Configuring S7-315-2DP MPI/DP Port as Profibus DP in STEP 7

Overview of the CPU 315-2DP Communication Interfaces

The Siemens SIMATIC S7-315-2DP (order number 6ES7 315-2AG10-0AB0) is a 128 KB work-memory CPU in the S7-300 family that ships with two physical Profibus-compatible connectors on its front panel. The exact protocol capability of each connector depends on the MLFB variant, and confusing the two ports is the single most common reason an integrator cannot find the DP configuration where they expect it.

For the specific part number 6ES7 315-2AG10-0AB0:

The CPU 315-2DP implements Profibus DP per IEC 61158 / IEC 61784 (CP 3/1, CP 3/3). Maximum supported baud rate on IF2 is 12 Mbit/s; IF1 is limited to 187.5 kbit/s MPI. Up to 32 stations can be addressed per Profibus segment, expandable to 125 stations through up to four RS-485 repeaters or optical link modules (OLM/OBT). The CPU itself counts as one station in the master system whether it operates as master or slave.

Prerequisites

Before opening HW Config, gather the following so the configuration download succeeds on the first attempt and a re-engineering loop is avoided:

• SIMATIC STEP 7 V5.5 SPx with HW Config installed (or STEP 7 Professional / TIA Portal V13 or later for V13+ projects). The S7-300 hardware support package (HSP) matching your CPU MLFB must be installed via Options > Install HW Updates. Without the correct HSP the catalog will not list the CPU or its interface submodules.
• GSD/GSDML files for any third-party Profibus slaves (ET200S, drives, valves, third-party sensors). GSD files are the device description files STEP 7 uses to represent slaves in the hardware catalog. They must be installed before the slave can be inserted into the DP master system.
• Profibus cable (twisted, shielded, violet sheath per the Profibus cable recommendation) terminated with D-sub 9-pin connectors wired per the Profibus standard. Terminating resistors ON only at the two physical ends of the segment; intermediate stations must have resistors OFF.
• Pre-planned bus addresses. The CPU's DP interface (IF2) defaults to address 2 for the master. Slave addresses typically start at 3 and increment; addresses 0, 1, and 126 are reserved for special functions (1 = class-2 master or diagnostic repeater; 126 is the default "no address" auto-assign for unconfigured slaves).
• CPU firmware version compatible with STEP 7 version. CPU 6ES7 315-2AG10-0AB0 ships with firmware V2.x; older STEP 7 versions (V5.1, V5.2) may not recognize the device without an HSP. The firmware version is visible in the CPU's diagnostic buffer at power-up.
• Online connection via MPI cable (PC-Adapter USB, CP 5512, CP 5711) or Ethernet CP for downloading the hardware configuration to the CPU. The download path uses the MPI port (IF1); Profibus is for production traffic only after configuration.

Identifying the MPI/DP Port in HW Config

The single most common reason integrators cannot locate the DP configuration is that they opened the CPU properties dialog and assumed it represents both interfaces. The CPU 315-2DP has two distinct interface objects in the rack:

1. The CPU module itself (labeled, e.g., "CPU 315-2DP"). Its properties dialog shows module-level information (cycle time, retentive memory, diagnostics buffer, and only the MPI/DP port for IF1 on this MLFB).
2. The interface submodule objects attached to the CPU slot, represented as small labeled blocks to the right of the CPU in the rack. These are individual objects: IF1 (MPI only on this CPU) and IF2 (DP master/slave).

To view the Profibus DP configuration on IF2:

1. In SIMATIC Manager, open your project and double-click Hardware to launch HW Config.
2. In the rack view, locate the CPU 315-2DP on slot 2. To its right you will see two small DP-capable blocks labeled for the two physical interfaces.
3. Close any open properties dialog for the CPU itself (click OK or Cancel).
4. Double-click the lower interface block — this is IF2 / X2 / Profibus DP. The "Properties — PROFIBUS Interface" dialog opens.
5. Click the Properties button on the General tab to access the address, subnet, and operating-mode settings.

Step-by-Step Configuration in STEP 7 HW Config

This is the canonical procedure for setting IF2 of the CPU 315-2DP to Profibus DP. For combined MPI/DP variants (such as the 315-2 PN/DP with MLFB 6ES7 315-2EH13-0AB0), the same procedure applies to whichever interface block is labeled "MPI/DP". Reference: STEP 7 V5.5 System Manual.

