Resolving S7-300 CPU 313C-2DP PROFIBUS DP Preset/Actual Mismatch

Problem Description

When commissioning a peer-to-peer link between two SIMATIC S7-300 CPU 313C-2DP controllers, the most common failure mode is a diagnostic event on the DP master reading:

The error is reported in the Module Information dialog (accessible via PLC → Module Information → Diagnostic Buffer) and is also exposed as diagnostic interrupt (OB 82) on the master. The CPU 313C-2DP, while advertising a fully integrated DP master/slave interface, only carries one PROFIBUS-capable port (X2, the combined MPI/DP socket). Therefore only one of the two CPUs can be a slave in any single PROFIBUS subnet - the second must be the master, or the two PLCs must be on different physical segments bridged by a repeater.

Because the message lists the expected slave's order number as "???", STEP 7 cannot resolve the GSD identity of the device it is polling. This almost always points to a configuration or wiring problem, not a defective CPU.

Hardware Overview: CPU 313C-2DP (6ES7313-6CF03-0AB0)

The CPU 313C-2DP is the compact controller with integrated digital and analog I/O plus a PROFIBUS-DP interface. The version relevant for current STEP 7 V5.x and TIA Portal V13+ projects is order number 6ES7313-6CF03-0AB0 (Firmware V3.3). Key parameters that matter for the link:

The full data sheet is published in the Siemens Industry Online Support entry SIMATIC S7-300 CPU 31xC and CPU 31x – Technical specifications. The 313C-2DP cannot perform S7 basic communication (GET/PUT/BSEND/BRCV) on its PROFIBUS port - only DP, MPI, and routing. Data exchange between the two CPUs must therefore be implemented as DP master/slave I/O transfer, not as S7 connection.

PROFIBUS DP Master-Slave Architecture Used Here

The simplest topology that fits a single 313C-2DP port on each side is a bus segment with two stations:

• Station 1 (Master): CPU 313C-2DP, PROFIBUS address 1, polled by engineering station, runs the application logic.
• Station 2 (Slave): CPU 313C-2DP, PROFIBUS address 2, exposes a process image that the master reads/writes cyclically.

Cyclic data exchange is the only mechanism that does not require S7 basic communication. The master holds the configuration of the slave as a "virtual" rack - each row in the slave's slot table corresponds to a fixed-size I/O area the master polls in its DP cycle. The slave's user program reads its assigned input area (data coming from the master) and writes the output area (data the master will read).

Root Cause Analysis of the Preset/Actual Mismatch

The diagnostic text is unambiguous: the master is asking the slave for a module type that the slave does not report. The four most common root causes, ordered by frequency in field commissioning, are:

A fifth, less frequent cause is the absence of the GSD file on the master side when the slave CPU is a different family (e.g. ET 200S), but for two 313C-2DP controllers the GSD is part of the STEP 7 installation - no manual import is required.

Prerequisites

• STEP 7 V5.5 + SP2 or later (V5.7 is the final V5 release, still supported for S7-300). The same steps work in TIA Portal V13+ but this article uses STEP 7 V5.5 notation.
• Two functional CPU 313C-2DP (6ES7313-6CF03-0AB0) with firmware ≥ V3.0. Older firmware ("ES" or V2.x) cannot enter DP slave mode on the X2 port.
• PROFIBUS DP cable 6XV1830-0EH10 (purple, standard type) or 6XV1830-0EH50 (highly flexible).
• Two PROFIBUS connectors 6ES7972-0BA12-0XA0 (with PG socket) for termination at the ends. Switch the terminating resistor ON on the two end connectors, OFF on any station in the middle.
• Two MPI/DP programming cables (USB or PC adapter) to download the hardware configuration to each CPU. The 313C-2DP shares its only port between MPI and DP, so the configuration download on the slave must occur via MPI - it cannot be done over the active DP cable while the CPU is set to DP slave mode.

Step-by-Step Configuration - Slave CPU First

Always configure and download the slave project before the master. If the master is brought online against a non-existent slave, STEP 7 caches the error in the master's diagnostic buffer and you will chase ghost faults for the rest of the day.

1. Open SIMATIC Manager and create a new project, then insert a SIMATIC 300 Station - rename it to Slave_313C2DP.
2. Open HWConfig. Insert a RACK 300, then drop a PS 307 in slot 1, the CPU 313C-2DP in slot 2, and any SM modules required. If you do not need additional I/O, leave the rest of the rack empty - the integrated DI/DO of the 313C-2DP are still usable in user program but do not consume DP I/O area.
3. Double-click slot 2 (the CPU row) to open its properties, then switch to the DP tab. The Interface field shows MPI/DP - click Properties.
4. In the new dialog select DP slave as the operating mode. Set the PROFIBUS address to 2. Confirm with OK.
5. Back on the DP tab, click Configuration. This is the heart of the slave configuration. STEP 7 shows a table with up to 32 slots. Each slot will hold a "universal module" of a fixed I/O size that the master must match exactly.
6. Click Row 1 in the configuration table. The catalog on the right shows Universal modules in sizes ranging from 1 byte to 32 bytes. As a starting point, select 1 byte input on slot 1 and 1 byte output on slot 2. This creates a 1-byte PI/1-byte PQ exchange on the slave. Click OK to close the configuration dialog.
7. Compile and download to the slave CPU. Switch the CPU to STOP first - download HWConfig is only allowed in STOP.
8. After the download, switch the slave CPU to RUN. The SF LED should be off. The DP interface LED (the one labelled "DP" next to X2) will be off because the slave is passive on the bus and only the master sends frames.

