PROFIBUS DP is a serial RS-485 fieldbus: one line, up to 12 Mbit/s, where the master polls each slave in turn and every station has a numeric address. PROFINET IO is Industrial Ethernet: 100 Mbit/s full duplex through switches, where every device sends its data each cycle and is identified by a device name and IP address. PROFINET carries more data, gives better diagnostics and shares the cable with IT traffic; PROFIBUS remains common in existing plants and in process instrumentation (PA). Both come from PI and use the same PROFIdrive and PROFIsafe profiles.
- PROFIBUS polls slaves one after another; PROFINET devices all send in parallel every cycle.
- PROFIBUS needs terminators at both line ends; PROFINET needs a correct device name on every device.
- Drive and safety profiles are the same, so STW1/ZSW1 logic moves from PROFIBUS to PROFINET unchanged.
- The two coexist: an IE/PB Link lets a PROFINET controller keep existing PROFIBUS devices.
Topology: Line Bus vs Switched Ethernet
The most visible difference is the shape of the network. PROFIBUS is a single shared line that every station taps into, terminated at both ends. PROFINET is built from point-to-point Ethernet links meeting at switches — either separate switches or the two-port switch built into most PROFINET devices.
This shape explains many practical differences. On PROFIBUS a broken cable in the middle of the line takes down every station beyond it; on PROFINET a broken link affects only that branch, and a ring with media redundancy (MRP) survives a single break.
How Data Moves: Polling vs Provider/Consumer
PROFIBUS DP is master/slave: the master sends a request to slave 3, waits for its response, then moves to slave 4, and so on. The bus cycle is the sum of all these exchanges, so it grows with each slave and each byte. PROFINET uses provider/consumer: the controller and every device send their cyclic data on their own every send clock, and full-duplex links let both directions travel at the same time.
Protocol Stack Side by Side
PROFIBUS is a compact fieldbus stack: the DP application, the FDL data-link layer and the RS-485 or MBP physical layer. PROFINET sits on standard Ethernet but gives cyclic I/O its own real-time channel that bypasses TCP/IP, plus an isochronous channel for motion — while normal IT traffic uses TCP/IP on the same cable.
Addressing: Station Address vs Device Name
A PROFIBUS station is identified by a number from 0 to 125, set on the device with switches or a parameter and repeated in the PLC configuration. A PROFINET device is identified by its device name; the controller finds the device by that name at start-up and gives it its IP address.
For drives, the addressing changes but the data does not. A SINAMICS G120 uses the same telegrams and the same STW1/ZSW1 bits on both networks — see SINAMICS G120 STW1 and ZSW1 and G120 Telegram Selection in TIA Portal.
Full Comparison Table
| Feature | PROFIBUS DP | PROFINET IO |
|---|---|---|
| Physical layer | RS-485 twisted pair (purple), fiber; PA uses MBP | Industrial Ethernet 100BASE-TX (green), fiber, wireless |
| Speed | 9.6 kbit/s – 12 Mbit/s (PA: 31.25 kbit/s) | 100 Mbit/s full duplex |
| Access method | Master/slave polling, token between masters | Provider/consumer, every device sends cyclically |
| Topology | Line bus; tree via repeaters | Star, line, tree, ring |
| Stations | Addresses 0–125; 32 per segment without repeater | No fixed address limit — set by the controller |
| Addressing | Station address on the device | Device name + IP address (DCP) |
| Typical cycle | A few ms, grows with slaves and lower baud rate | RT ≈ 1 ms (down to 250 µs); IRT down to 31.25 µs |
| Cyclic data per device | Up to 244 bytes in and 244 out | Up to 1440 bytes per direction |
| Distance | 100 m at 12 Mbit/s up to 1200 m at ≤ 93.75 kbit/s per segment | 100 m per copper link; kilometres with fiber |
| Termination | Required at both ends of each segment | Not needed |
| Device description | GSD file (text) | GSDML file (XML) |
| IT on the same cable | No | Yes — TCP/IP, web server, OPC UA, SNMP |
| Diagnostics | DP-V1 channel diagnostics | Channel + network diagnostics, topology, neighbour detection |
| Redundancy | Redundant bus with special hardware | Media redundancy (MRP ring), system redundancy |
| Safety / drives / motion | PROFIsafe · PROFIdrive · DP-V2 isochronous | PROFIsafe · PROFIdrive · IRT isochronous |
| Organisation | PROFIBUS & PROFINET International (PI) | PROFIBUS & PROFINET International (PI) |
Work on both networks with real hardware
Configure PROFIBUS DP and PROFINET IO side by side in TIA Portal and diagnose faults on each. Pune classroom or live online.
