Resolving CPU 314C-2DP to MP277 Profibus DP Disconnect Error

Problem Overview

This technical reference addresses a recurring commissioning fault on SIMATIC S7-300 systems: a CPU 314C-2DP that loses its connection to a SIMATIC MP277 HMI panel over Profibus DP. The fault is reported inside the WinCC Flexible runtime as "PLC rack 0 slot 2 disconnected" and is confirmed by every process tag on the MP277 displaying "#####" (out-of-range / no connection). Communication is restored the moment a PC Adapter (PC Adapter USB A2 or PC Adapter MPI A2) is plugged into the CPU; the instant the adapter is unplugged, the HMI drops again within one bus cycle.

This signature — "communication only with PG attached" — is one of the most frequently reported Profibus problems on S7-300 cells. It is almost always a physical-layer, bus-termination, or shielding problem rather than a CPU firmware defect. Walking through the diagnostic decision tree below isolates the root cause in 90% of cases without replacing any hardware. The article is written for the engineer who has just been paged at 02:00 and needs to know which of the seven checks to do first.

Affected Hardware and Software

Confirm the part numbers on site match the table below before applying the diagnostics. Profibus behaviour differs between the older 314C-2DP (6ES7 314-6CG03-0AB0) and the later 314C-2 DP (6ES7 314-6EH04-0AB0), and between the 8" MP277 (6AV6 643-0CD01-1AX1) and the 10" MP277 (6AV6 643-0DD01-1AX1). Mixing these part numbers without aligning the GSD file is a typical source of "PG sees the CPU, panel does not" symptoms.

The CPU 314C-2DP exposes two physical ports: X1 (MPI) and X2 (DP). The MP277 in this scenario is wired to the DP port only. The PC Adapter typically connects to the MPI port. If both devices are landed on the same Profibus segment — for example, by an MPI-to-DP bridge that was supposed to be a Y-cable — the topology changes and the termination rules change with it. Always draw the bus before debugging it.

Authoritative documentation to keep on hand during the procedure:

• SIMATIC S7-300 CPU 314C-2DP device manual (entry ID 8863991)
• SIMATIC MP277 HMI device manual (entry ID 26996247)
• Profibus DP connector 6ES7 972-0BA12 manual (entry ID 1117740)
• WinCC Flexible 2008 communication manual (entry ID 18796052)
• Profibus network design guidelines (entry ID 29042356)

Error Symptoms and Indicators

The fault presents a consistent set of indicators that, taken together, point to a physical-layer / bus-parameter issue rather than a CPU logic fault.

1. WinCC Flexible runtime error: "PLC rack 0 slot 2 disconnected" appears in the alarm line or status bar of the MP277 project.
2. Tag display: All process tags that reference the S7-300 CPU show "#####" (out of range / no connection). Internal HMI tags continue to work.
3. Communication works only with PG attached: When the PC Adapter A2 is connected, the MP277 establishes and maintains its DP connection. This is the diagnostic signature of the fault.
4. Communication drops on adapter removal: Unplugging the PC Adapter causes the HMI connection to drop within one bus cycle (typically 1–2 seconds at 1.5 Mbit/s, the standard Profibus DP rate for short segments).
5. Accessible Nodes (STEP 7): With the PC Adapter attached, "Accessible Nodes" lists both the S7-300 CPU and the MP277. The bus is physically present and addressing is correct; the issue is link stability, not node visibility.
6. CPU 314C-2DP LED status: SF (red) is typically OFF. BF (red, DP, on port X2) is the most useful indicator. Solid ON means no token; flicker means intermittent noise; OFF means the link is healthy.
7. MP277 status bar: Cycles between "Connecting…" and "Disconnected". The HMI's internal diagnostic view (Control Panel → System → Profibus Diagnostics) shows the bus error counter incrementing on each drop.

Symptoms #3 and #4 are the most diagnostic. They rule out PLC logic, project download integrity, and address conflict as the primary cause, and they point to signal integrity — bus termination, cable reflections, or shield/ground potential differences between the PLC cabinet and the HMI cabinet.

Root Cause Analysis

The central clue is that the PC Adapter "fixes" the communication. The PG is not a Profibus terminator, so it does not stop reflections in the electrical sense. What the PG does is provide a parallel signal path on the bus that can mask marginal cable and termination problems. The PG is, in effect, a stub of intermediate impedance that the master and slave see as a small additional load. On a marginal bus, that load is enough to dampen the reflection pattern enough for the bus to lock.

