Overview
This reference documents a complete field-tested procedure for establishing communication between a Siemens MP 277 Multi Panel and a SIMATIC S7-300 CPU 314 using either the MPI interface or a PROFIBUS DP segment. Both devices ship with an integrated MPI/PROFIBUS interface (X1, 9-pin sub-D female) on the front face, so a direct peer-to-peer link can be built with nothing more than a single PROFIBUS FC cable, two PROFIBUS bus connectors, and a properly configured HMI project.
For most stand-alone machines the HMI-to-CPU link is exactly that simple: a bus segment with terminators enabled at both ends, no DP slaves, no repeaters, and no STEP 7 / NetPro required when the WinCC Flexible project is authored as a stand-alone (non-integrated) application. When WinCC Flexible is launched from inside STEP 7 Professional, the connection is generated automatically from NetPro. The procedure below covers both paths, the resistor theory, the connector pinout, the WinCC Flexible 2008 connection dialog, the area pointers that must be enabled, and a diagnostic matrix for first-power-on faults.
System Identification
Confirm the exact catalog numbers on the front panels before you wire anything. The same MLFB is often re-released with several firmware ranges, and not every firmware accepts every baud rate on the X1 port.
| Device | MLFB (typical) | Firmware range | Interface X1 | Default address |
|---|---|---|---|---|
| S7-300 CPU 314 | 6ES7 314-1AG14-0AB0 | V3.3 (and later) | MPI / PROFIBUS DP master | 2 |
| S7-300 CPU 314C-2 DP | 6ES7 314-6CG03-0AB0 | V3.3 | MPI/DP combo + integrated DP | 2 |
| MP 277 8" Touch | 6AV6 643-0CB01-1AX1 | WinCC Flexible 2008 | RS-485 (MPI/PROFIBUS DP slave) | 1 |
| MP 277 10" Touch | 6AV6 643-0CD01-1AX1 | WinCC Flexible 2008 | RS-485 (MPI/PROFIBUS DP slave) | 1 |
| MP 277 10" Key | 6AV6 643-0CD01-1AX2 | WinCC Flexible 2008 | RS-485 (MPI/PROFIBUS DP slave) | 1 |
Verify the CPU 314 firmware by selecting PLC → Module Information → Online in STEP 7 or by reading the diagnostic buffer entry on cold start. If the panel is a 177- or 277-class device, the project must be compiled in WinCC Flexible 2008 SP3 or later; older WinCC Flexible versions (e.g. 2005) cannot generate runtime files for the MP 277. Use the menu Project → Compiler Version to confirm.
Selecting the Physical Link: MPI or PROFIBUS DP
Both devices expose a single shared RS-485 port (X1). The protocol selected at the CPU is independent of the protocol selected at the HMI, but they must agree:
| Link type | CPU port mode | HMI port mode | Default baud rate | Max segment length | Notes |
|---|---|---|---|---|---|
| MPI peer-to-peer | MPI (default) | MPI | 187.5 kbps | 50 m at 1.5 Mbps, 1000 m at 187.5 kbps | No programming device required; works out of the box |
| PROFIBUS DP single master | DP master | DP slave | 1.5 Mbps | 200 m at 1.5 Mbps, 1200 m at 93.75 kbps | CPU must be reconfigured in HW Config |
| PROFIBUS DP shared master | DP master with HMI on same bus | DP slave | 1.5 Mbps | 200 m at 1.5 Mbps | Combine drives and I/O on the same segment |
For a brand-new machine with no DP slaves, leave the CPU port set to MPI. The CPU ships in MPI mode; the panel ships in MPI mode; nothing has to be reconfigured in HW Config. Switch the CPU to DP master only if you intend to add ET 200S / ET 200M stations or inverters on the same segment later, because that path requires deleting and reinserting the CPU 314 in the STEP 7 project.
