Overview: What PROFIBUS DP Is and Why It Exists
PROFIBUS DP (Decentralized Peripherals) is the high-speed variant of the PROFIBUS fieldbus family designed for time-critical communication between automation controllers and distributed I/O. The "DP" suffix is an abbreviation of Decentralized Peripherals in EN 50170 / IEC 61158 terminology (sometimes rendered "Distributed Periphery" in legacy Siemens manuals), and it refers to the operational model in which field I/O is no longer wired back to a central I/O rack in the controller cabinet but is instead distributed throughout the plant and connected via a single serial bus.
Two collaborating bodies govern the technology: PROFIBUS International (PI), the umbrella organization headquartered in Karlsruhe, Germany, owns the trademark and certification program, while IEC maintains the standard under IEC 61158 (Industrial communication networks – Fieldbus specifications) and IEC 61784 (Industrial communication networks – Profiles). The DP protocol itself is defined in IEC 61158 Type 3.
PROFIBUS DP is distinguished from the two other principal PROFIBUS variants:
- DP (Decentralized Peripherals) – discrete I/O, drives, valves, HMIs. RS-485 or fiber. Cycle times down to 1 ms with DP-V2.
- PA (Process Automation) – process instrumentation on the same bus, intrinsically safe, MBP (Manchester Bus Powered) physical layer per IEC 61158-2.
- FMS (Fieldbus Message Specification) – largely deprecated, peer-to-peer data exchange for cell-level communication.
The "DP" Acronym: Decentralized vs Distributed Peripherals
The acronym "DP" has two coexisting expansions depending on the documentation era and translation source:
| Expansion | Origin | Typical Source |
|---|---|---|
| Decentralized Peripherals | English translation of German Dezentrale Peripherie | Current Siemens, PI, IEC documentation |
| Distributed Periphery | Alternative English translation of the same German term | Legacy Siemens SIMATIC manuals (S7-300/400, ET 200) |
| Distributed I/O | Functional descriptor (not literal translation) | Marketing material, English-language training courses |
Regardless of the spelling, the technical meaning is identical: a master/slave fieldbus where the slaves are remote I/O stations (digital/analog inputs, outputs, drives, valves) located away from the controller. The acronym is not a reference to "data processing" or any version number; the protocol versions are designated DP-V0, DP-V1, and DP-V2.
PROFIBUS DP Protocol Architecture
PROFIBUS DP implements layers 1, 2, and 7 of the OSI model. The application layer and lower layers are profiled separately, which is what makes DP interoperable across vendors.
| OSI Layer | Function | PROFIBUS DP Implementation |
|---|---|---|
| 7 – Application | User data, diagnostics, parameterization | DP-V0/V1/V2 services (Read, Write, Data_Exchange, RDWR, MSAC1/MSAC2) |
| 2 – Data Link | Bus access, frame format, error detection | Fieldbus Data Link (FDL) – token passing for masters, polling for slaves |
| 1 – Physical | Electrical/optical transmission | RS-485 (DP), MBP (PA), fiber optics |
The Data Link layer combines two access methods: a token ring between masters (so that multi-master configurations can share the bus deterministically) and a master-slave poll for cyclic data exchange with the slaves. Only the master holding the token may initiate a request; slaves respond only when polled.
Physical Layer (RS-485)
PROFIBUS DP uses a shielded twisted-pair cable (Siemens designation 6XV1 830-0EH10 or successor 6XV1 830-3EH10) with the following characteristics:
| Parameter | Value |
|---|---|
| Cable impedance | 150 Ω ± 15 Ω at 3–20 MHz |
| Capacitance | ≤ 30 pF/m |
| Loop resistance | ≤ 110 Ω/km |
| Core cross-section | 0.34 mm² (22 AWG) |
| Color code | Red (B), Green (A) |
| Max segment length (9.6 kbit/s) | 1 200 m |
| Max segment length (12 Mbit/s) | 100 m |
| Bus terminators | 220 Ω + 390 Ω, both ends of every segment |
| Connector | 9-pin D-sub, IP 20 (or M12 B-coded for IP 65/67) |
Baud rates are selectable from 9.6 kbit/s up to 12 Mbit/s. The maximum number of nodes per segment is 32 (1 master + 31 slaves, or up to 126 stations total across 4 repeaters in series); the maximum address space is 0–127 with the convention that address 126 is reserved for commissioning tools and address 127 is broadcast.
DP Device Classes
Three device classes are defined. The same physical hardware can implement one or more of these roles depending on firmware capability.
