Siemens 6ES7972 PROFIBUS Connector Selection: 0BA41 vs 0BB41 and Related Variants

Siemens SIMATIC PROFIBUS DP bus connectors in the 6ES7972 family share an identical RS-485 transceiver footprint but differ in cable-exit angle, programming-device (PG) interface presence, on-board termination behavior, and the maximum bus baud rate they support. Choosing the wrong variant for a CP 443-5 communications processor, a SIMATIC Field PG, or a third-party PC interface card rarely prevents operation outright, but it produces recurring field issues: lifted pins, lost segments, intermittent bus faults at 12 Mbit/s, and reflection errors at the segment terminator. This reference consolidates the field-tested differences between the -0BA41-0XA0, -0BB41-0XA0, -0BA12-0XA0, -0BB12-0XA0 and the related -0BA30 / -0BA40 / -0BA50 MLFBs, and gives concrete rules for CP 443-5, SIMATIC PG, and external PC connections.

1. PROFIBUS Bus Connector Family Overview

The 6ES7972-... series is Siemens' standard PROFIBUS DP RS-485 bus connector. Every variant in this family has the same D-sub male (DB9M) mating face that plugs into the PROFIBUS port of a CPU, CP, IM, or operator panel. The functional differences are in the connector body, the orientation of the incoming PROFIBUS cable, the optional programming-device (PG) socket, the supported transmission speed, and the on-board termination network. Siemens also manufactures higher-end IP20 connectors and the FastConnect system (6GK1500, 6ES7972-1A..., 6ES7972-1B...). The 6GK1500-0EA02 MLFB cited in the original question is not a bus connector at all; it is the SIMATIC NET OLM G12 Optical Link Module for fiber-optic segments, a full DIN-rail repeater with ST/BFOC optical ports and a 9-pin D-sub for the electrical side. A copper PROFIBUS segment cannot use the OLM as a passive connector.

2. Mechanical Variants: Cable Exit Angle

Two cable-outlet geometries dominate the family. The 35°/45° axial exit (-0BA12, -0BA30, -0BA40, -0BB12, -0BB40) routes the cable diagonally out the back of the device. The 90° exit (-0BA41, -0BB41, -0BA50, -0BB50) routes the cable straight down. In a control cabinet with a back panel and a 35 mm DIN rail populated with S7-400 modules, the 90° connector is almost always preferred because the cable drops directly into the wire duct below the rack without crossing the modules. The 35°/45° exit is more useful when the connector is mounted on a CP, IM, or operator panel on the right-hand side of the rack and the cable must continue along the same plane. The choice is mechanical only – the electrical characteristics of the trunk pins are identical for 0BA40/0BA41, 0BB40/0BB41, and 0BA50/0BB50.

3. The PG Interface Explained

The "PG interface" on 0BBxx variants is a second DB9 female socket mounted on the top of the connector body. The internal wiring of this socket is connected in parallel with the bus only through a short stub, and the socket is wired to the same A and B lines. Functionally it allows you to plug a programming device, a PROFIBUS analyzer (Procentec ProfiHub, Softing PROFINET/PROFIBUS analyzers, Indu-Sol ProfiTap), or a SIMATIC Field PG into the running segment without unplugging the device or disturbing the trunk. The trunk-side termination switch on the 0BB connector still works normally – the PG socket is electrically independent of the on-board terminator.

Key limits of the PG stub:

• Maximum stub length: The cable between the 0BBxx connector and the PG / analyzer is treated as a stub by PROFIBUS physical-layer rules. The PG socket is designed for cable lengths of ≤ 2 m; longer stubs cause reflection on the trunk, especially at 1.5 Mbit/s and above.
• Maximum baud rate on the stub: The PG stub is not rated for 12 Mbit/s; the analyzer must be configured for a baud rate ≤ 1.5 Mbit/s while connected through this socket, or bus errors will be observed on adjacent nodes. The trunk itself continues at the configured line speed.
• No termination on the PG socket: The PG socket does not carry a termination resistor. The bus must already be terminated on the trunk side or the analyzer will see a collapsed differential signal.

