PROFIBUS uses two physically distinct cable systems depending on which protocol variant is deployed:
- PROFIBUS DP (CP 3/1 in IEC 61784-1) — RS-485 twisted-pair copper, violet jacket, 150 Ω characteristic impedance
- PROFIBUS PA (CP 3/2 in IEC 61784-1) — MBP (Manchester-coded, Bus-Powered) twisted-pair copper, blue/green jacket, 100 Ω characteristic impedance
Both carry digital signals over shielded two-core cable. The color coding is the fastest way to tell them apart in the field. Everything else — conductor size, impedance, maximum segment length, connector type — differs between the two. This article covers both in full.
Table of Contents
PROFIBUS DP Cable Types
PROFIBUS International (PI) defines four standardized cable types for RS-485 use, labeled Type A through Type D. Type A is the reference standard. Types B, C, and D are legacy variants that predate the standard — still found in older installations but no longer recommended for new projects.
Type A — The Correct Choice for New Installations
Type A is the cable specified by IEC 61158-2 Clause 22 for PROFIBUS DP. All new installations must use Type A.
| Parameter | Value |
|---|---|
| Characteristic impedance | 135–165 Ω (nominally 150 Ω) |
| Loop resistance | ≤ 110 Ω/km |
| Capacitance per unit length | ≤ 30 pF/m |
| Wire cross-section | 0.34 mm² (22 AWG) |
| Conductor | Solid copper, 0.64 mm diameter |
| Insulation | Foamed polyethylene (PE) with hard skin |
| Screen | Aluminium foil + tinned copper wire braid (≥ 60% optical coverage) |
| Jacket color | Violet |
| Jacket material | PVC (standard), PUR, LSZH, or outdoor-rated options |
The dual screening — aluminium foil bonded to the conductor pair plus an overlapping tinned copper braid — is what separates industrial PROFIBUS cable from generic RS-485 cable. The foil provides 100% coverage against high-frequency interference; the copper braid provides mechanical protection and low-resistance contact for the shield termination.
Do not substitute standard RS-485 cable. Generic RS-485 cable may have the right impedance but lacks the screening and capacitance specification needed for reliable PROFIBUS DP operation, especially at high baud rates.
Type B, C, D — Legacy Only
These types existed before the current IEC specification was finalized.
| Type | Impedance | Notes |
|---|---|---|
| B | 150 Ω | Older standard, single shield, less defined capacitance |
| C | No specification | Multi-core cables used in early installations |
| D | No specification | Multi-core with varying constructions |
Types C and D may work at low baud rates but are not guaranteed to perform at 1.5 Mbit/s or above. If you encounter them in an existing installation running intermittent errors, cable type mismatch is worth investigating.
PROFIBUS DP Cable: Baud Rate vs. Maximum Segment Length
This is the most critical parameter for network design. PROFIBUS DP cable length is not a fixed number — it shrinks as the baud rate increases, because higher frequencies attenuate more rapidly over the same cable.
The entire network must run at a single baud rate. Every device must support it. Choose your baud rate based on the device with the lowest capability, then verify the cable lengths are achievable.
| Baud Rate | Max. Segment Length (Type A Cable) |
|---|---|
| 9.6 kbit/s | 1,200 m |
| 19.2 kbit/s | 1,200 m |
| 93.75 kbit/s | 1,200 m |
| 187.5 kbit/s | 1,000 m |
| 500 kbit/s | 400 m |
| 1.5 Mbit/s | 200 m |
| 3 Mbit/s | 100 m |
| 6 Mbit/s | 100 m |
| 12 Mbit/s | 100 m |
Segment length = trunk cable length + all spur/drop lengths combined. It is not just the trunk.
With repeaters, multiple segments can be chained. A maximum of 9 repeaters in series are allowed, potentially extending total network reach to 10,800 m at low baud rates. Each repeater counts as one device on each segment it bridges.
Stub Lines: The High-Speed Killer
One mistake that catches engineers off guard: stub lines (short spur connections off the trunk) are not permitted at high baud rates.
