Resolving PROFIBUS Repeater T-Bus and Termination Errors on S7-300

When a Phoenix Contact PS-REP-Profibus/12MB or compatible DIN-rail PROFIBUS DP repeater silently breaks cyclic I/O between a SIMATIC S7-300 CPU 317-2DP and a remote DP slave, the root cause is almost always one of three configuration errors: a T-Bus jumper left in place between adjacent repeater modules, segment terminators left ON at mid-segment ports, or a baud rate versus cable-length pair that exceeds the PROFIBUS RS-485 segment budget. This field reference documents the diagnostic path, the underlying electrical rules, and the verified fix used to recover a 12 Mbps DP segment on a two-master S7-300 panel.

1. Problem Summary

Reported field case: an S7-300 panel (CPU 317-2DP plus CP 343-1 Lean) carries two Phoenix Contact PS-REP-Profibus/12MB repeaters. One DP segment terminates at 11 remote slaves physically split into two cable groups (3 slaves and 8 slaves) that return to the panel. Direct connection from the CPU's integrated DP port to the field devices works at commissioning. Inserting either repeater into the segment causes all slaves to drop from the bus, and STEP 7 HW Config online view returns a station failure for every device downstream of the repeater.

Symptom signature:

• Cyclic I/O exchange collapses as soon as a repeater is inserted between CPU and first slave.
• Repeater segment activity LEDs show traffic on the CPU-side port but no echo on the field-side port (or vice versa), indicating that the repeater is not bridging the segments.
• SF (system fault) LED on the CPU 317-2DP and BF (bus fault) LED on the DP port illuminate.
• STEP 7 V5.x online scan shows "Station failure" or "Slave not found" for the affected slaves, even when addresses are unique and within the master's poll list.
• A second PROFIBUS line on the CPU's MPI/DP port (the six-device bus) keeps working normally because the repeater is only inserted on the second DP line.
• Removing the repeater restores the bus, confirming that the repeater is the failure point and not the slaves or the trunk cable.

2. PROFIBUS RS-485 Segment Fundamentals

A PROFIBUS DP segment is a single RS-485 trunk terminated at both physical ends. Every device on a segment, master or slave, sits on the same electrical bus. The segment has three hard limits that the repeater does not change - it only bridges two segments together while remaining itself a transparent node in the electrical chain.

Per IEC 61158-2 / EN 50170 the trunk length per segment is a function of baud rate:

Cable selection is not free-form. Use PROFIBUS cable Type A (twisted pair, 150 Ω characteristic impedance, 1.5 mm² solid conductor, 0.8 mm insulation, defined velocity factor 0.78 c). Type B is permitted for short jumps but reduces the segment budget by 30 % to 50 %; field installations consistently use Type A. The shield must be a braided or foil/braid combination with a surface transfer impedance below 20 mΩ/m at 30 MHz; aluminium-foil-only cable is not acceptable for industrial PROFIBUS.

The PROFIBUS user organization documents the standard repeater topology and cascadable repeater options in the PROFIBUS Repeater B1 product page.

3. Repeater Function and Limits

A PROFIBUS repeater regenerates the RS-485 physical layer between two segments. It re-shapes the bit timing, re-drives the differential voltage, and re-synchronises the start-of-frame delimiter. From the master's perspective the repeater is transparent; the slaves on the second segment look like slaves on the first segment, and the master's poll list does not change.

Three things the repeater does not do:

• It does not extend the 100 m / 12 Mbit/s electrical budget of either segment - both segments are still subject to Table 1.
• It does not change the PROFIBUS address space - each segment still allows up to 32 device addresses (one master + 31 slaves, or 32 slaves with no master on the segment), and 126 are reachable across a multi-segment topology with the address accounting rules from IEC 61158-2.
• It does not isolate a short circuit on one segment from the other. A failure on segment 2 is normally still visible to segment 1; segment-level galvanic isolation is a job for an active RS-485 isolator or a fiber converter (OLM, OBT, PSM-EG), not a standard repeater.

For the S7-300 system in this case, the Siemens catalogue repeater 6ES7972-0AA01-0XA0 (RS485 Repeater for PROFIBUS/MPI) applies the same physical-layer rules as the Phoenix Contact PS-REP. The two products are interchangeable for the diagnostics described below; the Siemens 6ES7972-0AA01-0XA0 product page documents the segment termination requirement and resistor network that is identical on the Phoenix Contact device.

