Resolving TP700 Comfort to S7-300 CPU 315-2 DP MPI Communication Failures

The SIMATIC TP700 Comfort (6AV2124-0GC01-0AX0 and related variants) is one of the most widely deployed 7" HMI panels in mid-range Siemens automation cells. When paired with an S7-300 CPU 315-2 DP (6ES7315-2AH14-0AB0), the typical field wiring path is a PROFIBUS RS-485 segment terminated with 9-pin D-sub bus connectors, even when the project labels the network as MPI. The recurring failure mode is: the panel powers up, the connection editor in TIA Portal V13 (or V13 SP1 / V14) is configured, the PLC and panel are both reachable from the PG via the PG/PC interface, yet the runtime shows ###### on every tag and no cyclic data is exchanged. The bus is physically intact but logically broken. This reference walks through every layer of the failure - cable type, port selection, bus parameters, termination, panel configuration, and the integrated connection approach - so the engineer can isolate and repair the link in a single shift.

1. Problem Description and Field Symptoms

The reported symptom chain is consistent across S7-300 + Comfort Panel field retrofits:

• TP700 Comfort panel boots, shows the configured start screen, but every value field shows ###### instead of the requested DBW, MW, IW, or PEW.
• The connection status indicator in the runtime (when enabled) shows Connection interrupted or No connection to PLC within 3-5 seconds of startup.
• From the PG/PC, the panel is reachable through Online > Accessible nodes with the correct MPI/PROFIBUS address and baud rate, and the CPU 315-2 DP is reachable from the same interface.
• The PLC diagnostic buffer contains no entry related to the HMI; the HMI does not appear in Accessible nodes on the PLC side when queried from STEP 7 classic.
• Manually matching baud rate (e.g., 187.5 kbps for MPI, 1.5 Mbps for PROFIBUS) and Highest Station Address (HSA) between the panel and the PLC has no effect.

The cause is almost always a mismatch between the physical RS-485 segment and the logical bus profile selected in TIA Portal, or a port assignment error on the CPU 315-2 DP, which exposes two interfaces - X1 (MPI/DP combined) and X2 (PROFIBUS-DP) - and requires an explicit selection of Interface type in the device configuration.

2. Hardware Layer: MPI Cable vs. PROFIBUS RS-485 Cable

The single most common root cause in retrofit cells is cable and connector confusion. The terms MPI cable and PROFIBUS cable are often used interchangeably in field vocabulary, but they are not the same product.

2.1 Cable Specification

PROFIBUS DP / MPI networks use a shielded twisted-pair cable with the following characteristics (per IEC 61158 / IEC 61784):

An "MPI cable" sourced separately is often a pre-assembled PC-Adapter cable (e.g., 6ES7901-0BF00-0AA0) terminated at one end with an RS-232 or USB PC-Adapter and at the other with a 9-pin D-sub. This is a point-to-point programming cable, not a PROFIBUS trunk cable, and it does not carry the PROFIBUS 150 Ω characteristic impedance required for multi-drop segments. If the panel has been wired into the trunk using such a cable, every reflection and signal-integrity error will cause cyclic frames to fail.

2.2 PROFIBUS Bus Connectors

Use only PROFIBUS D-sub bus connectors with switchable termination and integrated inductance (Siemens 6ES7972-0BA12-0XA0, 6ES7972-0BB12-0XA0, or 6GK1500-0EA02). The connector has three positions: ON (terminated, entering cable), OFF (terminated, not last node), and OFF (not last node, no termination).

• The two physical end nodes of the RS-485 segment must have termination ON.
• Every node in between must have termination OFF.
• Terminator is 220 Ω pull-up on pin 6 (+5V) to pin 3 (B), 220 Ω pull-down on pin 6 to pin 8 (A), and 390 Ω between pins 3 and 8.

