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Tell me more: maximum message sizes for various SECS standards

SECS remains the communication workhorse of the semiconductor industry. While more modern standards, such as EDA, exist for Big Data collection, SECS remains a viable solution for collecting data from equipment. How much data can be transferred with each message depends on precisely which standard is being used.

To send data, the SECS channel uses a binary communication protocol between the factory automation (FA) host and equipment for control messages and simple data collection. As equipment processing becomes more precise, as more parameter settings are configured, and as data collection needs grow, the information sent through SECS messages can also grow very large, very quickly. When sending SECS data messages, maximum sizes can apply.

SECS-II Item Size

  • The SEMI E5 (SECS-II) standard states that everything in a SECS-II message is either a list or item data structure. (See SEMI E5 – Section 9 – Data Structures.)
  • An item is an information packet that defines the length and format, along with the data value.
  • The maximum allowable length of a data item is approximately 16MB. (See SEMI E5 – Figure 2 – Item and List Header, three binary length bytes = 2^24 bytes.)

HSMS Message Size

  • The SEMI E37 (HSMS) standard specifies that the maximum message size at the protocol level is implementation-specific.
  • The largest theoretical message size is approximately 4GB. (See SEMI E37 – Section 8.2 – General Message Format: 4 byte message length = 2^32 bytes.)

SECS-I Message Size

  • The SEMI E4 (SECS-I) standard specifies that the maximum message size at the protocol level is approximately 7.9MB. (See SEMI E4 – Section 9.2.1 – Message Length: maximum data length allowed in one message = 244 x 32767 bytes.)
  • This SECS-I definition means that the maximum length of a SECS-II data item will be restricted to under 7.9MB, i.e., a SECS-II message sent with the SECS-I message protocol cannot specify a data item larger than 7.9MB, even though the SECS-II protocol allows for a larger data item size.

Multi-Block Messages

  • SEMI E5 requires that certain messages be sent in a single block or packet by the message transfer protocol. (See SEMI E5 – Section 6.3.2 – Multi-Block Messages.)
  • For compatibility with SEMI E4, the maximum length allowed for a single block SECS-II message is 244 bytes. SECS-II messages that are longer than 244 bytes are referred to as multi-block messages.
  • SEMI E5 specifies Multi-Block Inquire/Grant SECS-II messages that the sender can use to check if the receiving entity gives permission to send a multi-block message. Implementations supporting newer message protocols (such as SEMI E37 (HSMS)) find they do not need this Inquire/Grant handshake since the underlying message protocols have no issues handling multi-block messages.

With the adoption of TCP/IP to connect the FA host to equipment (rather than through serial cables), many previous maximum message sizes have disappeared. However, end user demands have driven implementations against the upper boundaries and it is very easy for an event report message to include so much analysis data that it cannot be represented in a single SECS message.

Several techniques can be used to break the data into different SECS messages (e.g., Stream 13 (Data Set Transfer) or Stream 21 (Item Transfer) messages. Additionally, Equipment Data Acquisition (EDA) supports high-speed data collection over a separate communication channel.

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