Hardware Prerequisites
- CPU 314 with firmware V3.3 or later. Earlier firmware supports the same X1 pinout but lacks some of the later MPI optimizations.
- MP 277 8" or 10" touch or key version, loaded with the WinCC Flexible 2008 runtime that matches the compiled project (touch or key).
- PROFIBUS FC cable (6XV1 830-0EH10, violet jacket) or the original grey RS-485 PC/MPI cable that ships with some panel kits (max length 1.5 m, no terminators, capped at 187.5 kbps).
- Two PROFIBUS bus connectors with terminating resistor switch (6ES7 972-0BA12-0XA0, 35° cable outlet, or the 90° 6ES7 972-0BA52-0XA0). One at each end of the segment.
- STEP 7 V5.5 + SP3 (or STEP 7 Professional in TIA Portal V13+ for integrated projects) and WinCC Flexible 2008 SP3 or later.
PROFIBUS Cable and Connector Pinout
The 9-pin sub-D X1 socket on both devices is wired per the PROFIBUS DP RS-485 standard. Pins 3 and 8 carry the differential bus signal; the shield is bonded through the connector housing; pins 5 and 6 provide the 5 VDC isolated bus terminator supply sourced by the connector itself.
| Pin (sub-D, female) | Signal | Function | Wire color in PROFIBUS FC |
|---|---|---|---|
| 1 | Shield | Bonded via housing; do NOT connect at multiple points | Braid / foil |
| 2 | M24 | 24 V ground (not used for data) | — |
| 3 | RxD/TxD-P (B) | Positive data line | Red |
| 4 | RTS-P | Request-to-send, TTL | — |
| 5 | DGND | Data ground (isolated) | — |
| 6 | VP (+5 V) | Terminator supply | — |
| 7 | P24 | +24 V (not used for data) | — |
| 8 | RxD/TxD-N (A) | Negative data line | Green |
| 9 | RTS-N | Request-to-send, TTL inverted | — |
Strip the FC cable jacket 20-25 mm, fold the shield back over the jacket, and secure the connector strain-relief clamp onto the folded shield; this is the only way the 360° shield bond is made. The shield must not be soldered to pin 1; the housing clamp is the engineered bond.
Terminating Resistor Theory and Practice
RS-485 segments are high-impedance, edge-rate-sensitive busses. Without termination the rising and falling edges reflect from the open ends of the cable and corrupt the differential signal at 1.5 Mbps and above. The PROFIBUS bus connector integrates the standard 3-resistor network:
| Resistor | Value | Function |
|---|---|---|
| Pull-up to VP (pin 6) | 390 Ω ± 5% | Biases line A to +5 V when bus is idle |
| Pull-down to DGND (pin 5) | 390 Ω ± 5% | Biases line B to 0 V when bus is idle |
| Series between A and B | 220 Ω ± 5% | Dampens reflections at the cable end |
The switch on the top of the connector inserts or removes this network. Activate the switch only on the two physical end nodes of the segment — in this case, the CPU 314 and the MP 277. If the panel sits in the middle of a future daisy-chain, leave its switch in the OFF position; otherwise the bus voltage levels are pulled out of spec and the master will report "Bus fault" or "Slave diagnostic".
Bus Topology Diagram
STEP 7 / NetPro Configuration (Integrated Projects Only)
Open NetPro only when WinCC Flexible is integrated with STEP 7 — that is, the panel station is visible in the SIMATIC Manager project tree and the WinCC Flexible ES is started from the "HMI Station" object. In that case the connection is already modeled and you do not need to set addresses manually.
- Open SIMATIC Manager → HMI Station → HMI_1 → Edit. The HMI station opens with its X1 MPI/PROFIBUS interface preconfigured.
- Open NetPro on the S7 project. A purple S7 connection line is drawn automatically between the HMI station and the CPU 314 sub-net. Verify the connection endpoint parameters:
- Local ID: any unused integer, e.g. 1
- Partner address: 2 (CPU MPI default)
- Slot: 2 (CPU 314 default)
- Rack: 0
- Click Compile and Save All. The connection SDBs are downloaded with the S7 program on the next PG upload to the CPU.
