Semi E4

SEMI E4 defines SECS-I, the original serial point-to-point protocol that carries SECS-II messages between a fab host computer and a piece of equipment over an RS-232 cable, and it is the older of the two transport options a GEM-compliant tool can use — the other being HSMS over Ethernet, defined separately in SEMI E37. SECS-I itself does not define what a message means; that is SECS-II's job (SEMI E5). SECS-I's entire responsibility is narrower and more mechanical: breaking a SECS-II message into fixed-size blocks, wrapping each block with a checksum and block-sequence information, sending it byte-by-byte down a serial line, and handling the low-level handshaking (ENQ/EOT/ACK/NAK) that confirms each block arrived intact before the next one is sent.

SECS-II message assembled
Message content and structure defined by SEMI E5
SECS-I transport (SEMI E4)Serial, point-to-point, block-by-block
Message split into fixed-size blocks
Each block gets a checksum and sequence number
ENQ/EOT/ACK/NAK handshake per block
Confirms each block before sending the next
Complete message reassembled at receiver
Equipment or host now has the full SECS-II message

SECS-I predates Ethernet-based factory networking by design, which is exactly why it is still documented and supported today even though most modern 300 mm fabs run HSMS instead — a serial RS-232 link only supports one equipment-to-host connection per cable, has a modest practical throughput ceiling, and that block/checksum/handshake overhead exists specifically to make a single unreliable serial line behave reliably, none of which is a constraint HSMS over Ethernet inherits. A tool built before Ethernet-based GEM connectivity became standard, or a simple single-purpose piece of equipment that never needed more than one host connection, is the typical place SECS-I still shows up in practice, and because SECS-I and HSMS both carry the identical SECS-II message content underneath, equipment and host software written against the SECS-II message definitions does not need to change at all when the transport underneath is swapped from one to the other.

SECS-I (SEMI E4) — Serial Block Transport same SECS-II message content, carried byte-by-byte over RS-232 Host computer Equipment RS-232 serial cable, point-to-point Block 1 Block 2 Block 3 each block: checksum + sequence number + ENQ/EOT/ACK/NAK handshake SECS-I (E4) serial, single connection older, still supported HSMS (E37) Ethernet, modern default same SECS-II content on top

SECS-I is purely a transport-layer standard — the message content it carries is always defined by SEMI E5 (SECS-II), and the equipment behavior that generates and consumes those messages is defined by SEMI E30 (GEM) regardless of whether SECS-I or its modern Ethernet-based replacement HSMS (SEMI E37) is the cable actually carrying them.

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