1. Open HW Config. From SIMATIC Manager, expand your S7-300 station, then double-click the Hardware icon. HW Config loads the rack with the CPU on slot 2.
2. Locate the DP interface object. In the rack view, identify the interface submodule below the CPU. Right-click and select Object Properties (or simply double-click). Confirm the dialog title reads "Properties — PROFIBUS Interface" (not "Properties — MPI Interface").
3. Assign a Profibus subnet. In the General tab, the "Subnet:" field shows the current subnet binding. To create a new Profibus subnet, click New..., set:
   • Subnet name (e.g., "DP-Master-System-1")
   • Transmission rate (typically 1.5 Mbit/s for mixed-vendor networks; 12 Mbit/s for Siemens-only)
   • Highest station address (HSA, default 126)
   • Profile: "DP" (master or slave mode is selected in the next step)
   Click OK. The new subnet appears as a vertical line attached to the IF2 interface object.
4. Set the master DP address. Switch to the Parameters tab (or "Address" tab in some STEP 7 versions). Set:
   • Profibus address: default 2 for the master. Valid range 1 to 125, but addresses 0, 1, and 126 are reserved for special functions.
   • Operating mode: select DP Master or DP Slave. The selection unlocks or hides the Configuration tab accordingly.
   Click OK.
5. For DP Master mode only: Switch to the Configuration tab. The right pane ("Configured Slaves") is empty until you insert DP slaves. The "Operating Mode Parameters" section exposes the master's bus timing — keep defaults unless you have specific cycle-time requirements. Click OK to commit.
6. Insert DP slaves. Open the hardware catalog on the right side of HW Config. Navigate to PROFIBUS DP > [Vendor] > [Device family]. If the slave vendor is not listed, install its GSD file via Options > Install GSD Files... and rebuild the catalog. Drag the slave icon onto the Profibus subnet line. In the resulting "Properties — PROFIBUS Interface" dialog for the slave, assign a unique bus address (3, 4, 5, ...) and confirm.
7. Map slave I/O. Double-click the inserted slave icon to open its properties. Note the configured slot and the I/O addresses STEP 7 reserves (e.g., PIW 256 / PQW 256 for a 4-word analog slave). Adjust the start address to avoid overlap with your program's I/O usage.
8. Save and compile. Press Ctrl+S, or Station > Save and Compile. STEP 7 generates the system data blocks (SDB) that the CPU needs at startup.
9. Download to CPU. Connect online via MPI or Ethernet CP. Select the CPU in HW Config and click the Download to Module icon (or PLC > Download). Confirm the target module. The CPU goes to STOP briefly during download.
10. Restart the CPU. After successful download, switch the CPU to RUN. The DP interface initializes; the BF (bus fault) LED should extinguish if the slave is present and correctly addressed.

Configuring DP Master Mode

In DP master mode the CPU 315-2DP polls its configured slaves cyclically, exchanges process data, and handles diagnostic events. The master accepts up to 32 slaves per segment (124 with repeaters) and supports acyclic services (DP-V1 read/write) provided the slaves are DP-V1-capable and the master is configured accordingly. The master automatically detects the protocol version of each slave through the parameterization frame and uses DP-V0, DP-V1, or DP-V2 features as the slave supports.

When the DP master is operating, the CPU exposes per-slave diagnostics through the following system functions and blocks:

• SFC 13 ("DPNRM_DG") — read the standardized diagnostic frame from a DP slave (DP-V0)
• SFB 52 ("READSLV") — record-oriented reading of slave diagnostics (DP-V1)
• SFB 53 ("WRITSLV") — record-oriented writing to a slave (DP-V1)
• SFC 7 ("DP_PRAL") — trigger a DP interrupt to itself
• SFC 11 ("DPSYC_FR") — synchronize / freeze / unsync groups of slaves (DP-V0 broadcast)
• Diagnostic buffer entries with slave address, station status (Prm-Fault, Not-Ready, Fault), and slot-level diagnostics

Configuring DP Slave Mode

If the CPU 315-2DP must operate as a DP slave to a higher-level master (e.g., a SIMATIC S7-400H, a WinAC, or a third-party controller), select DP Slave in the operating-mode field on the Address tab of the IF2 properties dialog. The dialog then exposes a Configuration tab where you declare the I/O data blocks the master will exchange with this CPU.