At 12 Mbit/s, signal wavelength is short enough that even a 20–30 cm stub causes a significant impedance discontinuity and reflection on the bus. This degrades signal quality and causes communication errors — especially for devices farther along the trunk from the stub.
| Baud Rate | Maximum Stub Length |
|---|---|
| ≤ 1.5 Mbit/s | 6.6 m |
| 3 Mbit/s | 0 m (no stubs) |
| 6 Mbit/s | 0 m (no stubs) |
| 12 Mbit/s | 0 m (no stubs) |
At 3 Mbit/s and above, every device must be connected directly to the bus trunk via its built-in connector — no spur lines. PROFIBUS connectors with integrated T-junction are designed exactly for this purpose.
Jacket Material Options
Standard Type A cable comes in PVC, but several alternative jacket materials are available for specific environments:
PVC — standard for control cabinet and general factory floor use. Good flexibility, adequate chemical resistance. Not suitable for outdoor use or applications with oil/solvent exposure.
PUR (Polyurethane) — significantly better resistance to mechanical abrasion, oils, and solvents. The correct choice for cables subject to movement (drag chains), outdoor routing in conduit, or oily machine environments.
LSZH / FRNC (Low Smoke Zero Halogen / Flame Retardant Non-Corrosive) — mandatory in many enclosed installations (tunnels, rail vehicles, buildings) where toxic fumes from burning PVC are unacceptable. Produces very low smoke and no hydrogen chloride gas when exposed to fire. Often labeled “PROFIBUS FC FRNC” cable.
Armoured (SWA — Steel Wire Armoured) — for direct burial or areas with high risk of mechanical damage. The steel wire armour provides crush and rodent resistance. Heavier and less flexible; typically used for inter-building runs or outdoor trenching.
Torsion-rated — for applications with continuous cable movement and rotation (rotary tables, cable festoons). Uses finely stranded conductors and a specially designed sheath to withstand millions of flex cycles without fatigue failure.
PROFIBUS PA Cable Specification
PROFIBUS PA uses a different physical layer entirely — MBP (Manchester-coded, Bus-Powered) as specified in IEC 61158-2. The cable carries both communication signals and device power on the same two wires.
| Parameter | Value |
|---|---|
| Characteristic impedance | ~100 Ω |
| Wire cross-section | 0.8 mm² (recommended) |
| Conductor | Stranded copper |
| Screen | Aluminium foil + copper braid |
| Jacket color | Blue (international) or green (some regions) |
| Max trunk length | 1,900 m (total including spurs) |
| Max spur length (standard) | 60 m |
| Max spur length (IS / FISCO) | 30 m |
| Transmission rate | Fixed 31.25 kbit/s |
PA cable must support both signal transmission (31.25 kbit/s Manchester-encoded signal) and DC power distribution to field instruments. The 0.8 mm² conductor cross-section is typically used to limit voltage drop along the trunk — especially important in long segments where devices at the far end must still receive adequate supply voltage.
PROFIBUS PA cable must not be substituted with PROFIBUS DP cable. The impedance (100 Ω vs 150 Ω), conductor size, and power-carrying requirement are all different. Using DP cable on a PA segment will cause impedance mismatches that degrade signal quality and may fail to deliver adequate power to distant devices.
Termination: Why It Matters and How to Do It Correctly
PROFIBUS is a transmission line. Without proper termination at both ends of the bus, signal reflections travel back down the cable and overlap with valid data, corrupting communication. This is not a theoretical concern — improper termination is one of the most common causes of intermittent PROFIBUS errors in the field.
PROFIBUS DP Termination
Both ends of the RS-485 trunk must be terminated. The standard PROFIBUS DP terminator circuit consists of:
- A pull-up resistor of 390 Ω connected from the B-line (positive data) to +5V
- A pull-down resistor of 390 Ω connected from the A-line (negative data) to GND
- A terminating resistor of 220 Ω connected between the A and B lines
This RC network sets the bus to a defined idle state (logical “1”) and absorbs signal reflections. The equivalent impedance of the terminator (~110 Ω differential) matches the 150 Ω cable characteristic impedance closely enough to prevent reflections.
Most PROFIBUS connectors include a built-in terminator with a slide switch. The switch must be set to ON only at the first and last physical devices on the bus trunk. All other devices must have termination switched OFF.
