PROFIBUS is the serial fieldbus of PROFIBUS & PROFINET International, specified in IEC 61158 and IEC 61784. Its DP variant runs over shielded twisted-pair RS-485 at 9.6 kbit/s to 12 Mbit/s, with one master cyclically polling the slaves on each segment and addresses set on the devices themselves.
Where PROFIBUS shows up on the plant floor
PROFIBUS is one of the legacy installations most often met on the plant floor: many lines commissioned between the mid-1990s and the mid-2000s carry a DP segment. You recognise it by the purple cable, the nine-pin D-sub connectors with bus-in and bus-out in the same housing, and the termination switch at each end of the segment. In continuous process plants (chemicals, paper, cement, water treatment) PROFIBUS PA is common too: the variant that runs at 31.25 kbit/s over a pair that also powers the instrument and is suitable for hazardous areas, joined to the DP segment through a coupler or link. It is a supported, living bus rather than a leftover, and the reason to digitalise it is almost never to replace it, but to read it.
PROFIBUS versus PROFINET
The confusion comes from the name. PROFINET is not PROFIBUS carried over Ethernet; it is a different mechanism. PROFIBUS is a shared serial bus where a master grants turns and bandwidth is split among all nodes on the segment, up to 32 stations per segment and addresses 0 to 125 across the network. PROFINET gives every device its own switched link, is configured by station name and IP address, and shares infrastructure with IT. What did carry over is the engineering model: PROFIBUS GSD files became GSDML, and the slot-and-module structure stayed, which is why migration feels closer to a change of medium than to a functional redesign.
Getting data out of a PROFIBUS segment
PROFIBUS has no IP addressing and nothing resembling a network connection: it is an RS-485 differential line where only the node holding the token may talk. That shapes every extraction option.
The least invasive route is to leave the bus alone and read from the master, that is, from the PLC, which already holds the full process image and normally has its own Ethernet channel. It reaches only what the program maps, but it changes neither the bus cycle nor its timing.
When you do need to go down to the bus, the standard itself provides a mechanism: a class 2 master joins the token rotation to perform acyclic DP-V1 reads and diagnostics without interfering with the class 1 master’s cyclic exchange. The trade-off is that an extra master lengthens token rotation time and therefore the bus cycle, which depending on baud rate, node count and bytes exchanged ranges from a few milliseconds to a few tens. On an installation already running at its timing limit that headroom may not exist, so it is sized before connecting.
The purely passive alternative is a bus monitor that listens without transmitting. It does not change timing, but the delicate part is physical: the D-sub connectors are daisy-chained, so unplugging one breaks continuity for every downstream node and can stop the line. It is connected on a running bus only if a spare connector exists at a segment end; otherwise it waits for a planned stop. The segment must also keep its powered termination at both ends, and a long or badly made spur degrades signal quality. As for interpretation, what you capture is raw bytes: you need each slave’s GSD file and the master configuration to know which offset belongs to which module at which address.
Related terms
PROFIBUS preceded PROFINET and shares a generation with Modbus RTU, which also runs on RS-485 but without device profiles. In process instrumentation it coexists with HART, and its path to a data layer normally goes through an industrial gateway or the PLC itself. At Captia Connect it belongs to heterogeneous systems interoperability: reading equipment of different brands and generations without replacing it.