DP Master Class 1 (DPM1)
A Class 1 master is the central controller. It runs the deterministic cyclic data exchange with its assigned slaves in the Operate state, exchanging I/O data with each slave in a fixed poll cycle. Examples: Siemens S7-300/400/1500 CPUs with PROFIBUS interface, ET 200S IM 151, Allen-Bradley ControlLogix 1756-DHRIO/RM, Schneider M340 BMX NOE 6110/61101.
DP Master Class 2 (DPM2)
A Class 2 master is an engineering, diagnostic, or HMI tool. It has its own bus access rights (it joins the token ring) but does not drive the cyclic I/O. Typical uses: Siemens PG/PC with COM PROFIBUS / SIMATIC Manager, PACTware, FieldCare, panel devices with commissioning function. The DPM2 reads diagnostic buffers, writes parameters, and uploads GSD revisions.
DP Slave
A slave returns its process data only when polled by the active DPM1. It does not actively arbitrate the bus. Examples: Siemens ET 200S, ET 200M (IM 153), SINAMICS drives, SIMOCODE pro, third-party valve manifolds, weighing terminals, ident systems (RFID), PanelView terminals.
DP Protocol Versions: V0, V1, V2
| Version | Year | Key Features | Typical Use |
|---|---|---|---|
| DP-V0 | 1993 | Cyclic master-slave I/O exchange, diagnostics, parameter assignment | Standard remote I/O |
| DP-V1 | 1997 | Acyclic Read/Write (MSAC1) for parameterization and diagnostics while bus is in Operate; alarms | Drives, intelligent field devices |
| DP-V2 | 2002 | Slave-to-slave broadcast (DXB), isochronous mode (equidistant), time stamping, clock synchronization, upload/download | High-speed motion, synchronized drives |
DP-V2's isochronous mode is the basis for synchronized drive control on PROFIBUS: the master broadcasts a global control (GC) frame at a constant interval, all slaves freeze their inputs and outputs simultaneously, and the jitter between stations is held below 1 µs. Siemens implements this on SIMOTION and SINAMICS S120; the bus cycle is configured with the same time base used by the controller's position loop.
Cyclic I/O Exchange and Bus Cycle
The bus cycle on a single-master system is the sum of all per-slave poll times plus inter-frame gaps. For a 1.5 Mbit/s bus with 16 slaves each exchanging 4 bytes in and 4 bytes out, the typical cycle is 1.5–3 ms. The master's bus cycle timer (TTR) defines when the token is passed; on Siemens controllers this is the TDP parameter in HW Config (TIA Portal: PROFINET/DP interface → constant bus cycle time).
A slave has the following internal states (FDL view of the slave's state machine):
- Power_On – after supply, awaiting parameterization.
- Wait_Prm – parameterization telegram (Set_Prm) received; slave checks configuration.
- Wait_Cfg – configuration telegram (Chk_Cfg) received; slave validates the I/O length and structure.
- Data_Exch – cyclic I/O is active.
- Diag.Flag – diagnostic pending, master must read Diag buffer before re-entering Data_Exch.
GSD Files: The Interoperability Contract
A GSD (General Station Description) file is a machine-readable text file that describes a slave's identity, baud rates, I/O module catalog, diagnostic information, and timing parameters. Every PROFIBUS DP slave shipped for production must have a GSD; without one the master cannot integrate the device.
Two GSD revisions exist:
| Revision | Filename extension | Features |
|---|---|---|
| GSD (original) | *.GSD | Text format, ASCII, up to DP-V0/V1 |
| GSD Revision 5 | *.GSDML-v2.42-* | XML, DP-V2 support, multilingual, multi-port devices |
Example GSD header (Siemens ET 200S IM 151-1 HF, abridged):
; GSD File for Siemens ET 200S
#Profibus_DP
GSD_Revision = 5
Vendor_Name = "Siemens AG"
Model_Name = "ET 200S IM 151-1 HF"
Revision = "V3.0"
Ident_Number = 0x08EC
Protocol_Ident = 0
Station_Type = 0
FMS_supp = 0
Hardware_Release = "HW 4"
Software_Release = "FW V3.0"
; Supported baud rates
9.6_supp = 1
19.2_supp = 1
45.45_supp = 1
93.75_supp = 1
187.5_supp = 1
500_supp = 1
1.5M_supp = 1
3M_supp = 1
6M_supp = 1
12M_supp = 1
; Maximum responder time at 12 Mbit/s
MaxTsdr_12 = 60The Ident_Number is the unique PID that allows the master to verify the connected device matches the configured one. If the user enables "Check Ident Number" in the master configuration (TIA Portal: Slave properties → Identification) and a mismatched device is plugged in, the master refuses to enter cyclic exchange and raises diagnostic interrupt 0x0C "Configuration Fault".
