Overview
This reference covers the correct application of the integrated terminating resistor on Siemens S7-300 PROFIBUS DP connectors. The 9-pin D-sub connector used on S7-300 CPUs and IM modules ships with a slide switch that engages or disengages a 220 Ω termination network, plus 390 Ω pull-up and pull-down bias resistors that hold the bus at a defined idle level. Misconfiguration of this switch is one of the most common field causes of intermittent PROFIBUS faults, mysterious station dropouts, and the dreaded "BF" (Bus Fault) LED on the CPU front panel. This guide consolidates the rules from the SIMATIC S7-300 manual, the SIMATIC NET PROFIBUS network manual, the PI Installation Guidelines, and field troubleshooting practice into a single commissioning checklist.
Two non-negotiable rules govern the bus:
- Termination is active at exactly the first and last node of every electrical segment, and nowhere else.
- The 220 Ω / 390 Ω network is powered from the host device's 24 V. A terminator on an unpowered host is functionally absent.
Everything else in this document is a corollary of those two rules.
Prerequisites
Before touching the connector switches, confirm the following items are available and current on the engineering workstation:
- STEP 7 V5.5 + SP2/SP3 (or TIA Portal V13 SP1 / V14 / V15) with HW Config and the "Set PG/PC Interface" tool installed.
- Siemens 6ES7972-0BA12-0XA0 or 6ES7972-0BB12-0XA0 PROFIBUS connector with diagnostics LED and slide switch. Older 6ES7972-0BA11-0XA0 and 6ES7972-0BA41-0XA0 variants are functionally identical for termination purposes.
- PROFIBUS cable, type 6XV1830-0EH10 (standard violet) or 6XV1830-3EH10 (FC fast-connect, food-grade outer jacket).
- PROFIBUS stripping tool 6GK1905-6AA00 and FC stripping tool for FC cable.
- For network analysis on suspect segments: a BT200 bus tester or a Softing PROFIusb / PROFIline diagnostic tool. These devices measure the differential bus voltage and decode traffic to confirm a clean signal eye.
- Topology print that shows the physical sequence of nodes, including the position of repeaters (6GK1500-0AA00 RS-485 repeater) and OLM (6GK1502 / 6GK1503) where fiber segments exist.
- A spare 6ES7972-0DA00-0AA0 passive PROFIBUS terminator for installation at the end of any backbone segment whose end device is occasionally de-energized.
PROFIBUS Topology Fundamentals
PROFIBUS DP and MPI both operate on an RS-485 electrical layer with a strictly defined line topology. The IEC 61784 / EN 50170 standard allows a maximum of 32 stations per segment — one master plus 31 slaves, or 32 masters in a multi-master arrangement — with line lengths that scale inversely with baud rate. The repeater, or RS-485 segment coupler, regenerates the signal so that segments can be cascaded; each new segment brings its own termination rules.
| Baud rate | Max segment length (Type A cable, 6XV1830-0EH10) | Typical use |
|---|---|---|
| 9.6 kbit/s | 1,200 m | Slow sensor integration, legacy segments |
| 19.2 kbit/s | 1,200 m | Same as above |
| 45.45 kbit/s | 1,200 m | PROFIBUS PA profile |
| 93.75 kbit/s | 1,200 m | STEP 7 default low speed |
| 187.5 kbit/s | 1,000 m | ET 200 standard |
| 500 kbit/s | 400 m | Common mid-speed |
| 1.5 Mbit/s | 200 m | Default STEP 7 DP profile |
| 3 Mbit/s | 100 m | DP-V1 fast |
| 6 Mbit/s | 100 m | DP-V2 fast |
| 12 Mbit/s | 100 m | DP-V2 maximum, drives and motion |
Line Topology Rules
- Strictly linear. No stars, no branches, no T-junctions, except for very short PG/PC service stubs that are not recommended on running production networks.
- Maximum 32 stations per segment in the address range 0-31.
- Terminating resistors active at exactly the first and last node of every segment.
- Shield continuity maintained along the entire run; shield bonded at the cabinet entry using PROFIBUS shield clamps (6GK1905-0AA00) on a clean ground bar.
- Baud rate is set per segment, not per node. All stations on a segment must operate at the same rate; the master in HW Config controls this for DP-V0/V1 segments.