4. Segment Termination, Shielding, and EMC

Every PROFIBUS DP segment must be terminated at both physical ends. The terminator is not a single resistor - it is a resistor network that matches the cable's characteristic impedance (150 Ω typical for PROFIBUS DP cable) and biases the bus lines to a defined idle voltage.

Termination rules for a repeater topology:

1. Each of the two segments has exactly two physical ends. Each end is terminated.
2. The repeater's two DP ports are mid-segment on both segments, not ends. Both ports must have termination OFF.
3. The first slave (or master) on segment 1 and the last slave on segment 1 have their built-in bus connector terminators set ON. The first slave on segment 2 and the last slave on segment 2 also have their terminators set ON.
4. If the repeater is placed at the physical end of segment 1 (the CPU is alone on segment 1 and the repeater is the next device), then the repeater's port 1 may be terminated and the CPU-side connector on the repeater's port 1 is the segment 1 end. This is the only exception - the repeater is an end on one segment, mid-segment on the other.
5. Termination ON at any mid-segment port damps the signal and shortens the achievable cable length; the master will start seeing telegram errors and slave-station failures even at 50 m of cable.

The Phoenix Contact PS-REP-Profibus/12MB exposes a slide switch on the front panel for each port. Position "ON" engages the on-board terminator; position "OFF" disables it. The factory default is OFF.

Shielding and grounding rules add a second failure category that often overlaps with the repeater symptoms:

• Apply a low-impedance shield bond at one end only (panel side) using a 360° EMC gland or a Phoenix Contact SKH shield clamp. The far end (last slave cabinet) is bonded through the cable capacitance, not a hard ground.
• Never bond both ends to ground if the two grounds are at different potentials. A 1 V ground-loop potential drives continuous common-mode current through the shield, raising the noise floor and corrupting the bus differential signal.
• Route the PROFIBUS cable in a separate tray, minimum 200 mm parallel separation from VFD power cables and 100 mm from 480 V AC trunk. Cross at 90° if a parallel run is unavoidable.
• Do not pass PROFIBUS through a multi-conductor cable that also carries 24 V DC power. The mutual coupling between the data pair and the power pair couples noise into the bus.

5. T-Bus Architecture and the Field-Specific Failure

Phoenix Contact DIN-rail PROFIBUS repeaters accept a T-Bus connector on the underside of the module. The T-Bus is a five-pin backplane jumper that snaps onto the DIN rail and carries 24 V power between adjacent Phoenix Contact devices in the same family. In a PS-REP-Profibus the T-Bus is used to supply the repeater's 24 V logic power from a neighbouring PS-REP-Profibus or from a Phoenix Contact power module, avoiding an extra wire run for the 24 V terminals.

The T-Bus is not a PROFIBUS signal path. It is a backplane power bus. However, two adjacent PS-REP-Profibus modules that share a T-Bus segment will see their power rails and internal reference grounds tied together. If both modules are also wired so that their DP terminals bridge the same two trunks (for example, segment 1 entering repeater A port 1, leaving repeater A port 2, entering repeater B port 1, leaving repeater B port 2, with a cable returning to the master), the result is a parallel termination network and a phase-shifted bit-recovery path - the two repeaters are now driving the same pair of segments against each other.

This is the exact failure mode reported in the field: the user connected both repeaters through their T-Bus and observed that "when I connect repeater, no working" and that the master could see "some devices from first cable, some from second." The T-Bus was energising both repeater modules from a common rail, and the segments were cross-coupled through the wiring on the terminals rather than through the dedicated DP ports.

The correct wiring is:

1. Each repeater powered individually from a 24 V terminal block (or from a single power module without a shared T-Bus jumper across the two repeaters).
2. Each repeater's port 1 wired to one segment, port 2 wired to the next segment. The segments are independent trunks, not paralleled at the panel.
3. The two DP cables coming back from the field (one carrying the 3-device group, one carrying the 8-device group) are wired to the two repeaters' port 2 terminals, and the CPU's DP cable is wired to one repeater's port 1 only. If the panel layout requires the two field trunks to converge on the same CPU port, the field trunks first run to two repeaters' port 2, and the two repeaters' port 1 are tied together with a short patch cable - that patch cable is a third segment with both ends terminated.

