Resolving CPU 315-2DP and IM 151-1 PROFIBUS Communication Errors

Problem Description

When commissioning a Siemens S7-300 CPU 315-2DP (6ES7315-2AFxx-0AB0) as the PROFIBUS DP master with an ET 200S IM 151-1 interface module (6ES7151-1AA05-0AB0) as a DP slave, the diagnostic buffer in STEP 7 / TIA Portal reports "Error in lower-level component" and the CPU's SF (System Fault) LED lights solid while the BF (Bus Fault) LED flashes. The IM 151-1 BF LED also flashes at approximately 0.5 Hz, which Siemens defines as "DP slave not found / bus short circuit or missing termination," rather than a logical parameterization error.

The most common diagnostic buffer entries that produce this LED pattern are listed below. Event IDs follow the STEP 7 v5.x / TIA Portal V11 diagnostic structure for an S7-300 CPU.

When a CPU 315-2DP and IM 151-1 combination produces simultaneous SF-on / BF-flashing on the master and BF-flashing on the slave, the fault is in roughly 90 % of field cases the physical layer (cable, connector, shield, or termination) rather than the GSD-based parameterization. Logical causes (GSD version mismatch, wrong slot assignment, watchdog time-out) are usually indicated by BF-on solid on the slave and a different diagnostic text such as "Station return with error" or "DP slave defective."

Affected Hardware and Identification

For the IM 151-1 variant 6ES7151-1AA05-0AB0, the supported PROFIBUS transmission rate is 9.6 kbps to 12 Mbps (auto-detect). At 1.5 Mbps the segment may be up to 200 m with FC standard cable; at 12 Mbps the maximum is 100 m. Reference: Siemens Support entry 1402443 – ET 200S IM 151-1 manual.

Root Cause Analysis

Six independent physical-layer causes have been observed in field reports of this exact CPU 315-2DP / IM 151-1 error pattern. Each is verifiable without additional hardware beyond a small screwdriver and a multimeter.

1. Incoming cable connected to A2/B2 instead of A1/B1. The Siemens FastConnect (FC) connector has two pairs of insulation-displacement contacts. A1/B1 is the incoming bus segment (marked with an arrow on the connector housing pointing IN); A2/B2 is the outgoing bus to the next device. Wiring the master or the slave into A2/B2 while leaving A1/B1 open makes the slave invisible to the master.
2. Insulation stripped from the green and red PROFIBUS cores. The FC connector uses insulation-displacement contacts (IDC). The contact blades are designed to pierce the violet PROFIBUS cable jacket and the inner core insulation simultaneously. If the inner insulation is removed manually the wire is too thin to make a gas-tight IDC, the contact resistance rises, and intermittent or permanent BF faults result.
3. Transparent wire guide not seated. After inserting the cores into the IDC, the plastic colour-coded insert (red/green) must be pushed down fully and the transparent cover locked. If the cover is loose, the spring pressure on the IDC is lost and the contact opens.
4. Shield (screen) not terminated. The braided shield of the PROFIBUS cable must be clamped 360° in the connector back-shell to form a low-impedance screening contact. A shield that is "tied to ground at one end only" is not acceptable on PROFIBUS DP – the shield must be continuous and bonded to the connector shell at every node, and the connector shell must make full metal contact with the D-sub housing.
5. Terminating resistor OFF at both end nodes, or ON in the middle. A PROFIBUS segment must be terminated at exactly the two physical end stations. The IM 151-1 must therefore have its terminator ON if it is the last node, and the CPU 315-2DP's connector must have its terminator ON if no further slaves are downstream. Intermediate nodes must have the terminator OFF.
6. PROFIBUS address DIP switch set incorrectly. An 8-position DIP switch on the IM 151-1 sets the bus address. Switch positions are binary-weighted, switch 1 = 1, switch 2 = 2, switch 3 = 4, switch 4 = 8, switch 5 = 16, switch 6 = 32, switch 7 = 64, switch 8 = 128. Address 3 = switches 1 + 2 ON, all others OFF. Position 8 on some IM 151-1 hardware revisions is reserved and labelled "not used" – do not set it.