Cables, Connectors and Distances
On PROFIBUS the maximum segment length depends on the baud rate — the faster the bus, the shorter the cable. Repeaters start a new segment when you need more length or more than 32 stations.
| Baud rate | Max segment length (cable type A) | Typical use |
|---|---|---|
| 9.6 – 93.75 kbit/s | 1200 m | Long runs, slow data |
| 187.5 kbit/s | 1000 m | Long runs |
| 500 kbit/s | 400 m | Medium plants |
| 1.5 Mbit/s | 200 m | Most common default |
| 3 – 12 Mbit/s | 100 m | Short, fast machine buses |
On PROFINET each copper link may be up to 100 m, regardless of speed, because every link runs from one port to the next. Fiber links cover longer distances.
| PROFIBUS DP | PROFINET | |
|---|---|---|
| Cable | 2-core shielded, purple, lines A and B | 4-core (2 pair) Cat 5e shielded, green |
| Connectors | 9-pin D-sub with switchable terminator; M12 B-coded | Industrial RJ45 (FastConnect); M12 D-coded; SC-RJ fiber |
| Line end | Terminator ON at first and last station only | Nothing — each link is point-to-point |
| Stub lines | Avoid at 1.5 Mbit/s and above | Not applicable |
| Tools | Bus tester, oscilloscope for signal quality | Wireshark, TIA topology view, switch diagnostics |
PROFINET RT, IRT and Conformance Classes
PROFINET offers three communication channels on the same cable. Most PLC-to-drive and remote I/O traffic uses RT; IRT is reserved for synchronised motion.
| Channel | Used for | Typical update | Needs |
|---|---|---|---|
| NRT (TCP/IP) | Parameters, diagnostics, web server, OPC UA | ≈ 100 ms | Standard Ethernet |
| RT (real time) | Cyclic I/O — drives, remote I/O, valves | 1 – 10 ms, down to 250 µs | PROFINET devices; managed switches recommended |
| IRT (isochronous) | Synchronised motion, high-speed axes | ≤ 1 ms, down to 31.25 µs | IRT-capable devices and switches, topology configured |
Devices are also certified to a conformance class, which tells you what network features they support.
| Conformance class | Adds | Typical devices |
|---|---|---|
| CC-A | RT communication, alarms, basic diagnostics; standard switches and wireless allowed | Simple I/O, building automation |
| CC-B | Network diagnostics via SNMP, topology and neighbour detection | Factory and process automation — most PLCs, drives, I/O |
| CC-C | IRT with bandwidth reservation and isochronous operation | Motion control, synchronised axes |
Mixed Plants and Migration
Few plants switch overnight. The usual approach is a PROFINET backbone that reaches existing PROFIBUS DP segments through an IE/PB Link, and PROFIBUS PA instruments through a DP/PA coupler.
| Option | How it works | When to use |
|---|---|---|
| Keep PROFIBUS | No change; maintain spares and cabling | Stable plant, no new functions needed |
| IE/PB Link (PN/PB proxy) | PROFINET controller sees PROFIBUS slaves through a gateway | New PLC or new PN devices, existing DP slaves stay |
| CPU with PN and DP ports | One controller runs both networks side by side | Retrofit where the old DP line must stay |
| Replace segment by segment | Swap interface modules (e.g. ET 200 DP → PN), keep I/O | Planned shutdowns, gradual upgrade |
| Full PROFINET | New devices and cabling throughout | Greenfield or major rebuild |
Which One Should You Choose?
Work down the questions. The first "yes" gives you the answer.
