Data link / software link

ISA 5.1-2009/2022 (instrument line symbols)

Drawn as

A thin line with small open circles on it at intervals.

Meaning

Internal system or communication link: software connections inside the DCS (e.g. a cascade setpoint) or a bus between systems (DCS to PLC, Modbus, fieldbus). Not modelled.

Variants

ISA 5.1-2009 separates shared-system links from fieldbus links with different marks; many legends use filled dots or diamonds for fieldbus
Very old drawings: o-o-o meant "software or data link" only

Not physical hardware. Read it, then skip it in the 3D model.