To confirm the configuration is correct before touching the master, open PLC → Module Information → Diagnostic Buffer on the slave. It should show the most recent entries as Download OK and Restart (cold). If the buffer shows Parameter assignment error, the slave rejected its own DP slave parameters - this happens when slot 0 (the "interface slot") is configured with non-zero data; reset slot 0 to "Empty slot".

Step-by-Step Configuration - Master CPU

1. Add a second SIMATIC 300 Station to the same STEP 7 project - rename it to Master_313C2DP. Build the same rack (PS 307, CPU 313C-2DP, slot 2).
2. Double-click slot 2 of the master, then the DP tab, click Properties next to MPI/DP. Select DP master (the default is "DP master (DPM1)"). Set the PROFIBUS address to 1.
3. In HWConfig, the right-hand catalog tree now has a PROFIBUS DP branch. Open Configured Stations → S7-300/ET 200U → CPU 31x. Drag the CPU 313C-2DP onto the master's DP subnet. STEP 7 places a new icon on the bus representing the slave.
4. Right-click the slave icon and choose Object Properties. Verify the PROFIBUS address is 2 (matching the slave CPU). Click the Configuration tab.
5. Click Edit in the configuration tab. You will see a list of slots. Insert the same modules in the same order as the slave project: Slot 1 = 1 byte input, Slot 2 = 1 byte output. The order and size of every slot must match the slave exactly - this is the "preset" the master sends to the slave during parameter assignment. Any mismatch produces the diagnostic event described at the top of this article.
6. Compile and download the master HWConfig to the master CPU.

Program Structure for Master and Slave

With the slot configuration above, the exchange uses the following I/O areas:

Note the reversal: from the master perspective, the slave's input area is its own input area. STEP 7 maps the slave's slot 1 (1 byte output from the slave) to the master's peripheral input IB 0, because what the master reads is what the slave writes. The full mapping is visible in HWConfig → Master DP → slave icon → Configuration → I/O Addresses.

Sample S7 program in the master (OB 1, STL):

// Master: read 1 byte from slave, increment, write back
      L     IB    0         // read slave input (slave output slot 1)
      +     1
      T     QB    0         // write to slave output (slave input slot 2)
      SET
      =     M     10.0      // heartbeat flag, also useful for loopback test

Sample S7 program in the slave (OB 1, STL):

// Slave: copy the byte the master wrote to a flag word
      L     PIB   0         // read master output (slave input slot 2)
      T     MB    20
      L     MB    30        // application data produced by slave
      T     PQB   0         // expose to master (slave output slot 1)

To watch the exchange live, open Monitor/Modify on the master, enter IB 0 and force QB 0. The value you force on the master appears in the slave at PIB 0 on the next DP cycle. With 12 Mbit/s bus, the cycle is sub-millisecond; with 1.5 Mbit/s (typical default), budget 1–5 ms.

Diagnostic Buffer and Online Diagnostics

When the link is healthy, the master's diagnostic buffer contains these entries in chronological order after power-up:

1. Restart (cold) - master CPU start-up
2. DP: parameter assignment OK, address 2 - master has accepted the slave
3. DP slave: status OK, address 2 - cyclic polling began

If the link fails, the master reports the following event codes (visible in the diagnostic buffer as hex IDs):

Use PLC → Accessible Nodes (or Target system → Accessible Nodes in TIA Portal) to see the slave's PROFIBUS address. If the slave does not appear at all, the bus is physically broken or the slave CPU is in STOP with its DP interface still configured as MPI.

Verification and Commissioning

1. Power both stations. The slave's SF LED must be off. The master's BF LED must be off and the DP LED on each CPU must be on solid (not flashing).
2. Open the master project, go online, and check the diagnostic buffer. Confirm the three healthy events listed above.
3. Open Monitor/Modify on the master. Force QB 0 = 16#55 and confirm the value appears in the slave at PIB 0 in its own online view.
4. Write a value from the slave side (e.g. set MB 30 = 16#AA in the slave's VAT) and verify IB 0 = 16#AA on the master.
5. Disconnect the PROFIBUS cable on the master end. The BF LED must come on within the configured watchdog time (default 10 s). The master diagnostic buffer logs Station failure. Reconnect and confirm the link auto-restores within 1 cycle.

Troubleshooting Matrix

Field-Proven Caveats

• STEP 7 V5.5 SP2 and later auto-rebuild the slot table when you change the slave's project. If you switch from a 1-byte to 4-byte universal module on the slave, you must open the master HWConfig and update the master's matching row - the program does not propagate the change.
• The CPU 313C-2DP's integrated DI16/DO16 do not appear in the DP I/O area. They are accessible only via the standard I (PII) and Q (PIQ) image and via the process I/O. The DP exchange always uses the virtual slots you configured in step 6 of the slave setup.
• If the same project must connect a 313C-2DP (slave) to a third-party master (e.g. a Beckhoff CX or Allen-Bradley ControlLogix), the slot configuration is exported as a GSD file from STEP 7: HWConfig → Options → Create GSD File for Slave. Import this GSD into the third-party engineering tool and the slot table travels with it.
• Watchdog behaviour: when the master fails to poll the slave, the slave switches its outputs to a safe state after the configured DP watchdog (default 10 s). For fail-safe operation, wire the watchdog to 0 = "keep last value" or 1 = "output 0" depending on your process safety requirements.

FAQ