If the WinCC Flexible project is stand-alone (no STEP 7 integration), skip NetPro entirely. The runtime connection is defined inside WinCC Flexible and the CPU 314 just runs its default MPI settings (address 2, 187.5 kbps). The CPU does not need any project-side connection configuration to answer HMI requests because the panel uses standard MPI S7 communication that the CPU's firmware handles natively.
WinCC Flexible 2008 Connection Configuration
This section covers the stand-alone (non-integrated) flow, which is the more common path for retrofit and small-machine work.
- In the WinCC Flexible project tree, expand Communication → Connections. Right-click → New Connection.
- Set the connection parameters:
| Parameter | Value (MPI) | Value (PROFIBUS DP) | Notes |
|---|---|---|---|
| Name | S7-300_CPU314 | S7-300_CPU314_DP | Free-form |
| Driver / Communication type | MPI | PROFIBUS DP | Select from the drop-down |
| PLC device | S7-300 / S7-400 | S7-300 / S7-400 | Determines the protocol frame |
| Station address (HMI) | 1 | 1 | Must be unique on the bus |
| Station address (PLC) | 2 | 2 | Matches CPU MPI / DP address |
| Highest station address (HSA) | 15 | 15 | Default; do not exceed 126 |
| Baud rate | 187.5 kbps | 1.5 Mbps | Must match the CPU port rate |
| Rack / Slot of CPU | 0 / 2 | 0 / 2 | CPU 314 default slot |
- Enable the area pointers in the same dialog under the "Area Pointers" tab. At a minimum enable Coordination, Date/Time, and PLC Job. Screen Number is needed for advanced Activate Screen By Number use cases.
- Click Compile → Build Runtime and transfer the resulting
.fwxto the MP 277 via RS-232, Ethernet, or USB stick.
Area Pointer Layout in the CPU 314
WinCC Flexible reserves the following DB numbers in the CPU 314. Either reserve them in the STEP 7 symbol table or just leave DB numbers unassigned in the S7 program; the runtime will create them automatically on first transfer.
| Area pointer | Default DB | Length | Direction | Function |
|---|---|---|---|---|
| Coordination | DB 999 (typical, configurable) | 1 byte | HMI → PLC | Life bit, project ID, read-only handshake |
| Date/Time | DB 998 (typical) | 8 bytes | HMI → PLC | BCD-coded clock push to PLC |
| PLC Job | DB 997 (typical) | 4 bytes | HMI → PLC | Trigger job number (e.g. print, archive) |
| Screen Number | DB 996 (typical) | 4 bytes | PLC → HMI | Tell the HMI which screen to display |
| Data Record | DB 995 (typical) | 4 bytes | Bidirectional | Recipe / data record transfer mailbox |
If any of these DBs is already used by the S7 program, change the DB number in the connection properties of WinCC Flexible and use the "Update" button to repoint the pointer. Never remap an existing program DB — the HMI will overwrite the data on every cycle.
First Power-On and Verification
- With the bus connectors unplugged, power up the CPU 314. Confirm the SF and BF LEDs are OFF (BF will be ON if no bus partner is connected — that is normal for now).
- Verify the CPU in STEP 7: PLC → Module Information → Diagnostic Buffer. The first two entries should be "Restart (cold)" and "Restart (warm)" with no bus fault.
- Plug the bus connector into the CPU X1 and slide the terminator switch to ON. Plug the other end into the MP 277 X1 and slide its terminator switch to ON.
- Power up the MP 277. After the loading screen the panel should display its project startup screen and the "Connection Status" indicator (the small icon in the status bar) should turn green within 3–5 seconds.
- On the CPU, check the diagnostic buffer again. A new entry "Connection established: S7 connection to MPI address 1" should appear.