When the PG is removed, the underlying physical-layer fault is no longer masked and the HMI drops its connection. The five root causes that produce exactly this signature are listed below in order of statistical frequency from Siemens field service reports on this exact symptom.

Diagnostic Procedure

Follow this procedure in order. Each step produces a pass/fail result that determines whether to move to the next step or apply the fix from the current step. Do not skip steps — root cause #1 (termination) and root cause #6 (WinCC Flexible online column) are the only ones that can be diagnosed from the office; everything else requires a multimeter at the cabinet.

Step 1 — Capture the LED State on the CPU 314C-2DP

1. Power up the PLC with only the HMI on the DP segment. PC Adapter disconnected.
2. Observe the LEDs on port X2 (DP):
   • SF (red) — solid ON: CPU fault; no relation to the bus problem. Read the diagnostic buffer first.
   • BF (red, DP) — solid ON: bus fault, no token, cable or termination problem.
   • BF flickering: intermittent bus errors, suspect noise, termination, or baud rate.
   • BF OFF: DP link active; the problem is above the physical layer (configuration, project, address).
3. Open STEP 7 → Online → Accessible Nodes. If the CPU appears with the PC Adapter attached but the BF LED is solid ON without the adapter, the physical layer is open.

Step 2 — Verify the Profibus Connector Switch Positions

1. Locate the two 9-pin Sub-D Profibus connectors (one on CPU X2, one on MP277 DP port).
2. Each connector has a slide switch labelled "ON / OFF" for the integrated termination resistor.
3. For a 2-node segment, BOTH switches must be ON.
4. If a third device (PC Adapter) is inserted on the same segment, its switch MUST be OFF.
5. Open each connector by unscrewing the strain relief. Verify that the incoming cable cores are landed on A1/B1 and the outgoing cores on A2/B2 — this is the single most common wiring error in the field.

Step 3 — Check Shield Termination and Ground Bonding

1. Confirm the cable shield is clamped 360° in each connector (not just twisted to pin 8).
2. Confirm the shield is bonded to the cabinet PE bar at the entry point of each cabinet (PLC cabinet and HMI cabinet).
3. Measure AC and DC voltage between the two cabinet PE bars. Difference must be < 1 V. If higher, install an equipotential bonding conductor (≥ 16 mm² Cu) between the two PE bars.
4. Inspect the shield for current: with the bus running, a healthy shield has < 50 mA of AC current. Higher current indicates a ground loop — find and break the second ground path.

Step 4 — Inspect the Profibus Cable

1. Visually inspect the cable for kinks, crushed sections, tight bends (≤ 8× cable diameter), or proximity to VFD outputs and motor cables.
2. Measure the loop resistance of A–B with the far end disconnected: typical value is 110–130 Ω for 150 Ω cable terminated at the far end, or open if the far end is also open.
3. If the cable runs within 200 mm of a VFD output, replace with 6XV1 830-3FH10 (Profibus FC) which has a higher shield coverage ratio (typically > 80% braid coverage plus foil).
4. Check that the cable tray is bonded to PE at both ends. Floating trays are a common cause of intermittent BF flicker.

Step 5 — Verify Profibus Address and Baud Rate

1. Open STEP 7 → HW Config → CPU 314C-2DP → Properties → Profibus Interface. Note the configured baud rate and Profibus address.
2. Open WinCC Flexible → Project → Connections → Connection_1. Verify the Profibus address matches what the CPU expects and the baud rate matches.
3. Confirm no address conflict: each Profibus address on a segment must be unique. Default addresses: CPU = 2, MP277 = 1, PG = 0. The PG address must not collide with a slave address when the PG is attached.

Step 6 — Check the HMI Project Configuration

1. In WinCC Flexible, open the project, navigate to Project → Connections → Connection_1.
2. Set the "Online" column to ON. This is the #1 cause of the panel showing the configuration but never going online.
3. Confirm the area pointers (Job mailbox, Coordination, Date/Time, Project ID) are defined.
4. Save the project and re-transfer to the panel. A configuration change in WinCC Flexible must be re-transferred before it takes effect on the MP277.

Step 7 — Check PG/PC Interface Assignment (when using the PC Adapter)

1. Open SIMATIC Manager → Options → Set PG/PC Interface.
2. Select "PC Adapter (MPI)" for MPI access or "PC Adapter (PROFIBUS)" for DP access — never both at the same time.
3. If the access point is set incorrectly, STEP 7 will find the CPU only when the adapter is the only device on the bus, while the HMI on the DP port is invisible to the same adapter.
4. Confirm the PC Adapter's own Profibus address (default 0) does not collide with the HMI's address.