1. On the IF2 Properties → Address tab, set Operating Mode = DP Slave and assign a slave address (3 to 125, not used elsewhere).
2. Open the Configuration tab. Use the row editor to add one row per I/O data block:
   • Type: Input, Output, or Input/Output (combined)
   • Length: 1, 2, 4, 8, or 16 words
   • Consistency: Total length (recommended for analog data, ensures the master's read/write frame is not split mid-data), or Unit (byte/word consistency for discrete bits where consistency within a single byte is sufficient)
   • Address in CPU: the P/Q area starting address (e.g., PIB 100 for 1 byte input starting at byte 100)
3. Confirm and download. The slave is now visible to the external master via its GSD file (which you export from HW Config via Options > Create GSD File for Slave).

GSD File Installation for DP Slaves

Any third-party Profibus DP slave (or even Siemens devices added through GSD rather than the integrated catalog) requires its GSD file in HW Config before it can be inserted into the master system. The standard GSD filename pattern is SIExxxxx.GSD for Siemens or vendor-specific for other manufacturers, with companion bitmap files *.BMP for the device icon.

1. Download the GSD from the device manufacturer. Verify the GSD revision matches the slave's firmware — most vendors publish a compatibility matrix on their support site.
2. Close all STEP 7 components (HW Config, NetPro, SIMATIC Manager).
3. From SIMATIC Manager, choose Options > Install GSD Files...
4. Browse to the directory containing the *.GSD (and optionally *.NME, *.BMP) files. Select one or more and click Install.
5. HW Config rebuilds the hardware catalog. The new device appears under PROFIBUS DP > [Vendor] > [Device] in the catalog. Restart HW Config if the new device does not appear in the catalog.
6. Drag the device onto the Profibus subnet, assign an unused address, and configure its slot-level I/O.

For Profibus PA devices connected via a DP/PA coupler (e.g., Siemens DP/PA Link or IM 153-2 with PA capability), the GSD is for the DP side; the PA slave devices are discovered on the PA segment via the coupler's transparent mapping. DP-V0 slaves (legacy) do not support acyclic services; DP-V1 slaves do, and DP-V2 slaves add features such as isochronous mode and slave-to-slave broadcast.

Bus Parameters and Cycle Time Planning

The Profibus cycle time is the sum of each slave's configured slot transmission time plus master frame overhead. Use the HW Config built-in bus parameter calculator (DP Master System > Properties > Bus Parameters) to validate that the resulting cycle fits within your process control requirements.

Per-slave slot time:

Tslot = (Nbytes × 11 + overhead_bits) / baud_rate

Where overhead_bits ≈ 198 bits at 12 Mbit/s and ≈ 380 bits at 1.5 Mbit/s (includes preamble, delimiter, HDLC framing, SDx, FC, FCS, ED). STEP 7 computes this exactly when you click Recalculate in the Bus Parameters dialog.

Total cycle time:

Tcycle = sum(Tslot_i) + Tmaster_frame + Tidle + Tretry × Nretries

Typical default retry count is 1; raising this to 3 improves robustness against single-bit errors but triples the worst-case cycle time on a fault. For drives and time-critical I/O, target cycle < 5 ms at 1.5 Mbit/s. Above 5 ms, increase baud rate to 12 Mbit/s or segment the network.

Downloading the Configuration and Verifying Operation

After saving and compiling HW Config, download the system data to the CPU:

1. Connect the programming device to the CPU via MPI/DP cable (PC-Adapter USB or CP 5512 / CP 5711) or Ethernet CP.
2. In SIMATIC Manager, set the online interface to "PC Adapter (MPI)" or the Ethernet CP.
3. Right-click the S7-300 station > Download. The CPU should report STOP briefly, then return to RUN-F (if the program is valid) or STOP with diagnostic buffer entry if configuration download failed.
4. Open PLC > Diagnostics/Setting > Diagnostics Buffer. Look for entries such as "Parameter assignment of module ... OK" and "Distributed I/O: Station OK".
5. From HW Config online view (View > Online), the DP master system shows each slave with a green check if reachable or a red X if not.
6. Use the diagnostic repeater or STEP 7's PLC > Profibus Diagnostics to read per-slave diagnostics.