3. CPU 315-2 DP Interface Selection

The S7-300 CPU 315-2 DP exposes two physical interfaces on the front panel:

If the panel is wired to X1 and the project still has X1 set to MPI while the panel is configured as a PROFIBUS node (the default), the bus is logically two separate networks even though the wires are shorted together. In TIA Portal:

1. Open the device view of the CPU 315-2 DP.
2. Select the X1 interface.
3. In Properties > PROFIBUS interface / MPI interface, set Interface type to MPI or PROFIBUS, depending on the panel configuration.
4. Confirm the Address (typical default 2 for CPU X1 MPI).
5. Confirm the Highest MPI/PROFIBUS address (default 31 or 126; HSA must be ≥ the largest node address).
6. Confirm the Transmission rate matches the panel - 187.5 kbps for MPI, 1.5 Mbps or 12 Mbps for PROFIBUS.

4. Bus Parameter Matching (MPI Profile)

For an MPI link between the TP700 and the CPU 315-2 DP X1 port, both devices must share the same bus profile. The decisive parameters are:

If the panel is set to PROFIBUS profile while the CPU X1 is set to MPI, the tokens never get passed and the panel remains in a passive listen state, generating the ###### display.

5. Termination and Shielding

RS-485 termination is mandatory at both physical ends of a PROFIBUS/MPI segment. Mis-termination causes intermittent ###### displays, especially under vibration or EMC stress. Use the Siemens PROFIBUS bus connector diagnostic checklist:

1. Verify termination ON at the CPU X1 connector (or X2 if used).
2. Verify termination ON at the panel connector.
3. Verify the shield of the violet cable is clamped to the connector housing on both ends and bonded to ground at the cabinet entry with a low-impedance bond (≤ 5 Ω to PE).
4. Verify the cable shield is not carried through any intermediate terminals that interrupt the 360° bonding.
5. Verify no third device (a stray PG cable, a HMIsim connector, or a non-terminated tap) is on the segment.

6. TIA Portal V13: PLC Proxy and Integrated Connection

Manual bus parameter matching is error-prone. Siemens recommends using a PLC Proxy device in the HMI project so that the HMI configuration inherits the bus parameters from the PLC project automatically. The reference entry for this procedure is the Siemens Industry Online Support article with ID 73502293.

6.1 Prerequisites

• TIA Portal V13 SP1 (or V14, V15) installed.
• S7-300 CPU 315-2 DP added as a device in the project, with the X1 or X2 interface configured as MPI or PROFIBUS.
• TP700 Comfort (6AV2124-0GC01-0AX0) added to the project.
• HMI tags already created (or planned).

6.2 Step-by-Step: Integrated Connection Setup

1. In the project tree, expand TP700 Comfort > Connections and double-click to open the Connections editor.
2. In the upper toolbar, click HMI connection to create a new connection.
3. For Connection type, select Integrated connection if a PLC Proxy exists in the project. Otherwise select S7 connection and manually set the parameters.
4. Choose Partner: select the CPU 315-2 DP from the project tree. The interface, address, and bus profile are inherited automatically.
5. Verify Connection path shows TP700 Comfort > MPI/DP > CPU 315-2 DP > MPI/DP. Both endpoints must reference the same physical interface role.
6. Compile the HMI project and download to the panel.
7. Download the PLC project to the CPU 315-2 DP.
8. Power-cycle the panel or trigger a connection re-init from the panel service menu (Control Panel > Transfer > Connect PLC).

6.3 Manual S7 Connection Parameters

If a PLC Proxy cannot be created (e.g., the HMI project is shipped without the PLC project), use a manual S7 connection with these values:

7. Panel-Side Configuration on the TP700 Comfort

Outside of the project, the Comfort panel also has runtime-side network settings that must match the project. Open Control Panel > Network and Dial-Up Connections on the panel:

1. Tap PROFIBUS (the panel's only fieldbus port).
2. Confirm Address matches the project (default 1).
3. Confirm Transmission rate matches the project.
4. Confirm Highest station address matches the project.
5. Tap PROFIBUS Profile and select MPI if connecting to a CPU X1 MPI port, or DP if connecting to a PROFIBUS-DP port.
6. Save and reboot.