4. Baud Rate Limits

Most 6ES7972 connectors from -0BA30 onward are specified for the full PROFIBUS DP line speed of 12 Mbit/s. The single exception in common use is the 6ES7972-0BA30-0XA0, which is mechanically identical to the 0BA40 but uses an older termination-network design that is only characterized up to 1.5 Mbit/s. At 12 Mbit/s the on-board termination network of the 0BA30 produces a level mismatch and increases the bit error rate, especially with the termination switch ON. Field installations that have grown from a 1.5 Mbit/s fieldbus to 12 Mbit/s must replace any 0BA30 connectors with 0BA40 or 0BA41 units, even if the housing still physically fits and the screws still thread. The 0BA30 also cannot be installed at the segment end at line speeds above 1.5 Mbit/s for the same reason.

Beyond 32 stations, 12 repeaters per segment, or 126 stations per bus, a PROFIBUS network must use repeaters or RS-485 segment couplers. The CP 443-5 counts as one segment with up to 31 additional repeater segments downstream of its port. Verify every segment against this table before commissioning; a 12 Mbit/s segment that has a 110 m trunk is guaranteed to fail intermittently regardless of connector quality.

5. Termination Resistor Behavior

Every 6ES7972 connector contains a combined bus terminator with a 220 Ω resistor on each data line (P and N) to VP and a 390 Ω resistor to GND, plus a 5 V idle-bias generator. The terminator is enabled by a slide switch on the connector body:

• ON: Terminator active, suitable only for the first or last device on the segment. The switch position should be painted on the housing; on -0BA40/-0BA41 it is a small slider near the shield clamp, on -0BA50/-0BB50 it is a labeled toggle.
• OFF: Terminator inactive. All intermediate devices must use this setting.

Never enable termination on more than two devices per segment. Never enable termination on a device in the middle of a segment. The 0BA30 cannot be installed at the segment end at all if the bus runs above 1.5 Mbit/s. If the trunk continues past the last device (a tee, a dropped branch, or a service outlet), that device cannot be terminated regardless of position – only the two physical ends of the trunk carry the terminator.

6. Cable Specifications: PROFIBUS Type A and Type B

All baud rate and length calculations above assume PROFIBUS cable Type A (Siemens 6XV1830-0EH10 or equivalent). Type A has the following minimum characteristics:

• Impedance Z = 135 Ω to 165 Ω at 3 to 20 MHz (nominal 150 Ω).
• Capacitance ≤ 30 pF/m.
• Loop resistance ≤ 110 Ω/km (≈ 22 AWG conductor).
• Shield coverage ≥ 80 %, often aluminum foil + tinned copper braid.
• Core colors: red = B (D-sub pin 3), green = A (D-sub pin 8).

Strip the cable jacket about 35 mm back, fold the shield back over the outer jacket, cut the filler / non-woven strength members flush, and strip exactly 9 mm of insulation from the red and green cores. The red and green cores are not the polarity indication in the connector; the connector housing has a printed "A" and "B" which must be matched to the engraving on the cable jacket from the manufacturer. Mixing the two is the most common reason for a "bus short" diagnostic when a fresh connector is installed.

7. Connecting a CP 443-5

The SIMATIC S7-400 CP 443-5 (6GK7443-5DX02, -5DX03, -5DX04) provides a single PROFIBUS DP master interface. The CP is installed in the S7-400 rack like any other module and is addressed through the backplane bus. Because the CP occupies a vertical slot with the D-sub at the bottom of the module, the 90° outlet connector (0BA41 or 0BB41) is the only one that allows the cable to drop cleanly into the wire duct without crossing adjacent modules or putting mechanical stress on the connector. The 0BB41 is the strongly recommended variant for the CP 443-5 because the PG socket is invaluable for live diagnostics.

1. Configure the CP 443-5 in STEP 7 / TIA Portal under the rack: assign PROFIBUS address (default 2), baud rate (match to slowest slave), and bus profile (DP, FMS, S7 communication as required).
2. Insert the 6ES7972-0BB41-0XA0 into the CP 443-5 D-sub female port and tighten the two thumb screws until hand-tight. Do not use a screwdriver – the thumb screws are not designed for torqued installation, and a worn screw will lose ground bond quality.
3. Slide the termination switch to OFF if the CP 443-5 is not at the segment end. The first and last devices in the segment carry the termination; typically these are the CP 443-5 and a remote ET 200 station at the end of the line.
4. Strip and insert the two PROFIBUS cores (red = B / pin 3, green = A / pin 8) into the screw terminals of the connector – observe the marking "A" and "B" on the housing, not the wire color. The shield of the bus cable is clamped against the strain relief / shield plate on the connector body. Keep the drain wire as short as practical and use the grounding bracket to bond the cable shield to the cabinet PE bar at least at the cabinet entry point.
5. Tighten the strain relief clamp firmly. A loose shield defeats EMC and the bus becomes susceptible to VFD-induced noise on adjacent power cables.

