Detailed Description

SERCOS III motion-bus telegram + IDN codec (DETWS_ENABLE_SERCOS).

SERCOS III is the real-time drive/motion bus over Ethernet (raw L2, ethertype 0x88CD, on the shipped services/rawl2). The master cyclically sends MDT (Master Data Telegrams) carrying setpoints to the drives, and the drives answer with AT (Acknowledge / drive Telegrams) carrying actual values. Both carry a short SERCOS header then the cyclic device data; a separate service channel transfers parameters addressed by an IDN (IDentification Number):

Telegram header: [type MDT/AT : 1][phase/counter : 1][cycle count : 2] IDN (16 bit): S/P bit(1) | parameter-set(3) | data-block(12) -> "S-0-0100" style addressing

This provides the MDT/AT telegram framing + the IDN encode/decode (the addressing every drive parameter uses). The isochronous timing + the ring/line topology are the hardware-gated part. Pure, zero heap, no stdlib, host-testable.

Definition in file sercos.h.