S7-300 PROFIBUS Connector Hardware
The 9-pin D-sub connector used on S7-300 CPUs (e.g., 6ES7315-2EH14-0AB0), CP 343-1 (6GK7343-1EX30), IM 360/361, and ET 200M / ET 200S slaves carries both PROFIBUS signals and the 24 V isolated supply that powers the integrated termination network. The pinout follows the PROFIBUS standard EN 50170:
| Pin | Signal | Function |
|---|---|---|
| 1 | SHIELD | Cable shield / functional earth, capacitively coupled inside connector |
| 2 | M24 | 24 V return (ground reference for terminator power) |
| 3 | RxD/TxD-P (B line) | Receive / transmit data, positive |
| 4 | RTS | Request To Send, direction control for repeaters |
| 5 | M5V2 | Data reference potential (isolated 5 V) |
| 6 | P5V2 | Isolated +5 V for bus termination bias |
| 7 | P24 | +24 V for terminator power |
| 8 | RxD/TxD-N (A line) | Receive / transmit data, negative |
| 9 | NC / DPR | Not connected, or Data Parity Request on some variants |
The integrated terminating resistor is built from a 220 Ω resistor between pins 3 and 8, plus a 390 Ω pull-up to pin 6 (P5V2) on the B line and a 390 Ω pull-down to pin 5 (M5V2) on the A line. Powering pins 7 and 2 from the host CPU, CP, or ET 200 slave is what enables the network. Without 24 V on P24/M24, the termination is invisible to the bus regardless of the slide switch position.
References: Siemens FAQ 1097514 — PROFIBUS terminating resistor and shield; SIMATIC NET PROFIBUS Network Manual (Edition 2009); PI Installation Guidelines for PROFIBUS-DP/FMS (Order No. 2.141); S7-300 Module Data Manual (2013 edition).
Termination Rules and Switch Configuration
The rule is unambiguous and applies to PROFIBUS DP, PROFIBUS PA (with segment couplers), and MPI alike: termination is allowed at exactly the first and the last node of every electrical segment, and nowhere else.
- Mark the two physical extremities of the bus. The first node and the last node of a segment are defined by the cable run, not by the master/slave role or the CPU's MPI address.
- Set the slide switch on the 9-pin connector to ON at the first node and ON at the last node.
- All intermediate nodes — CPUs, slaves, CPs, HMI panels — must have the switch in the OFF position.
- If a repeater sits in the middle of a segment, both of its bus termination switches must be OFF. The repeater provides port-to-port galvanic isolation, not termination.
- If the segment ends at a repeater port, the repeater is the last node. That port's switch is then ON.
Connector Switch Configuration: One Cable vs Two Cables
The 6ES7972-0B.12 PROFIBUS connector has two cable entries on a 35° angled body: an "IN" port (the one stamped with the arrow ⇨ pointing into the connector body) and an "OUT" port. The slide switch on the top of the housing has the following logic:
| Cables connected to connector | Resistor switch | Reason |
|---|---|---|
| Two cables (one in, one out) | OFF | This node is in the middle of the segment. The bus continues past it; active termination would distort the signal. |
| One cable (only the IN port) | ON | This node is the segment's end point. The OUT port is empty; the 220 Ω termination closes the bus. |
| One cable (only the OUT port) | Do not use this configuration | The OUT port is the outgoing side. If only the outgoing side is connected, the cable is reversed; the segment flows in the wrong direction. Always bring the field cable in on the IN (arrow) port. |
| No cables (service connector) | OFF unless this is the only connection | A PG service connector is a stub, not a segment terminus. Terminating at a service connector strands the rest of the bus. |
Field shortcut: Two cables on the connector = switch OFF. One cable = switch ON, and that one cable is on the side with the arrow.
Topology Reference Diagram
Cable Routing Direction and End-Node Powering
Cable Routing Direction
The PROFIBUS connector has the cable entry and the internal screw-terminal block oriented so that the segment flows from the IN (arrow) port toward the OUT side. Reversing the cable — landing the field cable on the OUT port — causes the slide switch to engage the termination network on the outgoing side of the bus, leaving the entire upstream trunk unterminated. The signal then looks clean from the perspective of a BT200 bus tester connected at that node, because the local 220 Ω is still in the circuit, but every other node sees reflections proportional to the stub length.
End-Node Powering Requirements
The integrated 220 Ω / 390 Ω terminator network is powered from the host's 24 V supply via pins 7 (P24) and 2 (M24) of the 9-pin D-sub. If a node is designated as an end node (switch ON) and the host CPU, CP, or ET 200 slave is de-energized, the termination becomes an open circuit. The two extreme ends of the segment are then unterminated, and the bus can no longer operate at any speed above 9.6 kbit/s without reflections.
This is the field failure mode most often reported when a customer "added a piece of cable for the case that he might need a node" but did not keep the new endpoint's terminator powered. Symptoms are:
- BF (Bus Fault) LED lit on the CPU front panel; SF on slaves.
- Diagnostic buffer entries 0x337E "PROFIBUS DP frame error" and OB86 / OB85 trip counters incrementing.
- PG online "Accessible Nodes" shows the master but no slaves, or shows slaves but with red diagnostic icons.
- Intermittent operation at 1.5 Mbit/s and total failure at 12 Mbit/s.
