PROFIBUS DP: Connect Siemens PLC to 2 Slaves via 6ES7 972-0BB50

Overview

Connecting a Siemens SIMATIC PLC to two PROFIBUS DP slaves over a single DP segment is a routine field task, but it requires a clear understanding of the linear (daisy-chain) bus topology mandated by IEC 61158 / EN 50170. PROFIBUS DP is a strictly linear bus; it does not support star or tree wiring without the use of active repeaters. With three physical nodes — the PLC master and two DP slaves — the segment is a small but representative case: it forces the installer to decide whether the master sits at one end (two-node-internal trunk) or in the middle (which is the cleanest PROFIBUS-DP arrangement when the cabling layout permits it).

This reference covers the practical use of the Siemens 6ES7 972-0BB50-0XA0 PROFIBUS FastConnect connector (90° cable outlet, with PG socket, switchable termination, diagnostics LED) for a 3-node segment. It also documents the alternative path — a passive T-junction such as the Weidmuller VPB-PRO-DIST — for installations where the physical placement of devices prevents a clean daisy-chain pull. The third-party T-tee is the only sanctioned way to convert a PROFIBUS spur into a stub of acceptable length without exceeding the 1.5 Mbaud timing budget on short runs.

The Pro-face GP3000 Series PROFIBUS DP Slave Driver manual confirms the slave-side wiring convention used in this guide and is a useful cross-reference when one of the slaves is a Pro-face HMI configured as a DP slave on the same segment: Pro-face GP3000 Series PROFIBUS DP Slave Driver (PDF).

Prerequisites

Before pulling cable, gather the components and verify version compatibility. The parts list below is the minimum kit for a 3-node PROFIBUS-DP segment terminated with Siemens 6ES7 972-0BB50-0XA0 connectors.

PROFIBUS DP Topology: The Linear Bus Rule

PROFIBUS-DP defines a single linear bus with terminated ends. Every segment has exactly two end-nodes; every device between them is a middle-node. There are exactly two correct physical topologies for a 3-node network:

1. Master in the middle: PLC connected to slave 1 on one side and slave 2 on the other. This is the preferred arrangement because it gives two equal stubs and minimizes the distance from the master to the most-distant slave.
2. Master at one end: PLC terminates the segment at one end, and the bus daisy-chains from the master to slave 1, then to slave 2, which is the far end-node.

Both arrangements are electrically identical with respect to termination. The choice is purely a function of how the field devices are physically mounted and how the trunk cable can be routed through the cabinet or machine.

The 5 connectors specified in the original query (6ES7 972-0BB50-0XA0) are more than the 3 required for a clean daisy-chain. The spare connectors are typically used to:

• Replace a connector that has been damaged during crimp/strip work.
• Pre-terminate the spare cable on a service cart.
• Provide one of the two spare end-node terminators that should be left switched OFF in the middle-node position so they can be moved to a different node later if the segment is reconfigured.

Step-by-Step: Daisy-Chain a Siemens PLC to Two DP Slaves

Step 1 — Plan the cable run

Decide the physical path before cutting. PROFIBUS cable must be kept clear of VFD power cables (minimum 200 mm parallel, or use a grounded metal divider). Total segment length is dictated by baud rate:

For a typical 3-node machine cabinet, lengths are in the 5–30 m range, well within the 1.5 Mbaud envelope. If the run exceeds the value for the selected baud rate, the bus will not start; the diagnostic LED on the 6ES7 972-0BB50-0XA0 will flicker or stay off.

Step 2 — Prepare the PROFIBUS cable ends

Use the Siemens 6GK1 905-6AA00 stripping tool. Set the tool for the FC cable diameter and rotate it 3–4 turns; the tool simultaneously cuts the violet outer jacket, removes the inner filler, and exposes the shield braid without nicking it. Pull off the stripped section, fold the braid back over the cable jacket, and slide the connector's shield contact spring over the folded braid. Insert the two cores (red and green) into the FastConnect insulation-displacement contacts, then tighten the strain-relief screw to roughly 0.6 N·m. No soldering, no crimping.

Step 3 — Identify the master, slave 1, and slave 2

Assign DP addresses from STEP 7 / TIA Portal before powering up. Defaults are 1 for the master and 0/1 for slaves, but the master must use address 1 or 2 (Siemens convention) and slaves cannot share an address. For a 3-node segment typical addressing is:

• Master (e.g., S7-300 CPU 315-2 DP) — address 2
• Slave 1 (e.g., ET 200S) — address 3
• Slave 2 (e.g., Pro-face GP3000 HMI as DP slave) — address 4

The Pro-face GP3000 manual linked above specifies the same addressing convention: select "PROFIBUS DP Slave" in GP-Pro EX, set the rotary switch on the back of the display to the same address, and import the Pro-face GSD into the STEP 7 hardware catalog. See Pro-face GP3000 Series PROFIBUS DP Slave Driver for the switch position table.