- Open WinCC Flexible 2008 runtime simulation: Start → Programs → SIMATIC → WinCC Flexible 2008 → Runtime Loader. If the offline simulation can drive a tag and the online value changes, the path is healthy.
- Touch a button linked to a PLC tag, e.g. a bit in
DB1.DBX0.0. Use STEP 7 Monitor/Modify on the same tag and confirm the value toggles within one panel update cycle (typical 200–500 ms).
Diagnostic Buffer and LED Indicators
| LED | Device | OFF (healthy) | ON (fault) | Flashing (caution) |
|---|---|---|---|---|
| SF | CPU 314 | No system fault | Hardware or firmware fault; check diagnostic buffer | — |
| BF | CPU 314 | Bus is healthy | No bus partner at configured baud rate, broken cable, or wrong terminator | Bus partner missing or address conflict |
| DC5V | CPU 314 | Internal 5 V OK | Backplane / load supply failed | — |
| RUN | CPU 314 | — | CPU in RUN | CPU in STOP — check SF for cause |
| Communication icon (status bar) | MP 277 | — | Red: no connection | Yellow: connection being established; Green: connected |
Troubleshooting Matrix
| Symptom | Likely root cause | First check | Fix |
|---|---|---|---|
| Panel shows red "no connection" icon, CPU BF solid ON | Terminator OFF at one or both ends, or no bus terminator at all | Look at the slider on both bus connectors | Set both end-node terminator switches to ON |
| CPU BF solid ON, panel yellow "establishing" | Baud rate mismatch between CPU and panel | Read CPU port rate with PG (MPI Diagnostics) | Set WinCC Flexible connection to 187.5 kbps (default) or change CPU port to 1.5 Mbps in HW Config and re-download |
| CPU BF flashing, SF OFF | Duplicate station address on the bus | List every node's MPI address (CPU 2, panel 1, PG 0) | Assign unique addresses 0…15 on the MPI subnet |
| Tags do not update but no alarm on either device | Area pointer DB was overwritten by S7 program; wrong DB number mapped | Watch the HMI tags in WinCC Flexible "Tag Simulation" | Remap Coordination / Date/Time / PLC Job to free DBs |
| Panel starts in transfer mode, never leaves | Wrong .fwx project was transferred (e.g. 177 runtime on 277 panel) | Compare panel MLFB to project → Device → Properties | Recompile project for correct panel type and re-transfer |
| Connection drops after a few hours, BF flickers | Loose connector or shield not bonded | Physically wiggle connectors under power | Re-strip FC cable, reclamp shield, torque connector screws to 0.4 Nm |
| Connection fails only at 1.5 Mbps, works at 187.5 kbps | Use of grey programming cable without terminators | Inspect cable color and length | Replace with violet PROFIBUS FC cable and proper bus connectors |
| CPU enters STOP when HMI is plugged in | Coordination pointer area overlaps an SFC / SFB that the S7 program depends on | Compare the S7 program DB list to the WinCC Flexible pointer DB list | Reassign WinCC Flexible pointer DBs to unused DB numbers |
| Touched button does not toggle a tag, but I/O on the field still work | Tag points to wrong memory area (e.g. bit in IB 0 instead of DB1.DBX0.0) | STEP 7 Monitor/Modify on the HMI tag address | Correct the tag address in WinCC Flexible and re-compile |
Field-Proven Caveats
- Highest Station Address (HSA) must be at least one greater than the highest node on the bus. With CPU 314 at 2, MP 277 at 1, and PG at 0, set HSA = 15 (the MPI default). A mismatched HSA on the master side does not cause an immediate fault but slows token recovery after a node comes back online.
- RS-485 repeaters are unnecessary for a 2-node segment and add a hidden failure mode: the repeater must also be powered, must be assigned a unique address, and breaks the bus if its 24 V supply is removed.