Common Faults on Each Network
The typical faults differ because the physical layers differ. PROFIBUS problems are usually electrical — termination, stubs, grounding. PROFINET problems are usually configuration — names, IP addresses and device descriptions. Capturing PROFINET frames with Wireshark is a quick way to see whether a device is sending at all.
| Symptom | Network | Likely cause | Fix |
|---|---|---|---|
| BF LED on several slaves | PROFIBUS | Terminator missing, or ON in the middle of the line | Terminators ON only at the two ends |
| Slaves drop out at higher baud rate | PROFIBUS | Stub lines, long segment, poor shield grounding | Remove stubs, check length table, ground shields |
| Two stations never come up | PROFIBUS | Duplicate station address | Give every station a unique address |
| Device error "not reachable" | PROFINET | Device name not assigned or misspelled | Online → Assign name, match the configured name |
| Device comes up then trips | PROFINET | IP address conflict or GSDML version mismatch | Check IP range, update to the matching GSDML |
| Random RT communication errors | PROFINET | Unmanaged office switch, cable over 100 m, damaged RJ45 | Industrial switch, shorter run, re-terminate connector |
Step-by-Step Lab: Compare Both in TIA Portal
Hands-on- A CPU with both a PROFIBUS DP and a PROFINET interface (or a PN CPU plus a DP master module), one DP slave and one PROFINET device.
- A PC with TIA Portal and Wireshark on the PROFINET network.
- Estimated time: 60 minutes.
Configure the PROFIBUS slave
Add the DP slave, set its address in TIA Portal to match the address set on the device, and select 1.5 Mbit/s.
Configure the PROFINET device
Add the PN device, give it a device name, then use Online → Assign name to write that name to the real device.
Compare update times
Open the properties of each network and note the DP bus cycle time and the PN update time.
Break the cable
Disconnect the DP cable between the master and the first slave, then disconnect one PN device cable at the switch.
Capture PROFINET traffic
Start Wireshark on a switch port with port mirroring, filter pn_rt.
You understand the difference if you can explain why the DP line failed beyond the break while PROFINET lost only one device, why PROFIBUS needs terminators, and why the PROFINET device needed a name but no manually entered IP.
Both networks carry drives the same way — see Siemens PLC and VFD networking for how a PLC reaches a VFD on either one.
Frequently asked questions
Is PROFINET faster than PROFIBUS?
Yes in almost every practical sense. PROFINET runs at 100 Mbit/s full duplex and every device sends in parallel, while PROFIBUS DP runs at up to 12 Mbit/s and polls one slave at a time. PROFINET RT typically updates in about 1 ms and IRT goes down to 31.25 µs.
Can PROFINET and PROFIBUS work in the same plant?
Yes. An IE/PB Link or another PN/PB proxy lets a PROFINET controller address PROFIBUS DP slaves, and a DP/PA coupler reaches PROFIBUS PA instruments. Some CPUs also have both a PROFINET and a PROFIBUS port.
Do drive control words change between PROFIBUS and PROFINET?
No. Both networks use the same PROFIdrive profile, so STW1, ZSW1 and the telegram layouts are identical. A drive program written for PROFIBUS keeps its bit logic on PROFINET; only the addressing changes.
Is PROFIBUS obsolete?
No. It is still widely installed and supported, especially in process plants with PROFIBUS PA. New machines and plants, however, are mostly built on PROFINET because it carries more data, diagnostics and IT traffic on one cable.
Why does PROFIBUS need terminating resistors but PROFINET does not?
PROFIBUS is a shared RS-485 line, and the signal reflects at open ends unless both ends are terminated. PROFINET uses point-to-point Ethernet links between device and switch port, so each link is electrically complete on its own.
Can I use normal office Ethernet switches for PROFINET?
Basic RT communication can pass through standard switches in conformance class A, but industrial managed switches are strongly recommended for reliability, diagnostics and topology. IRT needs IRT-capable switches.
What replaces PROFIBUS PA in hazardous areas?
PROFIBUS PA is still common. The newer option is PROFINET over Ethernet-APL, a two-wire Ethernet physical layer designed for process instruments in hazardous areas.