Configure S7-300 CPU 315-2 PN/DP as PROFIBUS DP Slave to GE Fanuc

1. Problem Definition and Bus Topology

A common integration scenario couples a Siemens SIMATIC S7-300 station to a non-Siemens controller over PROFIBUS DP. In the canonical case the S7-300 carries CPU 6ES7315-2EH13-0AB0 (CPU 315-2 PN/DP, hardware/firmware release 13) and is configured as a DP slave, while the foreign controller (in this case a GE Fanuc / Emerson RX3i with a PROFIBUS master module such as IC695PBM300) acts as DP master. The CPU's integrated MPI/DP interface must be flipped from its default master-capable mode to slave mode, and the GSD description of the S7-300 slave must be imported into the GE Fanuc master configurator (Proficy Machine Edition).

Two workable topologies exist:

1. Option A — Integrated DP port as slave: No additional hardware, but the slave functionality of the integrated port is limited to DP-V0 cyclic I/O. If the GE Fanuc master expects DP-V1 acyclic services, or if the integrated port reports configuration rejection, this path fails.
2. Option B — Add CP 342-5 (6GK7342-5DA02-0XE0): The CP sits in slot 4 (or higher) of the S7-300 rack and acts as a dedicated DP slave interface. This is the proven fallback when Option A does not produce diagnostics on the master side.

2. Prerequisites and Component Identification

3. GSD File Selection and Verification

Siemens distributes GSD files with the standard naming SIExxxxxx.GSD, where xxxxxx encodes the PNO Ident Number of the device. For the CPU 31x-2 DP/PN-DP family the Ident Number is fixed across hardware releases, so the same GSD covers order numbers 6ES7315-2EH13-0AB0 through -2EH14-0AB0 (and the older -2AF00-0AB0, -2AH14-0AB0).

To verify the GSD matches the CPU:

1. Open the .GSD file in a text editor.
2. Confirm Vendor_Name = "Siemens" and Model_Name contains "CPU31x-2 PN/DP" or "CPU 315-2 PN/DP".
3. Confirm Ident_Number matches the value shown in STEP 7 → HW Config → right-click DP interface → Object Properties → DP-Slave tab.
4. Confirm Slave_Family = 0 (DP-V0 family) if the master is a DP-V0 master.

Hand the GSD file to the GE Fanuc engineer. The master configurator will display the CPU as a node with selectable module slots; those slots define the exchange table.

4. Configuring the Integrated PROFIBUS DP Port as DP Slave

Configure the integrated DP interface (X2) in STEP 7 HW Config:

1. Open the S7-300 station and double-click DP on the CPU sub-module.
2. In the General tab set Interface: PROFIBUS and Mode: DP-Slave (NOT "MPI/DP master" and NOT "DP-Master").
3. Click Properties next to the PROFIBUS interface. Assign an address (typical 3, separate from the master's address) and select the bus profile (default DP, 1.5 Mbps or 12 Mbps).
4. In the DP-Slave tab configure the slots that the master will see. Each slot must match a universal module of the correct length (input or output, byte/word). The master and slave must declare mirrored modules — see Section 5.
5. Compile and download to the CPU.

The CPU does not require NetPro/PC-internal S7 connections when acting as a pure DP slave to a non-S7 master; the S7-side SFB/SFC peer services are irrelevant to DP-V0 cyclic exchange. The master address is not declared anywhere on the slave side: the slave merely listens for a master that calls its address and supplies bus parameters at start-up.

5. Mirrored Exchange Table Construction

The exchange table is the heart of PROFIBUS DP cyclic data. It is a list of slots, each with a direction (input or output, from the master's perspective) and a length (1/2/4/8/16 bytes or 1/2/4 words). The master defines its slots in terms of what it reads (Input = data sourced by the slave) and what it writes (Output = data sent to the slave). The slave must declare the mirror image in its own configuration: what the master calls "Output" appears as an Input module in the slave's slot table, and vice versa. The number of slots, the slot order, and the byte/word lengths must be identical between the two sides.

Worked example (12 bytes total):

The exact I/O byte addresses (e.g., QW0, IB26) are auto-assigned by STEP 7 from the configured slot base. The user program then MOVEs application data between these peripheral addresses and the working DBs.