A solid BF-DP LED at power-up indicates either an incorrect slave address, a missing terminating resistor, or reversed A/B wires. Intermittent BF-DP flashes with periodic recovery usually indicates EMC problems or loose connectors.

Diagnostic Interrupt Handling (OB 82)

When a DP slave enters or leaves a fault state (e.g., a sensor wire break, an over-temperature trip, or module removal under power), the DP master signals an OB 82 diagnostic interrupt to the CPU. The local data of OB 82 includes the slave's logical base address, the event type (incoming/outgoing), and the slot-level diagnostic data. The standard reaction is to read the standardized diagnostic frame with SFC 13:

// OB 82 — DP diagnostic interrupt
// LADDR  = logical base address of the slave (WORD at OB82_MDL_ADDR)
// RET_VAL = return value from SFC 13 (e.g., W#16#0000 = OK)
CALL  SFC 13 // DPNRM_DG
  LADDR   := OB82_MDL_ADDR
  RET_VAL := MW100
  RECORD  := P#DB99.DBX0.0 BYTE 34   // 34-byte diagnostic buffer
  BUSY    := M101.0

Always allocate a DB for the diagnostic record before the SFC call — a temporary local area may be overwritten by the next OB call. If OB 82 is not loaded, the CPU goes to STOP on the first diagnostic interrupt. For diagnostic events on DP-V1 slaves, use SFB 52 (READSLV) with index 0 to read the slot-level diagnostic record.

Troubleshooting Matrix

Alternative: Configuring in TIA Portal

If you are migrating a project to TIA Portal (V13 or later), the procedure differs slightly:

1. In the project tree, expand Devices > [CPU 315-2DP].
2. Double-click Device view to open the hardware editor.
3. Locate the PROFIBUS interface port on the CPU. The combined MPI/DP port appears with a port label; on the 6ES7 315-2AG10-0AB0, this is IF2.
4. In the properties pane, select PROFIBUS address and click Add new subnet or assign to an existing subnet. Set the address and mode.
5. For DP slaves, drag devices from the hardware catalog to the Profibus subnet, similar to HW Config.
6. Compile the device configuration (Compile > Hardware (rebuild all)) and download.

TIA Portal hides some of the legacy DP master system settings behind the "Operating mode" and "Bus parameters" tabs. The behavior is equivalent to STEP 7 V5.5 HW Config, but the catalog structure is reorganized around devices rather than slots.

Field-Proven Tips

• Always export the slave GSD from HW Config and archive it with the project. This allows the master configuration to be reproduced exactly on a replacement CPU after a hardware swap.
• Keep a printed Profibus address plan with the cabinet layout. Address conflicts are the single most common commissioning fault.
• Use Profibus diagnostic repeaters (Siemens 6ES7 972-0AB01-0XA0) for permanent online diagnostics during commissioning and operation. They reveal per-segment signal quality and reflect attenuation on the live network.
• For drives and safety integration, use 12 Mbit/s only on a Siemens-only segment; mixing 12 Mbit/s with non-Siemens slaves can produce retry storms due to different minimum Tslot implementations across vendors.
• Replace the CPU's Micro Memory Card (MMC) when upgrading firmware. The MMC contains the firmware image and project; without the matching MMC, the CPU will not start.
• Document the bus parameter set (Tslot, Tset, Tqui, HSA) on the cabinet door. These numbers are diagnostic gold when chasing a fault three years after commissioning.
• Set the diagnostic repeater's monitoring threshold to 50% of nominal signal amplitude. Above that, intermittent failures are imminent but not yet user-visible.
• Keep a copy of the CPU's GSD export in the cabinet documentation binder. When a replacement CPU arrives, you can rebuild the slave-side config without re-engineering the entire master project.

Related Standards and Reference Material

The Profibus DP standard underpinning this configuration is defined in IEC 61158 / IEC 61784 (Communication Profile Families CPF 3/1 and CPF 3/3) and the Profibus Nutzerorganisation (PNO) implementation documents. When implementing interoperability with non-Siemens masters, reference the PNO implementation guide for DP-V1 and DP-V2 conformance.

The S7-300 system manual covers the hardware foundation; consult it for rack sizing, power-budget calculation, and slot rules. The STEP 7 V5.5 system manual covers HW Config and NetPro navigation. Both are available on the Siemens Industry Online Support portal.

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