Commissioning a PROFIBUS DP Network: Step-by-Step
Prerequisites
- Master controller with PROFIBUS interface (e.g., Siemens S7-1512C-1 PN with CM 1542-5, or S7-315-2 DP).
- Configuration tool: TIA Portal, SIMATIC Manager (classic), Step 7 V5.5 + SP, COM PROFIBUS, or third-party (e.g., PACTware, Acromag).
- GSD file for every slave, installed into the master configuration tool.
- PROFIBUS cable (6XV1 830), PROFIBUS connectors (6ES7 972-0BA52-0XA0 with PG socket, or 6ES7 972-0BB52-0XA0 without), terminating resistors, addresses pre-assigned.
- PROFIBUS tester or oscilloscope for commissioning (Softing PROFIusb, Indu-Sol PROFINET-INspektor, or HMS Anybus Tester).
Procedure
- Wire the segment. Connect the master port to the first slave, daisy-chain to the next, terminate the last connector (switch ON). Leave the terminator OFF on all intermediate nodes.
- Set slave addresses with the rotary switches on the slave (typical range 1–99, address 0 reserved for master class 1). Each address must be unique.
- Install GSD files. In TIA Portal: Options → Manage General Station Description (GSD) files. Restart of the master is normally not required.
- Add the DP master system. Insert the master CPU, then add a DP master system in the device configuration. Set the baud rate (default 1.5 Mbit/s) and the highest station address (HSA), default 126.
- Insert slaves from the hardware catalog, drop onto the DP master system. Assign each slave its bus address, plug in I/O modules consistent with the physical configuration.
- Configure bus parameters (HW Config → DP master properties → Bus parameters). The tool calculates TTR (target rotation time) and slot time; verify the TTR warning is not raised.
- Compile and download the hardware configuration to the CPU.
- Monitor online: right-click the master system → "Online & Diagnostics". Check that all slaves reach state "OK" and that diagnostic events clear.
Diagnostics and Troubleshooting Matrix
| Symptom | Likely Cause | Diagnostic Path | Remedy |
|---|---|---|---|
| All slaves unreachable (BF = Bus Fault on master) | Open circuit, missing terminator, wrong polarity (A/B swapped), baud rate mismatch | Master diagnostic buffer: "Bus fault, no response from station X" | Recheck wiring A/B, enable terminator at both segment ends, set identical baud rate on all stations |
| Single slave flaps between OK and Station Failure | Terminator in middle of segment, intermittent connector, EMI on cable | Slave diagnostic: "External diagnostic: short circuit" or "Slave not ready" | Move terminator to physical ends, re-seat connector, check shield bonding |
| BF on slave, no parameterization | Wrong slave address, no GSD, slave's highest supported baud rate lower than master's | Master diagnostic: "Parameterization fault, station X" | Verify rotary address, install correct GSD, lower master baud rate or upgrade slave firmware |
| Configuration fault (diag 0x0C) | I/O module plugged in DP slave does not match configured slot | Master diagnostic: "Configuration fault" | Match physical module to configuration; or disable "Check module ID" |
| Slow bus, slaves respond with retries | Bus cycle too tight, TTR warning, excessive retries | HW Config bus statistics, repeater diagnostic counters | Increase TTR, lower baud rate, add repeater (Siemens 6ES7 972-0AA01-0XA0) to split segment |
| CRC errors rise over time | Poor cable quality, missing shield bond, ground loop | Repeater LEDs (Siemens: red LED per port indicates per-port errors) | Replace cable run, ensure 360° shield bond at cabinet entry, verify equipotential bonding < 1 Ω |
| Ident Number fault (diag 0x0F) | Connected device differs from configured one | Master diagnostic: "Ident number mismatch" | Disable "Check Ident Number" or replace with correct module |
Field-Proven Commissioning Checklist
- Measure shield-to-ground potential: must be < 1 V AC and < 5 V DC at the cabinet entry.
- Verify both terminators are present, ON, and the bus is continuous (no dangling stubs beyond 1 m at > 1.5 Mbit/s).
- Confirm every station has a unique address in 1–125.