6. Baud Rate vs Cable Length

Field case reports an 800 m run to a remote device and a 1000 m run to another. At 12 Mbit/s the segment limit is 100 m, so neither of those runs is achievable on a single segment - the repeater does not make 800 m of cable behave like 100 m, regardless of whether it is a Phoenix Contact PS-REP-Profibus/12MB or a Siemens 6ES7972-0AA01-0XA0.

To reach 800 m at 12 Mbit/s the project must use one of three physical-layer options:

The PS-REP-Profibus/12MB auto-detects the baud rate on both ports, so reducing the baud rate is a configuration change in STEP 7, not a hardware change. At 1.5 Mbit/s the segment budget is 200 m, so 800 m still requires at least three repeaters in cascade. At 187.5 kbit/s the segment budget is 1000 m and 800 m is reachable on a single segment without any repeater - the cheapest fix, if the application can tolerate 187.5 kbit/s cycle times.

7. Diagnostic Procedure and Tools

Run the following sequence when a repeater insertion drops the bus. Each step is independent and can be skipped only if the previous step has already been verified.

1. Verify the DP port LEDs on the CPU 317-2DP. The BF (bus fault) LED flashes at 0.5 Hz if the master detects telegram errors on segment 1. A solid BF LED indicates a physical-layer fault (cable short, missing terminator, or double termination).
2. With the repeater in place, open STEP 7 HW Config and run PLC > PROFIBUS > Diagnose. The diagnostic buffer returns telegram error counts and station-failure flags. Note which slaves are flagged - all of them indicates segment-wide problems; a contiguous range indicates a partial-segment problem.
3. Open the CPU diagnostic buffer (PLC > Diagnostic Buffer) and look for entries with event ID 0x3931 (DP slave not reachable), 0x3942 (DP slave failure on bus), or 0x3842 (DP bus error). A burst of these entries within 100 ms of repeater insertion confirms the repeater is the trigger.
4. Remove power from the repeater. Disconnect the field cable from the repeater's port 2. Reapply power. Measure the DC voltage between pin 6 (P5V) and pin 5 (DGND) on the port 2 D-sub connector. A reading of 4.75 V to 5.25 V DC confirms the repeater's on-board terminator is OFF (a 0 V reading with the cable disconnected means the repeater is sourcing terminator voltage onto a cable that is now open - check the slide switch).
5. With the field cable still disconnected from port 2, plug a PROFIBUS bus terminator (e.g. Siemens 6ES7972-0DA00-0AA0) onto the port 2 D-sub. Power the repeater. The BF LED on the CPU should clear if the repeater itself is healthy. If the BF LED remains solid, the repeater is the fault - swap it.
6. Reconnect the field cable. Re-set the slide switches on every device between the master and the first slave to OFF, except for the master connector and the last slave connector which are ON. Re-test.
7. Connect a PROFIBUS diagnostics tool (Softing PROFINET/PROFIBUS Diagnostic Tool, Indu-Sol PROFINET-INspektor, or a Siemens BT 200 / BT 77 handheld) at the repeater's port 2. Run a wire test and a signal-quality test. Signal quality above 80 % (Indu-Sol scale) or signal-to-noise margin above the tool's threshold indicates the physical layer is healthy.
8. For a deeper look, attach a ProfiTrace or Amprolyzer at the master end and capture 1,000 to 10,000 telegrams. Look for the "Repeater Control telegram" (RCV) and verify the segment activity. A high number of "Bus error" frames from one specific slave indicates that slave's bus connector or cable drop is marginal.
9. Remove the T-Bus connector from the repeater (if present). Power-cycle. Repeat the BF LED check from step 1.

Cross-platform note: the same diagnostic path applies to Allen-Bradley SLC 500 / PLC-5 with a PROFIBUS scanner (SST-PFB-CLX or 1788-PFB), to Schneider Electric Modicon M340 with a BMX-NOM-0200 communication module, and to ABB AC500 with a CM589-PNIO. The STEP 7 menu paths above are for SIMATIC; the RSLogix 5000 / ControlLogix path is I/O Configuration > right-click 1788-PFB > Module Diagnostic.

8. Step-by-Step Resolution

The verified fix for the field case is a T-Bus configuration change, but the diagnostic order matters. Apply the changes in the sequence below; do not skip the termination check.