Common mistakes:
- Terminating at both ends AND at a middle device (three terminations causes signal loading)
- Forgetting to enable termination at one end entirely
- Enabling termination at a device that is not at the physical end of the trunk
- Removing a device from the middle of the bus without moving the termination to the new end device
A bus with both terminations powered and active is stable. If power is lost to one end device (which takes its terminator with it), the bus loses one termination — and communication errors follow. Consider using external standalone terminators at both ends in installations where end-device power cannot be guaranteed.
PROFIBUS PA Termination
PA uses a different terminator: a series RC circuit with C = 1 µF and R = 100 Ω. This matches the 100 Ω PA cable impedance. The same rule applies: one terminator at each physical end of the trunk. PA segment couplers often include built-in terminators that can be activated by a switch or jumper.
DB9 Connector: Pin Assignment
The 9-pin D-Sub (DB9) is the most common connector for PROFIBUS DP on PLCs, drives, and controllers in cabinet environments. It is defined in Annex I.2 of IEC 61158-2 (Type 3, asynchronous transmission connector specification for RS-485).
DB9 Male Connector Pin Assignment (Standard PROFIBUS DP)
| Pin | Signal | Description |
|---|---|---|
| 1 | Shield / FE | Functional Earth (cable shield) |
| 2 | — | Not used (some vendors use for 24V supply) |
| 3 | B / RxD+TxD+ | Positive data line (non-inverting) |
| 4 | CNTR-P | RTS (Request to Send) — used by some transceivers |
| 5 | DGND | Data ground (signal reference) |
| 6 | VP | Positive supply voltage (+5V for terminator) |
| 7 | — | Not used (some vendors use for 24V supply return) |
| 8 | A / RxD+TxD- | Negative data line (inverting) |
| 9 | CNTR-N | RTS complement — used by some transceivers |
The two data signals you always connect are Pin 3 (B, positive) and Pin 8 (A, negative). The shield goes to Pin 1. Pin 5 (DGND) and Pin 6 (VP) are needed only when using a connector with integrated termination.
Wire color convention for Type A cable:
- Green wire → Pin 3 (B, positive, non-inverting)
- Red wire → Pin 8 (A, negative, inverting)
This color convention is universally followed by all PROFIBUS Type A cable manufacturers. Getting it reversed (A-line to Pin 3, B-line to Pin 8) is the single most common wiring mistake on PROFIBUS. The network will not communicate at all with reversed polarity.
M12 Connector: The Field-Proven Alternative
The M12 circular connector has largely replaced DB9 for field-mounted devices — drives, remote I/O modules, sensors, and any equipment exposed to the industrial environment. The M12 offers IP65/IP67 sealing, vibration resistance, and a positive lock that DB9 cannot match.
M12 B-Coded Connector
PROFIBUS DP uses the B-coded M12 variant. The B-coding (a keyway position on the connector face) physically prevents accidental mating with the more common A-coded M12 connectors used for general I/O — eliminating a major source of installation errors.
M12 B-Code 5-Pin Assignment (PROFIBUS DP)
| Pin | Signal | Description |
|---|---|---|
| 1 | VP | +5V (terminator supply) |
| 2 | A / RxD+TxD- | Negative data line (inverting) |
| 3 | DGND | Data ground |
| 4 | B / RxD+TxD+ | Positive data line (non-inverting) |
| 5 | Shield / FE | Functional Earth (cable shield) |
Note the pin numbering is different from DB9, but the signal assignment logic is the same — one positive line, one negative line, ground, shield, and VP for powered termination.
M12 Connector Variants
| Variant | Application |
|---|---|
| M12 male (plug) | Typically used on cable side |
| M12 female (socket) | Typically used on device side |
| Straight body | General use, lower profile |
| Right-angle body | When cable exits parallel to device face |
| M12 to DB9 adapter | Connecting field cable to cabinet equipment |
When ordering M12 PROFIBUS cables, always specify B-coded explicitly. A-coded M12 cables will not physically mate with PROFIBUS device ports.
PROFIBUS PA Connector
PROFIBUS PA field devices most commonly use either:
- 7/8-inch circular connector (older European process instruments, 5-pin)
- M12 A-coded circular connector (newer installations, 4- or 5-pin)
- Screw terminals (direct field wiring without a pre-assembled connector)
The 7/8-inch connector is the traditional PA connection, similar in size and philosophy to the M12 but larger — suited for robust process instrument connections. The two data/power conductors connect to specific pins; exact assignment depends on whether the device is bus-powered or separately powered.