Step 4 — Wire the daisy-chain

Run the first cable from the PLC to slave 1. Run the second cable from slave 1 to slave 2. With the PLC at one end, the connector assignments are:

If the master is in the middle (preferred), swap the master onto a middle-node position with termination OFF, and place slave 1 and slave 2 at the two ends with termination ON. The PLC's two PROFIBUS ports on the CPU 315-2 DP / CPU 317-2 DP / ET 200S IM 151 are physically identical — the same 6ES7 972-0BB50-0XA0 connector plugs into either.

Step 5 — Set the bus parameters

In STEP 7 (classic) the DP master is configured under Hardware > PROFIBUS DP > Properties > Network Settings. Match the baud rate to the slowest slave's GSD. Confirm the slot time, target rotation time, and gap factor match the PROFIBUS-DP defaults unless the slaves request specific values via their GSD. Typical defaults that work for S7-300 + ET 200S + GP3000:

Tslot = 300 bit-times
Tset = 15 bit-times
Tqui = 0 bit-times
GAP factor = 10
Retry limit = 3
Baud = 1.5 Mbaud

In TIA Portal the same parameters live under Devices & Networks > PROFIBUS subnet > Properties > Network data > Bus parameters.

Step 6 — Download and go online

Download the hardware configuration. Open Online > Accessible Nodes; all three PROFIBUS addresses (2, 3, 4) should appear. The diagnostic LED on each 6ES7 972-0BB50-0XA0 should be steady green. If any LED is off, flickering red, or blinking yellow, the segment has a physical problem (see troubleshooting matrix below).

Step-by-Step: T-Junction Alternative (When the Daisy-Chain Fits the Floor Plan Poorly)

If the slaves are physically adjacent to the master cabinet but cannot be re-arranged in a clean daisy-chain, a passive T-distributor lets the installer bring a stub from the master to a central tee, then run two short stubs to each slave. The 6ES7 972-0BB50-0XA0 connector is not a T-connector — it is a straight or 90° inline connector. For a real T you need a passive distributor such as the Weidmuller VPB-PRO-DIST or the Siemens 6GK1 905-0AA00 (PROFIBUS T-connector) — though the Siemens unit is for the older PG-bus era and is rare on the field today.

Critical constraints for any passive T on PROFIBUS-DP:

• Each leg of the T (trunk-to-slave distance) must be ≤ 1 m at 1.5 Mbaud, and ≤ 10 m total at 1.5 Mbaud across all three legs combined, per the PROFIBUS Installation Guideline for short stub operation. The PROFIBUS user organisation is unambiguous: passive stubs longer than this create reflections that corrupt the token-passing handshake.
• Baud rate must be limited to 1.5 Mbaud when using passive stubs. 3, 6, and 12 Mbaud require fully linear segments only.
• The T itself does not contain a terminating resistor. The two end-nodes still need their 6ES7 972-0BB50-0XA0 termination switched ON.

For a Weidmuller VPB-PRO-DIST (catalog 8564340000), wire the trunk through the T's IN/OUT ports, and the two slaves to the SPUR ports. Keep cable lengths short, ground the T's shield clamp to cabinet PE, and use the same FC cable (6XV1 830-0EH10) on all three legs to maintain impedance continuity.

Termination Resistor Mechanics on the 6ES7 972-0BB50-0XA0

The 6ES7 972-0BB50-0XA0 integrates a four-pole switch under a small sliding cover on top of the D-sub housing. The switch positions are:

The diagnostic LED has three states:

• Steady green — node is on the bus, no errors.
• Slowly flashing green — node is configured but not yet in token rotation (usually because the master has not started cyclic data exchange yet).
• Off / red / fast flicker — see troubleshooting matrix.

Verification

Run these checks before handing the segment to commissioning:

1. With the bus unpowered, measure resistance between pins 3 and 8 at the far end of the segment with a multimeter. With both end-node terminators ON, the reading should be close to 110 Ω (220 Ω in parallel with 220 Ω — the two end-node resistors are in parallel as seen from the middle of the bus).
2. Power the segment. Each 6ES7 972-0BB50-0XA0 LED should be steady green within 5 seconds of the master cycling the bus.
3. Open STEP 7 / TIA Portal and run Online > Diagnostics > PROFIBUS > Bus Statistics. Look for zero Repeat Counter increments and a stable Bus Error Count over a 10-minute soak test.
4. Forcibly disconnect slave 2 mid-run. The diagnostic LED on the master's connector should remain green (the master still sees slave 1). Slave 1's LED should remain green if it is between the master and the disconnection, or report "slave not reachable" if the master is between slave 1 and the disconnect.

Troubleshooting Matrix

Notes for Mixed-Vendor Segments

When one of the slaves is a Pro-face GP3000 HMI configured as a DP slave, the HMI's PROFIBUS port is wired identically to any other DP slave — D-sub 9-pin, pin 3 = B-line (red), pin 8 = A-line (green), pin 6 = VP, pin 5 = DGND. The HMI's address is set with a rotary switch on the back of the display rather than via software. Refer to the GP3000 PROFIBUS DP Slave Driver for the exact switch table and the GSD file name. The same connector (Siemens 6ES7 972-0BB50-0XA0) fits the HMI's D-sub port without modification, because the PROFIBUS connector pinout is standardized by IEC 61158.

FAQ