SECS/GEM background

SECS/GEM is the family of SEMI standards that defines how factory equipment talks to a host computer. This section covers the concepts you need before you use the PDComm SDK. Each page tells you which standard and topic a concept comes from, so you can look up the details in the standard.

Where SECS/GEM is used

A semiconductor fab runs hundreds of process tools: lithography scanners, etchers, deposition chambers, metrology tools. The fab’s manufacturing execution system (MES) or a cell controller has to read each tool’s status, download process programs, and receive events and alarms. SECS/GEM is the common language for that conversation. When a tool vendor ships a SECS/GEM interface, the fab connects the tool the same way it connects every other tool instead of learning a vendor protocol.

+---------------------------+      +---------------------------+
|  MES / fab host           |      |  Cell controller          |
+-------------+-------------+      +-------------+-------------+
              | HSMS (TCP/IP)                    | HSMS or SECS-I
      +-------+--------+                 +-------+--------+
      |  Equipment A   |                 |  Equipment B   |
      |  (GEM)         |                 |  (GEM)         |
      +----------------+                 +----------------+

The same standards also run in factories that resemble semiconductor fabs, such as flat-panel display, LED, solar cell and electronics assembly lines.

The standards

“SECS/GEM” is shorthand for several SEMI standards:

Standard	Name	What it covers	Read more
SEMI E5	SECS-II	Message and data item structure; the meaning of streams and functions	SECS-II messages
SEMI E37, E37.1	HSMS, HSMS-SS	Carrying SECS-II messages over TCP/IP and managing the connection	HSMS
SEMI E4	SECS-I	Sending messages as blocks over an RS-232 serial line	SECS-I
SEMI E30	GEM	Standard equipment behavior: communication and control state models, data collection, alarms, remote commands	GEM state models, GEM capabilities
SEMI E39, E40, E87, E90, E94 and others	GEM300	Additional standards for 300 mm fabs: carriers, process jobs, substrate tracking	GEM300

You can buy the standards from SEMI. These pages cite standard names and topics; they don’t reproduce standard text.

How the layers fit together

SECS/GEM is easiest to understand as a stack. The lower layers move bytes; the upper layers give those bytes meaning.

+--------------------------------------------------+
|  Your application (host or equipment)            |
+--------------------------------------------------+
|  GEM (SEMI E30)                                  |
|  state models, data collection, alarms,          |
|  remote commands, process programs, spooling     |
+--------------------------------------------------+
|  SECS-II (SEMI E5)                               |
|  stream/function messages and data items         |
+------------------------+-------------------------+
|  HSMS (SEMI E37)       |  SECS-I (SEMI E4)       |
|  TCP/IP                |  RS-232 serial line     |
+------------------------+-------------------------+

Transport: HSMS or SECS-I delivers each message to the peer. Almost all new equipment uses HSMS.
Messages: SECS-II defines message names such as S1F3 and data formats such as lists, integers and strings.
Behavior: GEM defines which messages to send when, and how the receiver must react.

Host and equipment, active and passive

Every connection has two independent settings:

Role: the host manages equipment; the equipment is managed. GEM behavior depends on the role. For example, the equipment sends event reports and the host receives them.
Connection mode: the active side opens the TCP connection; the passive side listens on a port and waits.

Role and connection mode don’t depend on each other. A host is usually active and the equipment passive, but the reverse is just as valid.

Program A	Program B	Result
Host, active	Equipment, passive	The most common setup.
Host, passive	Equipment, active	The equipment opens the connection. Also valid.
Host, active	Equipment, active	Nobody listens, so no connection forms.
Host, passive	Equipment, passive	Nobody connects, so no connection forms.

What PDComm provides

The PDComm SECS/GEM SDK implements HSMS, SECS-I, SECS-II and GEM for both the host and the equipment side.

Component	What it is
C ABI	`pdcomm.h` (SECS-II items), `pdcomm_tcp.h` (HSMS), `pdcomm_secs1.h` (SECS-I), `pdcomm_gem*.h` (GEM) and `pdcomm_gem300.h` (GEM300 capability catalog).
C++ wrappers	The matching `.hpp` headers. They need C++14 or later and manage handle lifetimes with RAII.
.NET binding	`PDComm.dll` for .NET 10. The `PDComm` namespace contains `GemHost`, `GemEquipment` and asynchronous worker classes.
GUI simulator	`pdcomm-simulator`, a Qt program that plays the equipment or the host so you can test without real equipment.
Headless simulator	`pdcomm-simulator-headless` runs predefined scenarios without a display and prints JSON results.
MCP server	`pdcomm-simulator-mcp` exposes the simulator as Model Context Protocol (MCP) tools, so AI tools and automation scripts can drive it.

The C ABI never creates threads or callbacks of its own. Your application calls poll to make progress and takes results from an event queue. The .NET worker classes run that loop for you on a dedicated thread. For every function and type, see the API reference.