For PA, the shield connection and polarity (+/−) are the critical wiring parameters. Reversing polarity on a PA device typically causes it to fail to communicate and draw no current, which is how the problem shows up diagnostically.
Shielding and Grounding: The Rules That Prevent EMC Problems
Improper shield grounding is responsible for more PROFIBUS communication problems than almost any other single cause. The rules are straightforward but frequently misunderstood.
The Goal
The cable shield must be connected to protective earth (PE) at a low-impedance point in order to drain high-frequency noise currents induced by the surrounding environment (motor drives, switching power supplies, welding equipment). A shield that is connected at only one point or not at all provides minimal EMC protection.
IEC 61784-1 Recommendation
The standard states: “It is recommended to ground the shield of the fieldbus cable as often as possible. This is usually the best practice to improve EMC.”
This means: ground the shield at every cabinet entry, at every device that provides a shield terminal, and at both ends of every segment. Multiple grounding points are better, not worse — as long as all ground points are at the same potential (bonded together).
Practical Rules
Ground the shield at every panel entry. As the cable enters a control cabinet, connect the shield to the cabinet’s PE bar immediately at the entry point. Use a cable gland with an integrated 360° shield clamp — not a pigtail lead to a screw terminal. A pigtail creates inductance that reduces HF shield effectiveness.
Connect the shield at every device. Most DB9 PROFIBUS connectors connect Pin 1 (shield) directly to the device housing, which should itself be bonded to PE. M12 connectors do the same through the metal shell. Do not float the shield.
Both ends of every segment must be terminated AND grounded. Termination handles signal reflection. Grounding handles EMC. They are separate functions and both are required.
Avoid ground loops only at power frequency (50/60 Hz). The concern about “ground loops” is real but applies to low-frequency currents — not to HF shield grounding. A shield grounded at multiple points forms a low-impedance path for HF noise (which is what you want) but can carry 50 Hz ground current if there is a potential difference between grounding points (which causes problems). The solution is not to reduce HF grounding points, but to ensure all grounding points are at the same potential through proper equipotential bonding of the facility ground system.
For IS (intrinsically safe) applications — shield grounding rules are more restrictive. Follow the IS system documentation and barrier/isolator manufacturer requirements, as grounding arrangements affect IS certification.
Installing PROFIBUS Cable: Key Rules
Separate from power cables. Run PROFIBUS cable in separate cable trays from 230V/400V power cables, motor supply cables, and especially variable frequency drive (VFD) output cables. VFD cables carry high-frequency switching noise that couples directly into nearby signal cables. Minimum separation: 20 cm in parallel runs; cross at 90° when paths must intersect.
Minimum bending radius. Exceeding the bending radius cracks the foil shield or permanently deforms the conductor, changing its impedance. Typical Type A cable: minimum bending radius = 5× outer diameter for static installation, 10× for dynamic (moving) applications.
Maximum pulling tension. Pulling too hard during installation stretches the conductor or inner structure. Typical Type A cable: maximum pulling tension ≈ 100–150 N (varies by manufacturer — check the cable datasheet).
No parallel runs with high-current switching loads. Even with correct cable separation, VFD output cables running in parallel with PROFIBUS for tens of meters can cause communication problems. Where unavoidable, use armoured PROFIBUS cable or fiber optic extension.
Label every connector. Mark the device address and network segment at every connection point. On a PROFIBUS network with 32 devices and no labelling, tracing a faulty connection takes hours. With labelling, it takes minutes.
Document shield grounding points. Record exactly where the shield is grounded and how. When problems arise later, this documentation is invaluable for systematic fault finding.
Troubleshooting PROFIBUS Cable Problems
Most PROFIBUS cable faults produce one of three symptoms: complete communication loss on a segment, intermittent communication errors at high baud rates, or errors that appear only when nearby equipment operates.
