S5F1 — Alarm notification
• S6F11 — Event report

Each message also contains structured data elements such as integers, ASCII text, floating-point values, binary data, or nested lists.

These elements are organized in hierarchical structures so complex information can be transmitted clearly.

Because the message structure is standardized, equipment and host systems from different vendors can interpret messages consistently.

How These Standards Work Together

Now step back for a moment.

Each of these standards handles a different layer of the communication process.

• Transport: SECS-I or HSMS
• Message Structure: SECS-II (E5)
• Equipment Behavior: GEM (E30)

Together, they form the complete SECS/GEM communication architecture used in semiconductor manufacturing.

Once implemented, semiconductor equipment can exchange information with factory automation systems in a standardized way. Process data. Equipment status. Alarm events. Control commands.

Factories gain real-time visibility into production. Automation systems coordinate equipment operations. Engineers monitor performance across manufacturing lines.

Hundreds of machines across a fab stay connected. Talking. Reporting. Responding.

Not glamorous, maybe. But essential for modern semiconductor manufacturing.