8. PG/PC Interface Diagnostic Procedure

To prove which interface is healthy and isolate the failure:

1. Open Control Panel > Set PG/PC Interface on the engineering station.
2. Select the PC-Adapter (USB) or CP 5611/CP 5711 interface and confirm Accessible nodes finds both the CPU 315-2 DP and the TP700 Comfort.
3. Note the baud rate and HSA reported for each. They must be identical.
4. If one node reports a different baud rate, the panel's physical switch or runtime configuration is overriding the project - reset via Control Panel > Network.
5. If neither node is visible, the bus is physically broken: check terminator power (pin 6 = 5V) with a multimeter, then check A/B line continuity.

9. Configuration of the PG/PC Interface for MPI

The PG must be configured to the same bus parameters as the panel and PLC. In Set PG/PC Interface:

10. Verification Checklist

Walk through this list after every change:

1. PG/PC sees both nodes in Accessible nodes at the correct baud rate and HSA.
2. CPU diagnostic buffer shows no Distributed I/O fault or Partner not reachable entries.
3. Panel connection editor shows the S7 connection in the Online > Connections view with status Connected.
4. One PLC tag (e.g., DB1.DBD0 = a counter) is bound to an HMI tag of type Int or Real.
5. On the panel, the corresponding IO field shows a numeric value, not ######.
6. The connection-status indicator on the panel (if enabled in HMI > Settings > Connection status) is green.
7. Disconnection test: pull the PROFIBUS connector on the panel; the connection-status indicator turns red within 5 s and the IO fields return to ######.

11. Failure Mode and Effects Matrix

12. Alternative: Migrate from MPI to PROFIBUS-DP

If the MPI link remains unstable after addressing all of the above, the most reliable migration path is to re-profile the network as PROFIBUS-DP:

1. In the device configuration of the CPU 315-2 DP, change X1 from MPI to PROFIBUS (or use X2 if X1 must remain MPI for legacy reasons).
2. Set the CPU to DP master with address 2.
3. In the HMI project, change the connection partner interface to PROFIBUS and set the baud rate to 1.5 Mbps.
4. On the TP700, change PROFIBUS Profile to DP at 1.5 Mbps.
5. Recompile and redownload both devices.
6. Verify from the PG: both nodes visible, HSA 126, no diagnostic buffer faults.

PROFIBUS-DP offers higher diagnostic granularity, support for more slaves per segment, and compatibility with all Comfort, Basic, and Mobile panels.

13. Firmware and Compatibility Notes

Always confirm firmware versions in Online > Accessible nodes > Flash LED on the panel before assuming the panel will accept the project binary.

14. Long-Cable and EMC Field Practices

For segments approaching 500 m at 187.5 kbps or 150 m at 1.5 Mbps:

• Use a single uninterrupted violet cable run. Splices introduce reflections.
• Maintain ≥ 200 mm separation from VFD power cables (motor cables from SINAMICS G120, MicroMaster, etc.).
• Bond the cable shield at every cabinet entry using EMC cable glands (e.g., Lapp EMC brush glands).
• Avoid daisy-chaining more than 32 nodes on a single segment. Use an OLM/OLP or RS-485 repeater (Siemens 6GK1500-0EA02) to extend the bus.
• Verify that no unshielded patch cables are used between the trunk and the device D-sub connector.

15. Documentation References

• Siemens Support Entry 73502293 - Configuring a connection between HMI and PLC (TIA Portal)
• SIMATIC TP700 Comfort Operating Instructions
• S7-300 CPU 315-2 DP Manual
• TIA Portal Communication - SIMATIC HMI HTTP (Panels Comfort / RT Advanced / RT Professional)
• SIMATIC HMI Panels Commissioning

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