8. Connecting a PC or Field PG

A Field PG is a regular PROFIBUS DP node and the same rules apply. Use a 0BB41 if you intend to connect the analyzer later. If the PC only has a USB-to-PROFIBUS adapter (e.g. Siemens PC adapter USB A2 6GK1571-2BA00), connect through a small active PROFIBUS repeater or ProfiHub rather than directly into the bus connector, because most USB adapters do not implement a fully compliant RS-485 transceiver and can disturb the bus even on a stub. The PC adapter USB A2 is a slave device and will participate in DP token passing – assign it a unique address above 32 and below 126.

For a CP 5611 / CP 5611 A2 / CP 5621 PCI card inside an industrial PC, the DB9 male on the card plugs directly into a 6ES7972-0BA41-0XA0 (no PG interface needed on the PC side). Verify the PC card baud rate matches the line baud rate; the CP 5611 auto-detects but the PC adapter USB A2 requires manual selection in STEP 7 / TIA Portal under "PG/PC Interface". Set "PG/PC is the only master on the bus" if the PC is the sole DP master – otherwise the bus will time out on watchdog errors when no other master is active.

9. Segment and Repeater Rules

PROFIBUS DP allows up to 32 stations per segment, including repeaters. Each RS-485 repeater (e.g. Siemens 6ES7972-0AA02-0XA0 Diagnostic Repeater, 6GK1500-0AA10 RS-485 Repeater) counts as a station on both segments it joins. A CP 443-5 segment with one repeater and 30 slaves is fully loaded; the 31st and 32nd station are the CP and the repeater. Beyond 32 stations, a second repeater must be added.

When using a repeater, observe the following:

• The maximum number of repeaters between any two stations is 10 in series (a typical industrial design uses 3 or fewer).
• Each repeater segment has its own termination rules. The repeater provides segment-end termination on both sides by default; do not install a third terminator at the repeater location.
• For fiber segments, replace the OLM (6GK1500-0EA02) for the electrical-to-fiber conversion. The OLM does not require any external connector on the copper side other than a standard 0BA41 plugged into its D-sub.
• Maximum 126 stations per bus when repeaters and a higher-level master are used.

10. EMC, Shielding, and Grounding

PROFIBUS cable shielding is the single most important EMC measure on the bus. The shield must be bonded to the cabinet PE bar at least at the cabinet entry point, and ideally at every cabinet along the trunk. The 6ES7972 connector's strain-relief clamp is the shield bond point; it must clamp the cable's shield braid and the drain wire, not just the outer jacket. Use a PROFIBUS-specific EMC grommet at the cabinet entry to maintain 360° shield contact through the cabinet wall.

Route the PROFIBUS cable at least 200 mm from VFD power cables and at least 100 mm from 24 V DC power cables. If a crossover is unavoidable, cross at 90°. Use only Type A cable with ≥ 80 % shield coverage for the trunk; do not substitute lower-cost cable in the field to save a few euros per meter.

11. Decision Matrix: Which MLFB to Use

12. Installation Procedure (Field Step-by-Step)

1. Power down the bus segment if the device is currently a live segment end. Mid-segment connectors can be hot-swapped but only if the device is electrically out of the bus; otherwise expect a short burst of bus errors on adjacent stations.
2. Cut the PROFIBUS cable square, strip the jacket 35 mm back, expose the red / green cores.
3. Fold the shield back over the outer jacket; trim the filler / non-woven strength members flush with the jacket.
4. Strip exactly 9 mm of insulation from each core. Do not nick the copper strands.
5. Insert red into terminal "B" and green into terminal "A" per the housing marking. Do not follow wire color for the terminal assignment – follow the housing marking.
6. Clamp the shield under the strain relief / shield plate; tighten the strain relief firmly.
7. Set the on-board termination switch to OFF for mid-segment, ON for segment end. Only one device at each physical end of the segment should be ON.
8. Plug into the target device, hand-tighten the thumb screws, and verify a click. The connector should not be able to rotate or pull out with light hand force.