Termination Resistor Configuration Deep-Dive

The bus termination rule is non-negotiable. A Profibus DP segment is a single transmission line that must be terminated at both ends with 220 Ω in series with 390 Ω pull-up and 390 Ω pull-down, giving the network its characteristic 150 Ω match. Without termination, signal reflections corrupt every bit transition and the segment becomes unusable beyond a few metres.

For the topology in question — one CPU, one HMI — the wiring is direct, point-to-point, with the cable forming one segment whose two endpoints are the PLC and the HMI connectors. The slide switch on each connector engages the 220 Ω + 390 Ω network across the A and B lines.

To mechanically confirm the switch is engaging:

1. Remove both connectors from the devices.
2. With a multimeter on continuity, measure resistance from A1 to A2. With switch ON, you should read approximately 220 Ω. With switch OFF, you should read open (OL).
3. Repeat the measurement for B1 to B2.
4. If a reading is open with the switch in the ON position, replace the connector. The 6ES7 972-0BA12-0XA0 has a known failure mode where the slider detent wears and the resistor remains open-circuit.

Reference: Profibus DP connector 6ES7 972-0BA12 manual (Siemens ID 1117740).

Cable, Connector, and Shielding Verification

Profibus cable is 150 Ω twisted pair, shielded, with a defined capacitance (≤ 30 nF/km) and attenuation budget. The Siemens 6XV1 830-0EH10 cable supports the maximum segment lengths below at each baud rate.

Shield termination must be 360° in each connector. The shield drain wire soldered to pin 8 is not adequate on its own; the shield must be clamped to the connector housing by the strain relief, and the connector housing must be bonded to the cabinet PE at the panel-mount side. If the shield is broken at any point, the entire segment becomes a common-mode antenna and the BF LED will flicker at low rates and latch solid at higher rates.

Reference: Profibus network design guidelines (Siemens ID 29042356).

Profibus Address and Bus Parameter Verification

A Profibus segment requires unique addresses for every device. The CPU 314C-2DP defaults to address 2, the MP277 to address 1, and the PC Adapter to address 0. None of these may overlap.

If the HMI address is set to the same value as the CPU (for example, both at 2), the BF LED will be solid ON and no node will accept the master's tokens. The HMI will show "#####" because it is being polled at the master's address, which is itself. The address check from the panel side on the MP277 is straightforward:

1. On the MP277, go to Control Panel → Transfer → Channel Configuration.
2. Confirm the Profibus address and baud rate.
3. Confirm Enable Channel = ON.

The address check from STEP 7 is:

1. Open STEP 7 → HW Config → DP master system.
2. Confirm the MP277 is in the master system with the correct GSD file (Siemens AG → HMI → MP277 → 6AV6 643-0CD01-1AX1).
3. Confirm no address conflict: every slave must have a unique Profibus address.

HMI Project Configuration Checks

Beyond the physical layer, the WinCC Flexible project must be correctly configured to allow the panel to go online. The most common configuration error on the MP277 is that the connection is defined but not enabled.

1. Open WinCC Flexible → Project tree → Connections → Connection_1.
2. In the "Online" column, set the value to ON. If the column is empty or shows "--", the connection is configured but disabled.
3. Verify the "Communication driver" is set to "SIMATIC S7 300/400" (NOT "SIMATIC S7-200" — this is a frequent error when a different PLC has been used in the same project before).
4. Verify the "HMI station" address matches the panel's Profibus address.
5. Verify the "PLC station" address matches the CPU's Profibus address.
6. Verify the "Profile" is "Profibus" or "MPI/Profibus" — not "MPI only" if the CPU's X2 port is the DP port.
7. Verify the area pointers (Coordination, Job mailbox, Date/Time PLC) are defined and pointed to valid DB / bit memory in the CPU.

Reference: WinCC Flexible 2008 communication manual (Siemens ID 18796052).

PG/PC Interface and Adapter Behaviour

The "PG fixes the bus" symptom often confuses engineers who expect the PC Adapter to act as a terminator. The PC Adapter A2 has no on-board Profibus termination resistor. It is a pure repeater / transceiver that converts RS-485 levels to USB or RS-232. What it does contribute to the bus is:

• A small additional stub impedance at the connection point, which can dampen marginal reflection patterns.
• A second ground reference (via the USB or PC chassis), which can rebalance a marginal ground potential difference between the two cabinets.
• DC loading on the bus that pulls the idle voltage into a more centred range.