Complete Communication Loss on a Segment
Check in this order:
- Termination. Is termination active at exactly two physical end points on the trunk? Not one, not three — two. Use a multimeter to measure resistance across the A and B lines with all devices powered OFF. You should read approximately 110 Ω (two 220 Ω terminating resistors in parallel). Too high (>150 Ω) = one or both terminators missing or switched off. Too low (<80 Ω) = more than two terminations active.
- Cable polarity. Check that A-line connects to Pin 8 (DB9) or Pin 2 (M12), and B-line to Pin 3 (DB9) or Pin 4 (M12) throughout the segment. A single reversed connection anywhere in the segment silences the entire bus.
- Physical continuity. Use a cable tester to verify continuity of both conductors and shield through every segment. A broken conductor from mechanical damage, an over-tightened cable gland, or a pulled connector is easy to miss visually.
- Shield continuity. Check that the shield is continuous through the entire segment. A broken shield often does not cause errors by itself but leaves the bus vulnerable to EMC-induced problems.
Intermittent Errors at High Baud Rates
- Stub lines. At 3 Mbit/s and above, any spur off the trunk trunk is a problem. Find them and eliminate them.
- Cable type mismatch. If part of the network uses non-Type-A cable (higher capacitance, wrong impedance), signal quality at high frequencies degrades. A protocol analyzer (ProfiTrace, ProfiCore Ultra) can show the actual signal eye diagram and measure jitter.
- Connector quality. Poor-quality PROFIBUS connectors have higher contact resistance, loose clamps on the shield, or imprecise geometry that adds impedance discontinuities. Especially problematic at 12 Mbit/s where connector quality matters as much as cable quality.
- Baud rate too high for segment length. Recalculate the maximum allowed segment length for your chosen baud rate. If you are at or near the limit, drop the baud rate one step.
Errors That Appear When Nearby Equipment Operates
This is an EMC (electromagnetic compatibility) problem, not a cable problem per se — but the cable installation is usually the cause.
- Improve shield grounding. Add grounding at every panel entry and device housing.
- Increase separation from power cables. Move the PROFIBUS cable to a separate tray, or install a metal barrier between it and VFD cables.
- Check equipotential bonding. If there is a potential difference between control cabinet grounds and field device grounds, common-mode noise couples into the bus. Bond all grounds together.
- Check for damaged shield. A shield that is intact at connectors but nicked or cut in a conduit run has a point of failure that creates antenna-like behavior, picking up radiated noise efficiently.
Quick Reference: PROFIBUS Cable Selection Guide
| Requirement | Cable Choice |
|---|---|
| New PROFIBUS DP installation, general factory | Type A, PVC jacket, violet |
| PROFIBUS DP in oily or solvent environment | Type A, PUR jacket |
| PROFIBUS DP in fire-sensitive area (tunnels, buildings) | Type A, LSZH/FRNC jacket |
| PROFIBUS DP outdoor / direct burial | Type A, armoured (SWA) |
| PROFIBUS DP with moving cable (drag chain) | Type A, torsion-rated |
| PROFIBUS PA, standard process | MBP cable, 0.8 mm², blue jacket |
| PROFIBUS PA, intrinsically safe (FISCO) | MBP cable per IS specification, blue jacket |
Key Numbers Summary
| Parameter | PROFIBUS DP (Type A) | PROFIBUS PA |
|---|---|---|
| Characteristic impedance | 150 Ω (135–165 Ω) | ~100 Ω |
| Wire cross-section | 0.34 mm² (22 AWG) | 0.8 mm² (recommended) |
| Loop resistance | ≤ 110 Ω/km | — |
| Capacitance | ≤ 30 pF/m | — |
| Jacket color | Violet | Blue |
| Max segment length (12 Mbit/s) | 100 m | N/A |
| Max segment length (9.6 kbit/s) | 1,200 m | 1,900 m (fixed 31.25 kbit/s) |
| Max spur length (PA standard) | N/A | 60 m |
| Max spur length (PA IS/FISCO) | N/A | 30 m |
| Terminator (DP) | 390/220/390 Ω | — |
| Terminator (PA) | — | 100 Ω + 1 µF in series |
| Primary connector (cabinet) | DB9 male | 7/8-inch or M12 A-coded |
| Primary connector (field) | M12 B-coded | 7/8-inch or M12 A-coded |