To guarantee the bus stays terminated, either (a) keep the end node energized whenever any other node is communicating, or (b) install a passive 9-pin PROFIBUS terminator (6ES7972-0DA00-0AA0) with external 24 V wired directly from the cabinet 24 VDC distribution. The passive terminator is purely a 220 Ω / 390 Ω network in a small housing and is the recommended choice for long-distance backbone terminations because it eliminates the dependency on the end device being powered.
PG/PC Node Address Configuration
A programming device connected to PROFIBUS for online diagnostics must occupy a unique node address. The PG can sit anywhere on the bus — including physically piggy-backed on the connector of an existing CPU — but it must have a valid PROFIBUS or MPI address that is not already in use. The default for an MPI/DP interface on a Siemens Field PG is 0, with the highest MPI address typically 31.
Procedure (STEP 7 V5.5):
- Open the Windows control panel entry "Set PG/PC Interface" (PG/PC-Schnittstelle einstellen).
- Select the access point of the application — for STEP 7 use "S7ONLINE (STEP 7) -> TCP/IP" for Ethernet download, or "PC Adapter (MPI)" / "PC Adapter (PROFIBUS)" depending on the adapter.
- Click Properties and confirm the transmission rate matches the bus (e.g. 1.5 Mbit/s or 12 Mbit/s). Auto-detect is supported on CP 5512, CP 5611, CP 5613, CP 5711, and the PC Adapter USB.
- Set the PG address. 0 is the default, but on a bus with a CPU at address 2 the PG should use address 1 or another free slot.
- Test with "Accessible Nodes" (Erreichbare Teilnehmer). All live, addressed, and powered slaves should appear in the list within a few seconds.
References: Siemens: PG/PC Interface setup for STEP 7 V5.x; Siemens FAQ: PROFIBUS connector termination rules; Siemens: PC Adapter USB (6GK1571-0BA00) configuration.
Step-by-Step Termination Procedure
- Pull the bus segment print. Identify the two physical end nodes. Confirm there is no extra "spare" cable coiled at the cabinet — this is the single most common cause of unpowered termination in the field.
- Verify every CPU, CP, ET 200 slave, HMI, and repeater in the segment has a node address assigned. Address conflicts (two nodes with the same DIP) are not protected against by the protocol; the symptom is "address assignment conflict" diagnostic in the master and OB100 / OB86 entries in the diagnostic buffer.
- At end node 1, lift the slide switch to ON. Confirm the cable enters the connector on the IN port (arrow side). Confirm the node is powered (24 V present on the host, P24/M24 on the D-sub not open).
- At end node 2, repeat step 3. If end node 2 is a passive 6ES7972-0DA00 terminator, wire its 24 V to a fused cabinet supply, not to a node that may be switched off for maintenance.
- Walk the segment from end to end. At every intermediate node, confirm the slide switch is OFF, the cable enters on the IN port, and the outgoing cable leaves on the OUT port.
- Power up the segment in any sequence. The 24 V on the terminators must be present any time the master is active and the bus is expected to communicate.
- Run "Accessible Nodes" from the PG. All addresses should respond with their diagnostic data within 5 seconds at 1.5 Mbit/s.
- Optionally, run a BT200 / Softing scan to verify the differential bus voltage between pins 3 and 8 is ≥ 1 V at every node and that the signal eye is clean at 12 Mbit/s.
Verification and Diagnostics
After commissioning, run the following checks to confirm a robust PROFIBUS installation.
Visual and Mechanical Checks
- Slide switches in the correct position at end nodes only.
- No spare cable coiled at the end; coiled cable is an unterminated stub.
- Shield clamps in place at every cabinet entry; shield transitioned from 360° clamp to ground bar.
- All connectors torqued (screw terminals on the D-sub are typically 0.4 Nm; do not overtighten and crack the housing).
Electrical Checks
- Differential voltage V_AB (between pins 3 and 8) at every node, with no traffic, must be ≥ 1.0 V. A reading below 0.5 V indicates missing or unpowered termination.
- Idle voltage on pin 3 (B) ≈ +1.0 V, pin 8 (A) ≈ +0.7 V with the 220 Ω terminator as the divider. The exact levels depend on cable length and station count; ± 0.2 V variation across the segment is normal.
- Insert a BT200 between the master and the first slave. Read the bit error rate. A healthy 1.5 Mbit/s segment should be 0 errors over 24 hours of normal traffic.