- Cable length limit at 187.5 kbps is 1000 m without repeaters, dropping to 200 m at 1.5 Mbps. For machine cabinets the limit is never the cable; it is always the bend radius and strain relief.
- Shield bonding is single-ended: bond the shield at the cabinet entry plate and isolate it from the panel housing with a plastic sub-D hood if the panel is mounted on a separate, low-impedance machine frame.
- Firmware compatibility: WinCC Flexible 2008 SP5 is the most stable runtime generator for the MP 277 family. SP3 works; SP1 and earlier produce runtime files that fail to load on panels shipped after mid-2009.
- Time sync: the Date/Time area pointer pushes the panel clock into the CPU. If you instead need the panel to display the CPU clock, swap the pointer direction in the "Area Pointers" dialog and clear the HMI checkbox "Synchronize time with PLC" — the panel will instead pull the clock from the PLC.
-
Backup of the project: before any firmware update of the MP 277, copy the running
.fwxto USB. A failed firmware transfer can only be recovered from an external medium; the panel's internal flash is the only thing that fails to re-flash.
Related Configuration Path: TIA Portal V13+
For greenfield projects, the S7-300 + MP 277 pair can also be configured in TIA Portal V13 or later. Add the CPU 314 (V3.3 firmware) and the MP 277 to the Devices & Networks view, draw a connection between X1 on both devices, and the HMI connection parameters are set automatically. TIA Portal handles the WinCC Flexible / WinCC project under the same portal but expects a "WinCC Professional" or "WinCC Comfort" add-on for the MP 277 panel, not the legacy WinCC Flexible runtime. The wiring, baud rate, and terminator rules are identical to the STEP 7 flow described above.
Do I need NetPro if I am using WinCC Flexible stand-alone?
No. NetPro is only required when WinCC Flexible is launched from inside STEP 7 as an integrated project. For a stand-alone WinCC Flexible project the connection is defined entirely inside WinCC Flexible's "Communication → Connections" dialog and the CPU 314 needs no STEP 7-side configuration beyond the default MPI settings (address 2, 187.5 kbps).
What is the default MPI address of the CPU 314 and the MP 277?
The CPU 314 ships with MPI address 2 and the MP 277 with MPI address 1. Both must be left unique on the bus segment; a programming device is typically address 0. The Highest Station Address (HSA) should remain 15 unless the bus is extended past 15 nodes.
Why does the BF LED stay solid ON on the CPU 314?
A solid BF means the master cannot exchange telegrams with the configured partner. The two most common causes on an MP 277 link are missing or wrong termination (both end-node terminator switches must be ON) and a baud rate mismatch (CPU MPI default 187.5 kbps, panel must match). Verify the shield bond and that the cable is PROFIBUS FC (violet), not the grey programming cable.
How do I set the terminating resistor on a PROFIBUS bus connector?
On the 6ES7 972-0BA12-0XA0 connector (and its 90° variant 6ES7 972-0BA52-0XA0) a sliding switch sits on the top face. Slide it toward the cable strain relief to the "ON" position; that inserts the standard 390/220/390 ohm network. Switch ON must be set only on the two physical end nodes of the segment; all other connectors must be in the OFF position or the bus voltage collapses.
Can the original grey MPI cable that ships with the panel be used permanently?
It is rated for commissioning and short bench tests, not for fixed installation. It is approximately 1.5 m long, has no integrated terminating resistor, and the maximum usable baud rate is 187.5 kbps. For production use, replace it with violet PROFIBUS FC cable (6XV1 830-0EH10) and proper 6ES7 972-0BA12-0XA0 bus connectors terminated at both ends.
Which WinCC Flexible version supports the MP 277?
WinCC Flexible 2008 with at least Service Pack 3; Service Pack 5 is the most widely used stable release for the MP 277 family. Older WinCC Flexible versions (2004, 2005) cannot generate runtime files for the MP 277. In TIA Portal V13 and later, the MP 277 is supported by the WinCC Comfort or Professional option.