- Read the diagnostic buffer of the master after the first warm-up cycle. Clear it and run for 1 hour; the buffer should be empty.
- Capture a scope trace at 1.5 Mbit/s: idle line is high (~5 V between A and B), data toggles symmetrically around 0 V. Asymmetry indicates a ground reference issue.
- Log per-port error counters from any repeaters or diagnostic stations (e.g., Indu-Sol PROFINET-INspektor in PROFIBUS mode, Softing PROFIbus Tester 5).
PROFIBUS DP vs PROFINET: When to Choose
| Criterion | PROFIBUS DP | PROFINET |
|---|---|---|
| Physical layer | RS-485 or fiber, dedicated cable | Ethernet (100 Mbit/s, 1 Gbit/s, copper or fiber) |
| Topology | Line (daisy chain) | Line, star, tree, ring |
| Max devices | 126 per network | Practically unlimited with managed switches |
| Cycle time | ≥ 1 ms (DP-V2 isochronous) | ≥ 31.25 µs (IRT) or 250 µs (RT) |
| Existing install base | Massive (millions of nodes) | Growing, dominant in new builds |
| Tooling | Step 7 V5.5, TIA Portal, COM PROFIBUS | TIA Portal, PRONETA, multi-vendor |
Choose PROFIBUS DP when extending an existing DP plant, integrating legacy drives that have only DP ports, or operating in environments where the network must be deterministic on a single linear bus without managed switches. Choose PROFINET for new greenfield plants requiring high-speed motion, distributed I/O over standard Ethernet infrastructure, or integration with IT systems.
Standards and Further Reading (Official Sources)
The following official documentation expands on the technical material above. Consult them directly for normative detail and certification requirements.
- PROFIBUS & PROFINET International (PI) – the governing body, technology and downloads portal.
- IEC 61158 – Industrial communication networks – Fieldbus specifications.
- IEC 61784 – Industrial communication networks – Profiles.
- Siemens PROFIBUS product portal – SIMATIC ET 200, CM 1542-5, SINAMICS PROFIBUS option boards.
- Rockwell Automation 2711-UM005 – PROFIBUS DP Communications for PanelView Terminals.
- Schneider Electric DOCA0256 – TeSys Tera PROFIBUS DP network characteristics.
- Acromag Introduction to PROFIBUS DP – technology primer with master/slave protocol walkthrough.
FAQ
What does the "DP" in PROFIBUS DP stand for?
DP stands for Decentralized Peripherals (German: Dezentrale Peripherie). Older Siemens manuals use the alternative English translation Distributed Periphery or Distributed I/O. All three refer to the same master/slave protocol for connecting remote I/O stations over RS-485.
What is the difference between PROFIBUS DP and PROFIBUS PA?
DP uses RS-485 at up to 12 Mbit/s on shielded twisted pair, optimized for discrete I/O and drives with cycle times down to 1 ms. PA uses MBP (Manchester Bus Powered) at 31.25 kbit/s per IEC 61158-2, supports intrinsic safety for hazardous areas, and is used for process instrumentation (pressure, flow, temperature). A segment coupler or link (e.g., Siemens DP/PA coupler 6ES7 157-0AC00-0XA0) bridges the two physical layers.
What is a GSD file and where do I get one?
A GSD (General Station Description) is a text or XML file that describes a PROFIBUS DP slave's identity, supported baud rates, I/O module catalog, and diagnostic structure. The manufacturer supplies the GSD on its website or on the device CD; install it into the master configuration tool (TIA Portal: Options → Manage General Station Description Files) before adding the slave to the project.
What is the maximum number of PROFIBUS DP slaves and the maximum cable length?
Up to 126 stations (master + slaves) are addressable, with 32 stations per segment and up to 4 repeaters in series. Maximum segment length depends on baud rate: 1 200 m at 9.6 kbit/s down to 100 m at 12 Mbit/s. Use PROFIBUS cable type A (150 Ω, 0.34 mm²) and enable terminating resistors only at the two physical ends of each segment.
Is PROFIBUS DP still recommended for new installations?
For new greenfield plants, PROFINET (the Ethernet-based successor) is generally preferred. PROFIBUS DP remains an excellent choice when extending an existing DP installation, integrating legacy drives with only DP ports, or operating in environments where a single deterministic linear bus is preferred over managed Ethernet switches. Many vendors continue to ship DP masters and slaves with full support, and PROFIsafe on PROFIBUS is still used in safety applications.