Short answer
Pin 1 is +5V Supply, Pin 2 is Line A (Green), Pin 3 is 0V Ground, Pin 4 is Line B (Red), Pin 5 is Shield.
Under industrial standard IEC 61076-2-101, the keyway orientation governs terminal positions. Always verify whether you are inspecting male pins or female contact sockets; pin sequences mirror horizontally across mating faces.
Mating face vector schematic
Compare the male plug and female socket pin positions before terminating wires or taking multimeter readings.
Physical & electrical specifications
| Specification Parameter | Standard Value | Practical Field Implication |
|---|---|---|
| Governing Standard | IEC 61076-2-101 | Guarantees dimensional interoperability between manufacturers (Siemens, Phoenix Contact, Turck, Murr). |
| Mechanical Keyway | Single inverse polarizing keyway notch located at 6 o’clock position (opposite of standard A-coded). | Prevents accidental cross-mating into incompatible communication or power ports. |
| Coupling Nut Torque | 0.6 Nm | Excess torque crushes the internal elastomeric O-ring, causing coolant leaks and intermittent shorts. |
| Rated Voltage | 60 V AC / 60 V DC | Maximum dielectric insulation boundary between adjacent conductor pins. |
| Rated Current Limit | 4 A per contact | Continuous current carrying capacity per pin without thermal de-rating. |
| Recommended Wire Gauge | 22 AWG shielded single-pair Profibus cable (Type A) | Conductor strand cross-section required to maintain terminal contact and pull-out strength. |
| Ingress Protection | IP67 / IP68 | Submersion and high-pressure washdown seal integrity when fully threaded. |
Pin assignment & multimeter test values
Measure between the designated pin and reference ground with a digital multimeter in DC Volts mode.
| Pin # | Standard Color | Signal Designation | Operating Voltage | Multimeter Test Method |
|---|---|---|---|---|
| Pin 1 | VP (+5 V DC Term. Supply) | Bus Termination Positive (+5 V) | +4.75 to +5.25 V DC | DC Volts to Pin 3 (0V). Must read nominal +5V on active terminator nodes. |
| Pin 2 | Green (GN) - Line A | Profibus Data Line A (RxD/TxD-N) | +1.5 to +2.1 V DC idle | DC Volts to Pin 3. In idle state, Line A is biased ~0.5V lower than Line B. |
| Pin 3 | DGND (Data Ground / 0 V) | Bus Termination Return (0 V) | 0.00 V DC reference | DC reference ground for active bus terminators and bias networks. |
| Pin 4 | Red (RD) - Line B | Profibus Data Line B (RxD/TxD-P) | +2.9 to +3.5 V DC idle | DC Volts to Pin 3. Line B must measure higher than Line A when bus is idle. |
| Pin 5 | Shield / Braided Drain | Protective / Functional Earth (FE) | Chassis Earth Potential | Continuity to panel ground. Shield braid must maintain 360° shell contact. |
Meter-first 3-step field diagnostic sequence
Perform these 3 tests in order before swapping field devices or cutting cable bundles.
- Step 1: Check Active Termination Voltage
Action: Set meter to DC Volts. Measure Pin 1 (+5V VP) to Pin 3 (DGND) at the end of the line connector.
Expected Reading: +4.8 V to +5.2 V DC.
Fault Symptom: If 0V, the device providing active termination is powered down, corrupting whole-segment communication.
- Step 2: Verify Idle Differential Line Bias
Action: With the PLC in RUN but bus idle, measure DC Volts across Pin 4 (Line B Red) and Pin 2 (Line A Green).
Expected Reading: +0.8 V to +1.2 V DC differential (B positive relative to A).
Fault Symptom: If differential reading is 0V, termination pull-up/pull-down resistors are disconnected; receiver chips will interpret float as garbage data.
- Step 3: Unpowered Bus Impedance Test
Action: Power down all Profibus stations. Set meter to Ohms. Measure resistance across Pin 2 (A) and Pin 4 (B).
Expected Reading: 100 Ω to 110 Ω on a correctly terminated segment (two 220 Ω termination resistors in parallel).
Fault Symptom: 220 Ω indicates only one end is terminated; 55 Ω indicates 4 terminators are mistakenly active; ~0 Ω reveals a crushed wire short.
Field traps & common failure modes
The Red/Green Line A/B Reversal Trap
Observed Symptom: Profibus DP master shows red BF (Bus Fault) LED; no slave nodes establish communication.
Root Cause: Technicians often assume "Red is Positive, Green is Negative". In Profibus: Line A is Green (Negative/Inverting) and Line B is Red (Positive/Non-inverting). Reversing these at one junction drops the whole line.
Field Correction: Verify Pin 2 = Green (Line A) and Pin 4 = Red (Line B) at every M12 drop box and T-piece.
Forcing B-Coded Plug into A-Coded Socket
Observed Symptom: Crushed plastic keyway and shattered pin guide during fast changeover.
Root Cause: B-coded keyway is at 6 o’clock; A-coded is at 12 o’clock. Forcing them misaligns pins and shorts 24V into Profibus transceivers.
Field Correction: Align visual notch carefully. B-coded female faces have an internal raised key at 6 o’clock.
Industrial protocols & applications
| Network / Protocol | Standard Application | Reference Standard |
|---|---|---|
| Profibus DP (Decentralized Periphery) | Fieldbus communication up to 12 Mbps in manufacturing and process plants. | IEC 61158 / IEC 61784 |
| Interbus & Fieldbus Loop | Legacy European distributed I/O systems. | DIN 19258 |
Frequently asked field questions
Why is M12 B-coding used for Profibus DP instead of standard A-coding?
The B-coding keyway is reversed (located at 6 o’clock instead of 12 o’clock). This physical mechanical key prevents technicians from accidentally plugging standard +24V DC sensor cables into sensitive RS-485 Profibus communication ports.
What should the resistance be across Profibus Line A and Line B?
On a properly terminated, completely unpowered Profibus DP segment with termination switches ON at both ends, resistance between Line A (Pin 2) and Line B (Pin 4) must be approximately 110 Ohms (two 220 Ohm terminations in parallel).
Tape this inside the enclosure door.
No signal on the plant floor? Get the 2-page laminated pocket card. Contains 4–20 mA NAMUR thresholds, PNP/NPN PLC common logic, M12/M8 pin numbers, and 24V DC voltage drop rules.