PROFIBUS FastConnect Connector Wiring Procedure

The single most common root cause in the field is incorrect wiring of the FC connector. Use the procedure below for both the CPU 315-2DP end and the IM 151-1 end.

1. Strip exactly 35 mm of the violet outer jacket from the PROFIBUS cable. Do not nick the braided shield.
2. Fold the braided shield back over the outer jacket. Do not twist it into a pigtail – the shield must remain a continuous tube that the connector clamp can grip 360°.
3. Unwrap the foil shield and the inner filler; expose the red and green cores. The cores must remain insulated.
4. Insert the red core into terminal A1 and the green core into terminal B1 of the FastConnect connector. With the connector held cable-outlet-down and the D-sub facing you, A1 is on the right and B1 on the left of the inner contact row.
5. Close the IDC slider until it clicks. The slider blades must cut through the inner core insulation.
6. Place the folded shield braid under the metal clamp in the connector back-shell and tighten the strain-relief screw so the shield is gripped 360°.
7. Snap the transparent cover closed. The cover must be flush with the connector body.
8. Plug the connector into the device and torque the two retaining screws (UNC 4-40) finger-tight plus 1/4 turn.

Setting the IM 151-1 PROFIBUS Address

Address 3 (decimal) on the 8-position DIP switch of the IM 151-1 is binary 0000 0011. Therefore:

• Switch 1 = ON (value 1)
• Switch 2 = ON (value 2)
• Switches 3 – 8 = OFF

The address set in hardware (DIP switches) and the address configured in STEP 7 / TIA Portal under DP slave > Properties > PROFIBUS address must match. If the CPU's hardware catalog shows the IM 151-1 as a generic "IM 151-1" without a GSD-specific device name, install the GSD file for the IM 151-1 AA05 firmware via Options > Install GSD file in TIA Portal V11. The GSD file is named SIEM80D6.GSD (or the equivalent revision) and is bundled in the ET 200S GSD collection.

Terminating Resistor Configuration

For a single-master / single-slave segment with no further devices downstream:

For a multi-slave segment, only the first and last physical nodes are terminated; intermediate nodes must have the terminator OFF. The FC connector terminator switch is on the top of the connector housing, slides between "ON" and "OFF," and locks with a small screwdriver.

A floating or incorrectly terminated segment will typically produce BF-flashing at 0.5 Hz on the affected devices, exactly the symptom reported with this CPU 315-2DP / IM 151-1 combination. A 150 Ω resistance measured across pins 3 and 8 of the D-sub at each end of the segment (with the bus powered and the two end terminators ON) is the expected value for a healthy terminated segment.

LED Status Reference

LED behaviour on the CPU 315-2DP and IM 151-1 is the fastest field-level diagnostic. Siemens defines the states in the relevant device manuals.

For the specific case in this article (CPU SF solid, CPU BF flashing 0.5 Hz, IM 151-1 BF flashing 0.5 Hz) the symptom is a "no response from configured slave" – consistent with a wiring fault that prevents the master's poll frame from reaching the IM 151-1.

Step-by-Step Diagnostic Procedure

Follow the sequence below in order. Each step verifies or eliminates a single physical-layer root cause.