6. I/O Address Mapping and Consistent Data

For DP-V0 cyclic exchange the slave's process image addresses (IB/QB/IW/QW) are updated automatically each PROFIBUS cycle. The user program simply references them as ordinary I/O.

Typical user pattern (STL):

     L     IW    0          // read 4 bytes from master (input to slave)
     T     DB10.DBW   0      // mirror into working DB

     L     DB10.DBW 100      // get data to send to master
     T     QW    0           // place into output area (master reads as input)

Standard I/O access (L IW, T QW) is non-consistent — the CPU snapshot may straddle a bus update. If the application requires atomic read/write of > 4 bytes, use the consistent-data SFCs:

7. CP 342-5 Fallback Solution (6GK7342-5DA02-0XE0)

If the integrated DP port refuses the master configuration (CPU diagnostic buffer shows "DP slave configuration error" or "Ident_Number mismatch"), or if the master requires DP-V1 acyclic services that the integrated port does not support, drop in a CP 342-5. Its order number 6GK7342-5DA02-0XE0 (DP-V0/V1, master or slave) is the proven fallback in field integrations.

CP 342-5 configuration steps:

1. Drag the CP 342-5 from the hardware catalog (SIMATIC 300 → Communication → CP 342-5) into a free slot of the S7-300 rack.
2. Double-click the CP, open the DP-Slave interface, set the PROFIBUS address (e.g., 3) and baud rate.
3. Define the slot table: e.g., Slot 0 = 4 Word Output (to master), Slot 1 = 2 Word Input (from master).
4. Compile and download to the CPU.
5. In the S7 program, call FC1 DP_SEND (CPLS7 → DP_SEND or its variant in the CP 342-5 library) and FC2 DP_RECV cyclically in OB1.

     CALL  FC    1           // DP_SEND
      CPLADDR  := W#16#100    // logical base address of CP
      SEND     := P#DB20.DBX0.0 BYTE 8   // 8 bytes to master
      DONE     := M10.0
      ERROR    := M10.1
      STATUS   := MW12

     CALL  FC    2           // DP_RECV
      CPLADDR  := W#16#100
      RECV     := P#DB21.DBX0.0 BYTE 4   // 4 bytes from master
      NDR      := M11.0
      ERROR    := M11.1
      STATUS   := MW14

The CP handles the PROFIBUS handshake independently; the application only feeds and drains the SEND/RECV buffers.

8. Bus Parameter Negotiation and Diagnostic Indicators

PROFIBUS DP-V0 performs parameterization at start-up: the master emits a Set-Param telegram to each slave containing bus timing, watchdog, and Ident Number. The slave accepts and enters cyclic data exchange, or rejects (returning a fault byte) and the master reports configuration mismatch. The slave does not need to be told the master's PROFIBUS address; the address is established by the master's initialization pass.

Diagnostic buffer event IDs (CPU 315-2 PN/DP, PROFIBUS related):

9. Verification and Commissioning Procedure

1. Power up the S7-300 station with the CPU in STOP. Confirm BF LED is OFF or flashing slowly (no traffic, but no physical fault).
2. Connect the GE Fanuc master to the bus. With the master in stop/idle, the S7 BF LED will start slow flashing — the slave is awaiting parameterization.
3. Place master into run. BF LED on the S7 must clear within one PROFIBUS cycle (~1 ms at 12 Mbps, ~6 ms at 1.5 Mbps). SF LED must remain off. If BF remains flashing, inspect master diagnostics for "slave diag not accepted" or "Ident Number mismatch".
4. Switch CPU to RUN. The user program begins executing; the cyclic I/O image is updated each bus cycle.
5. Force a test pattern in the master (write a known value to the master's output words). In STEP 7, open a VAT (Variable Table) online and monitor IW0 in the slave — confirm the value appears.
6. Force a test pattern in the slave by MOVE-ing a constant into QW0 in OB1 (or a cyclic OB). Monitor the master's input words — confirm the value appears.
7. Check CPU diagnostic buffer for any pending events (PLC → Module Information → Diagnostic Buffer).
8. Check master diagnostics on the GE Fanuc side for slave state, exchange errors, or configuration warnings.
9. Run a sustained test for ≥ 10 minutes to catch intermittent issues such as watchdog timeouts or EMC-induced faults.

10. Troubleshooting Matrix

Frequently Asked Questions