None of these effects is documented in the PC Adapter manual, and none of them is reliable as a permanent fix. They are diagnostic — they tell you the bus is marginal, not that the PC Adapter is the solution. Once the PC Adapter is removed, the underlying problem reasserts itself.

The second source of confusion is the PC Adapter's MPI/DP switch in the Set PG/PC Interface dialog. If the access point is set to "PC Adapter (MPI)" but the cable is wired to the DP port, STEP 7 will not find the CPU even though the HMI on the same segment is still visible. The reverse is also true. Always confirm the access point matches the physical port the adapter is plugged into.

Resolution and Verification

Apply the fixes from the diagnostic steps in order. After each fix, perform the verification below before moving to the next step.

Verification Procedure

1. Power down the PLC and HMI.
2. Confirm the PC Adapter is disconnected.
3. Apply the candidate fix (for example, flip the termination switch on the HMI connector to ON).
4. Power up the PLC first, then the HMI.
5. On the HMI, start the runtime. Watch the status bar.
6. Expected: "Connecting…" → "Connected" within 10 seconds. No "PLC rack 0 slot 2 disconnected" alarm.
7. Touch a tag with a live value (for example, MW0 set to 1234 in the PLC). The HMI should display 1234, not "#####".
8. Cycle power on the HMI three times. The connection must re-establish every time without manual intervention.
9. Power down the HMI, leave it disconnected for 30 minutes, then re-apply power. The connection must still re-establish.

Resolution Sequence (most common fix first)

1. Set both Profibus connector switches to ON (PLC end and HMI end). This alone fixes the majority of reported cases.
2. Enable the Online column in the WinCC Flexible connection. Often combined with #1.
3. Re-terminate the shield with 360° clamp at both connectors.
4. Install equipotential bonding between cabinets if PE-to-PE voltage is > 1 V.
5. Replace the Profibus cable if any visual damage or proximity to VFD is observed.
6. Lower the baud rate if the segment length is near the limit for the configured rate.
7. Replace the Profibus connector if the resistor network measures open with the switch in ON.

Diagnostic Tooling Reference

Several tools can speed up the diagnosis. The most useful in this scenario are listed in priority order.

Replacement CPU Migration Checklist

If, after all of the above, the CPU is replaced as a last resort (as in the original case history that motivated this article), perform the following steps to confirm the new CPU is healthy and to leave the panel in a maintainable state.

1. Confirm the replacement part number matches the original: for example, 6ES7 314-6CG03-0AB0 → 6ES7 314-6CG03-0AB0 (NOT -6CG04 or -6EH04 unless the project was migrated).
2. Confirm the firmware version is compatible with the project: STEP 7 V5.5 supports CPU 314C-2DP up to firmware V3.3. The 6EH04 variant requires STEP 7 V5.5 + SP4 or TIA Portal.
3. Download the hardware configuration to the new CPU using the PC Adapter.
4. Download the WinCC Flexible project to the MP277 (full compile → transfer). A recompile is required after any change to the S7 project that touches the address area.
5. Perform the verification procedure above.
6. Document the Profibus addresses of every device on the panel inside the cabinet door. This is the single most useful piece of documentation for the next engineer.
7. Update the as-built drawings with the new CPU serial number, firmware version, and Profibus topology.
8. Save a backup of the STEP 7 project, the WinCC Flexible project, and the new CPU's serial number to the plant's document management system.

Reference: SIMATIC S7-300 CPU 314C-2DP manual (Siemens ID 8863991).

Preventive Maintenance Recommendations

Once the fault is resolved, the following actions prevent recurrence.

1. Torque-check the Profibus connector strain reliefs at every scheduled maintenance. Loose strain reliefs cause intermittent shield contact, which produces exactly the symptom in this article.
2. Inspect the slide switch detents on every Profibus connector. Replace any connector with a worn switch that does not positively engage the ON or OFF position.
3. Measure shield current annually with a clamp meter. A reading above 50 mA indicates a ground loop that will eventually cause communication drops.
4. Verify the equipotential bonding between cabinets annually. PE-to-PE voltage above 1 V must be corrected by re-landing the bonding conductor.
5. Document the Profibus topology in the cabinet door, including addresses, baud rate, and termination switch positions. Engineers arriving on a future fault will be able to verify the topology in 30 seconds.

Frequently Asked Questions