1. Power down both devices. With the CPU in STOP or power removed, remove both PROFIBUS connectors and inspect each with a torch. The transparent IDC cover must be fully seated. The shield braid must be visible under the back-shell clamp, not stranded into a pigtail.
2. Verify the inner conductor insulation is intact. Cut a 30 mm section out of each connector and confirm the red and green cores still have their coloured insulating layer. If the insulation was stripped, rebuild the connector with a new FC plug and do not re-use the cable end.
3. Confirm A1/B1 wiring at both ends. Re-make the connector following the procedure above. The red core must land on A1, the green core on B1. A continuity check from CPU D-sub pin 3 (B-line) to IM 151-1 D-sub pin 3, and pin 8 (A-line) to pin 8, must show < 1 Ω.
4. Verify shield continuity. Measure resistance between the D-sub metal shell at the CPU end and the D-sub metal shell at the IM 151-1 end. It must be < 1 Ω and the shield must be continuous along the cable run.
5. Check PROFIBUS address. Switch 1 ON, switch 2 ON, all others OFF, on the IM 151-1 DIP. In the TIA Portal V11 hardware configuration, double-click the IM 151-1 in the DP slave list and confirm the PROFIBUS address is set to 3.
6. Set terminating resistors. CPU connector terminator = ON. IM 151-1 connector terminator = ON. If a future slave is later added in the middle of the segment, that slave's terminator must be OFF.
7. Power up the CPU, then the IM 151-1. Wait 5 seconds for the IM 151-1 to complete its bus initialisation.
8. Read the diagnostic buffer. In TIA Portal V11 go Online > Online & diagnostics > CPU 315-2DP > Diagnostic buffer. The expected entry after a successful restart is "DP slave: station return" with Event ID in the 0x381F / 0x3900 range and no entry for 0x381E.

Verification

A correctly commissioned CPU 315-2DP + IM 151-1 segment shows the following steady-state behaviour:

• CPU SF = OFF, CPU BF = OFF, CPU RUN = ON (green).
• IM 151-1 BF = OFF, IM 151-1 ON = ON (green), IM 151-1 SF = OFF.
• Diagnostic buffer last entry: "DP slave: station return" (Event ID 0x381F or similar), no further DP error entries.
• STEP 7 / TIA Portal Online > Monitor / Modify on the IM 151-1 input bytes shows the I/O updating at the configured scan rate.

A quick functional check is to insert a digital input module into slot 1 of the ET 200S, force the input, and read it back in a VAT table at I-address 0. If the input updates, the bus is in data exchange and the physical layer is healthy.

When the Problem Persists After Wiring is Corrected

If BF continues to flash after the connector, shield, address, and termination have all been verified, the fault is no longer physical-layer. Re-check the following in order:

1. GSD file revision. The IM 151-1 AA05 supports firmware versions V1.0.x and V2.0.x with different GSD files. Using a V1 GSD against a V2 IM 151-1, or vice versa, can cause parameterisation failure with BF-flashing. Read the firmware version from the label on the IM 151-1 and install the matching GSD.
2. PROFIBUS transmission rate. The CPU 315-2DP defaults to 1.5 Mbps. The IM 151-1 auto-detects from 9.6 kbps to 12 Mbps. If the master has been set to a rate the cable length cannot support (e.g., 12 Mbps over 150 m), reduce to 1.5 Mbps or 500 kbps and re-test.
3. Slot / module assignment in HW Config. The IM 151-1 power module and any I/O modules plugged into the ET 200S base unit must match the configuration in HW Config exactly. A missing power module in slot 0 causes the IM 151-1 to refuse parameterisation.
4. Watchdog time. A watchdog set to 10 ms while the bus is running at 9.6 kbps will time out. Increase the watchdog in the slave properties to 100 ms or 1 s for slow buses.
5. Bus cable length vs. baud rate. Siemens PROFIBUS cable length table: 1 000 m @ 9.6 kbps / 187.5 kbps, 400 m @ 500 kbps, 200 m @ 1.5 Mbps, 100 m @ 3 / 6 / 12 Mbps. Reference: Siemens PROFIBUS network manual entry 21859074.

Tools and Spare Parts Checklist

The FC stripping tool is strongly recommended. Hand-stripping the violet PROFIBUS cable almost always nicks the braided shield and produces intermittent shielding failures that are very difficult to trace.

Safety and Commissioning Notes

FAQ