13. Verification After Connector Installation

1. Power the segment. On the CP 443-5 the SF / BF LEDs should remain off. A steady red BF LED on the CP 443-5 means no bus activity – check termination, polarity, and baud rate in that order.
2. Open STEP 7 / TIA Portal, go to "Online → Accessible Nodes". The CP 443-5 and all configured slaves should appear with the configured addresses. A node that appears as "???" is a non-Siemens station that is reachable but not yet configured.
3. If you have a ProfiHub or a repeater with a signal-quality indicator, walk the bus from the CP 443-5 to the last slave and verify a stable "OK" or a good signal-quality bar at every drop. Negative values indicate termination or reflection issues.
4. Run a 24-hour diagnostic log. Look for sporadic diagnostic interrupts, retries, and station failures. A clean bus has zero retries per cycle. Persistent retries at 12 Mbit/s almost always point to a 0BA30 left in the segment or a missing terminator.
5. Touch the connector body. Any warm connector under normal load is a sign of an intermittent high-resistance contact in the screw terminals – re-strip the cable.
6. Measure the A–B voltage with a multimeter (key off / key on) at every segment end: expect approximately 1.0 V DC at 1.5 Mbit/s and below, 0.8 V DC at 12 Mbit/s. A reading below 0.4 V indicates missing or shorted termination; a reading above 1.5 V indicates a double-termination on the segment.

14. Common Pitfalls and Field-Proven Caveats

1. Misidentifying 6GK1500-0EA02 as a bus connector. The 6GK1500-0EA02 is the SIMATIC NET OLM G12 (Optical Link Module), a fiber-optic repeater for PROFIBUS, not a copper bus connector. It uses a 9-pin D-sub for the electrical segment but occupies an entire DIN-rail housing and has ST/BFOC optical ports. A copper bus segment cannot use the OLM as a passive connector.
2. Leaving termination on a mid-segment device. Two terminators in the middle of the segment halves the signal and corrupts 12 Mbit/s traffic within minutes. Three terminators will collapse the signal entirely.
3. Long PG stub on a 0BB. A 5 m analyzer cable plugged into the PG socket looks harmless but the bus fault counter on adjacent slaves will rise within minutes. Keep the PG stub ≤ 2 m and the analyzer at ≤ 1.5 Mbit/s.
4. 0BA30 at 12 Mbit/s. The older termination network cannot bias 12 Mbit/s correctly. Replace with 0BA40 / 0BA41 / 0BB41 before changing the line baud rate.
5. Stripping too much insulation. The FastConnect terminals (used on -0BA4x, -0BA5x) use insulation-displacement contacts. Strip exactly 9 mm of outer jacket; the standard 10-11 mm strip typical of Phoenix terminals will cause intermittent shorts and difficult-to-diagnose bus faults.
6. Forgetting the shield clamp. The strain relief on the connector is also the shield bond. A loose shield defeats EMC and the bus becomes susceptible to VFD-induced noise on adjacent power cables.
7. Mixing cable polarity at the connector. Wire color and connector marking are not always aligned. Always trust the connector's printed "A" / "B" for the terminal assignment and verify with a multimeter (A to pin 8, B to pin 3 of the device-side D-sub).
8. Swapping 0BA12 with 0BB12 across a 35° → 90° boundary. The mechanical differences are small. A 0BA12 / 0BB12 cannot be replaced one-to-one with a 0BA41 / 0BB41 without changing the cable routing – the duct below the rack will be too shallow or too deep. Plan a 5-minute routing adjustment when changing the outlet angle.

15. Summary

For a CP 443-5 plus two PC connections, the practical answer is:

• CP 443-5: 6ES7972-0BB41-0XA0 (90°, with PG, max 12 Mbit/s, modern termination).
• PC #1 (e.g. Field PG or engineering station): 6ES7972-0BB41-0XA0 (90°, with PG for analyzer access).
• PC #2 (HMI / SCADA, no diagnostics): 6ES7972-0BA41-0XA0 (90°, no PG, max 12 Mbit/s).

All three are mechanically and electrically interchangeable for the trunk; the only functional difference is the diagnostic PG socket on the 0BB41. Use 90° outlet throughout unless cabinet geometry forces 35°/45°. Replace any 0BA30 found in the field before enabling baud rates above 1.5 Mbit/s. The 6GK1500-0EA02 MLFB in your parts list is not a bus connector and should be removed from the connector list. Use Type A cable (135-165 Ω) only, terminate the segment at exactly two physical ends, keep the PG stub ≤ 2 m, and verify the bus with STEP 7 / TIA Portal "Accessible Nodes" plus a 24-hour diagnostic log before releasing the cabinet.

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