Diagnostic Buffer Entries
After power-on, inspect the CPU's diagnostic buffer (Online > Module Information > Diagnostic Buffer). The expected healthy entries are the start-up complete event and per-slave parameter-assignment confirmation. The following entries indicate a termination or wiring problem:
| Event ID | Text | Likely cause |
|---|---|---|
| 0x337E | PROFIBUS DP frame error | Reflection, unterminated stub, EMI |
| 0x00E0 | Failure of a DP slave (OB86) | Slave lost or unpowered |
| 0x01A1 | Diagnostic interrupt from DP slave | Slave reports a station problem |
| 0x3385 | PROFIBUS DP: parameter assignment error | GSD mismatch or address conflict |
Reference: Siemens FAQ 1097514 — PROFIBUS termination and shield; Siemens: Diagnostic buffer entries for S7-300/400; Siemens: PROFIBUS DP diagnostics in STEP 7.
Common Pitfalls and Field Notes
| Pitfall | Symptom | Fix |
|---|---|---|
| Spare cable coiled at the end node, terminator unpowered | BF at master, random slave loss, OB86 ticks | Remove the coiled cable or install a powered 6ES7972-0DA00 terminator |
| Cable landed in OUT port instead of IN | Clean at low speed, intermittent at 12 Mbit/s | Re-lug the cable on the arrow (IN) side |
| Both end nodes have switch OFF | Total segment failure above 9.6 kbit/s, BF on every node | Set switch ON at exactly two nodes |
| Three or more terminations active | Lower bus voltage, weak signal, master sees lower than 0.5 V | Check the segment end-to-end and disable intermediate switches |
| Repeater used as end node, switch left ON | Stubs from the repeater to the next node produce reflections | Disable the repeater's bus terminator unless the repeater is the absolute last device |
| Shield not bonded at cabinet entry | EMI susceptibility, CRC errors proportional to drive power | Install 6GK1905-0AA00 shield clamps on a clean ground bar |
| PG connected without an MPI/DP address | "Accessible Nodes" returns nothing, or PG address collides | Configure Set PG/PC Interface with a unique address |
| Service PG connector left on a running bus | CRC errors proportional to the stub length | Remove PG connector when not actively commissioning |
| Mixing 6XV1830-0EH10 and 6XV1830-3EH10 in one segment | Asymmetric impedance, reflections at the join | Use a single cable type end-to-end |
Troubleshooting Matrix
| Symptom | Primary check | Secondary check | Likely cause |
|---|---|---|---|
| BF lit on CPU, no slaves visible | Master's bus terminator switch | Power on end node 1 | End node 1 unpowered or terminator OFF |
| Random slaves drop out, BF flashes | Differential voltage V_AB at failing slave | Cable routing at the slave's connector | Cable landed on OUT port; signal reflected at stub |
| PG cannot see master | PG/PC Interface address | PG baud rate matches bus | PG and master at the same address, or rate mismatch |
| Intermittent at 12 Mbit/s, fine at 1.5 Mbit/s | Reflection: cable entry orientation | Spare coiled cable | Improper termination topology or cable stubs |
| Whole segment fails after maintenance | Slide switch state on each end node | Power to the end nodes | Maintenance left a switch in OFF position or 24 V removed |
| One specific slave always fails | Address DIP on the slave | Slave's 24 V supply | Address conflict or slave unpowered |
| CRC error count grows over time | Shield continuity at cabinet entry | Drive cabinet VFD grounding | EMI coupling from VFD or ungrounded shield |
Frequently Asked Questions
How do I know which two nodes are the "end" of the PROFIBUS segment?
The end nodes are the first and last devices on the physical cable run, not the master and not necessarily the highest-addressed slave. Trace the bus from one terminus to the other; whichever devices have only one cable connected to their D-sub connector are the ends. Activate the slide switch on those two and only those two nodes.
What happens if the end-node CPU or ET 200 slave is de-energized?
The 220 Ω / 390 Ω terminator network inside the connector needs 24 V on pins 7 and 2 of the D-sub. If the host is unpowered, the termination becomes an open circuit and the bus is left unterminated. This is the most common field cause of an S7-300 BF (Bus Fault) with no other changes. Use a passive 6ES7972-0DA00-0AA0 terminator powered from a fused cabinet supply to avoid this dependency.
Can I leave a coil of spare cable at the end for future nodes?
No. A coil of cable beyond the end node is a stub. It is unterminated and causes reflections, especially at 12 Mbit/s. Either install a powered passive terminator at the coil and treat it as the new end, or remove the coil. The PI Installation Guidelines forbid open stubs beyond the terminator.
Does the repeater need termination?
Only if the repeater is the absolute last device of the segment. Otherwise the repeater sits in the middle of the segment, has cable entering and leaving both ports, and both of its bus terminator switches must be OFF. The repeater regenerates the signal; it does not terminate the bus.
What is the correct bus termination resistance on PROFIBUS?
220 Ω across the two data lines (pins 3 and 8 of the D-sub), with a 390 Ω pull-up to P5V2 on the B line and a 390 Ω pull-down to M5V2 on the A line. The integrated network in 6ES7972-0B.12 connectors implements exactly this and is enabled by the slide switch combined with 24 V on the host.