1. Power off the S7-300 panel. Discharge the 24 V rail to 0 V.
2. Locate the two PS-REP-Profibus/12MB modules on the DIN rail. Inspect the underside of each module for a green T-Bus connector.
3. If a T-Bus connector bridges the two PS-REP modules, pull it off the rail between them. The two repeaters must each be powered from their own 24 V terminal pair, or from a separate power module that does not share a T-Bus segment with the repeaters.
4. On the first repeater, set the slide switch for port 1 to OFF (this port will carry the CPU-side segment) and the slide switch for port 2 to OFF (this port will carry the field cable to the 3-device group).
5. On the second repeater, set the slide switch for port 1 to OFF (this port will carry the CPU-side segment) and the slide switch for port 2 to OFF (this port will carry the field cable to the 8-device group).
6. Wire the CPU's DP connector to a short patch cable that lands on the first repeater's port 1. Wire a second short patch cable from the first repeater's port 2 back to a bus connector on the second repeater's port 1. The CPU side of this short patch cable is mid-segment and the field side is mid-segment - the bus connector on the second repeater's port 1 must have its terminator switch OFF, and the bus connector on the first repeater's port 2 must also be OFF. The CPU connector and the end of this short patch must be terminated by the bus connectors on the devices at each end (the CPU's bus connector is ON, the second repeater's port 1 connector is OFF because the patch ends at the second repeater).
7. Re-apply 24 V power. Power the CPU. Verify both repeater segment LEDs show green activity. Open STEP 7 HW Config and run PLC > PROFIBUS > Diagnose. All 11 slaves should now be visible.
8. If the segment still shows failures, swap the bus connector on the second repeater's port 1 from a standard connector to a 6ES7972-0DA00-0AA0 terminator and re-test. If failures clear, the CPU connector was not actually terminated - either the bus connector's switch was in the wrong position, or the connector was a feed-through without an integrated switch.

9. Topology Design Recommendations

For a panel that aggregates two or more field trunks to a single CPU port, the following topology rules apply:

1. One CPU DP port drives one segment. A segment is a single RS-485 trunk with one master and up to 31 slaves (or 32 slaves with no master on that segment).
2. Each field trunk is a separate segment. The two repeaters' port 1 (CPU-side) must not be paralleled onto the same D-sub. Either run two cables from the CPU's DP connector to a 2-port splitter, or use a multi-port repeater with galvanic isolation between ports (some PS-REP variants support this).
3. For 11 slaves in two groups at 800 m to 1000 m, design the baud rate first. At 12 Mbit/s the segment budget is 100 m, so the long trunks must use fiber or a cascade of repeaters. At 187.5 kbit/s a single segment reaches 1000 m and no repeater is needed on the long run.
4. Place the repeater as close as possible to the boundary between the short and long trunks. The repeater does not amplify a long trunk - it only isolates the electrical characteristics of two segments.
5. Allow a slack loop of at least 0.5 m at each repeater location so the bus connector can be removed for diagnostics without cutting the cable.

10. Verification and Commissioning

After the wiring change, run the following verification sequence:

1. STEP 7 HW Config online: PLC > PROFIBUS > Diagnose. All 11 slaves should be present with no "station failure" or "slave not found" entries.
2. CPU diagnostic buffer: PLC > Diagnostic Buffer. No bus-fault entries since the wiring change.
3. Live diagnostic: in HW Config, double-click each slave and open the Diagnostics tab. The "Station status" field should read "OK" and the cyclic I/O tab should show incoming and outgoing bytes.
4. Cycle-time check: in HW Config, Station > PROFIBUS > Bus Parameters. Verify the configured baud rate matches the actual bus speed, and check the target rotation time (TRT) - if the TRT exceeds the master's configured slot time, slaves will be reported as "not reachable".
5. Long-term test: leave the system running for at least 60 minutes. The CPU's diagnostic buffer should remain free of bus faults. A single intermittent fault per hour is acceptable; more than that indicates a marginal signal that needs further investigation with a PROFIBUS diagnostics tool.
6. Loop test: temporarily disconnect the field cable from one repeater's port 2. The slaves on the other repeater should continue to communicate. This confirms the two repeaters are on independent segments and not paralleled at the panel.

11. Common Fault Matrix

12. Specifications Reference

Both products are electrically compatible with PROFIBUS DP per IEC 61158-2. The diagnostic procedure, termination rules, and segment budget are identical. The PS-REP-Profibus/12MB additional T-Bus interface is the only functional difference; the Siemens 6ES7972-0AA01-0XA0 takes 24 V from its own terminals only.