Resolving SFB14 Missing Error on S7-300 GET/PUT Communication

This reference addresses a recurring STEP 7 / CFC compile-time failure when a user configures S7 communication between two Siemens SIMATIC S7-300 CPU 315-2 PN/DP controllers using the integrated PROFINET interface. The reported runtime/compile error is:

The error is not a hardware fault. It is a project-engineering defect caused by inserting the wrong block type (system function block SFB14) into a CPU whose firmware only provides the loadable function block FB14 "GET". The condition repeats on every 31x PN/DP, 31x-2 PN/DP, 31x-2 DP/PN, and 31xF-2 PN/DP generation that lacks a CPU-side SFB14 image. The fix involves correct block selection, correct project type, and proper NetPro binding.

1. Root Cause Analysis

The SFB14 / SFB15 naming convention differs between SIMATIC platforms. The S7-300 firmware contains a loadable copy of the GET/PUT logic, distributed as FB14 and FB15 in the Standard Library → Communication Blocks → CPU_300 folder. The S7-400 firmware contains those same functions as integrated system function blocks SFB14 / SFB15, located in the Standard Library → System Function Blocks folder and stamped into the CPU operating system at firmware build time.

When a CFC chart running on an S7-300 target references an SFB14 symbol directly, the SCL compiler cannot resolve it against the CPU's block catalog, because the catalog is delivered as a loadable FB, not a system SFB. The compiler emits the diagnostic above and refuses to download the resulting system data.

There are three engineering defects that produce the same error class:

1. Drag-and-drop of SFB14 from the S7-400 system function block library into an S7-300 chart (or into a shared library used by both platforms).
2. CFC chart compiled against the wrong target system family (target-system bit set to S7-400 in the block properties while the CPU is an S7-300).
3. PCS7 master data library binding that re-routes the GET call to an SFB14 wrapper in a PCS7-specific catalog that is not present on a standard STEP 7 CPU 315-2 PN/DP.

2. Hardware and Firmware Context: CPU 315-2 PN/DP

The relevant order numbers of the CPU 315-2 PN/DP family are:

On every CPU 315-2 PN/DP variant that supports S7 communication over PROFINET, the catalog delivered to STEP 7 contains FB14 "GET" and FB15 "PUT" as loadable function blocks in the S7-300 communication library. The catalog does not contain SFB14 or SFB15. Cross-referencing the CPU's online block folder (target system → Blocks → System Data) confirms the absence of any SFBxx with a 14/15 index when the S7-300 target is selected.

3. Block Selection Rules: SFB vs FB by Platform

For the S7-300 family the FB14/FB15 blocks must be present in the offline project under S7 Program → Blocks so the loader writes them to the CPU at download. The S7-300 firmware has no in-OS image of these blocks; the S7-400 firmware does have an in-OS image. Forgetting to drag FB14/FB15 from the Standard Library into the offline Blocks folder is the most common secondary cause of "block not found" errors at runtime, after the SFB14/FB14 confusion.

4. NetPro Configuration for S7 Connection

NetPro defines the S7 connection that the GET/PUT instance DB references. For two CPU 315-2 PN/DP units talking over the integrated PROFINET port, the procedure is:

1. Open NetPro from SIMATIC Manager (or open the project, then Options → NetPro).
2. Right-click the CPU 315-2 PN/DP node → Insert New Connection.
3. Partner: the second CPU 315-2 PN/DP. Type: S7 connection.
4. Local ID: the connection ID assigned by NetPro (e.g. 1). This ID must match the ID input of the FB14/FB15 instance.
5. Confirm the partner IP address, subnet, and rack/slot.
6. Compile and download NetPro to both CPUs. Connection establishment requires the connection configuration in both directions (active and passive).

The connection ID is critical: a mismatch between the NetPro-assigned ID and the ID wired to the FB instance causes STATUS = W#16#0001 (communication error, ID invalid).

5. CFC-Specific Considerations on S7-300

CFC is a Siemens optional package, order number 6ES7658-1EX08-0YA5 (CFC V8.0) at the time of the reported case. It supports S7-300 targets, but with platform-specific code generation. From the CFC for S7 manual, the target system selection in the block-type properties ("General" tab) drives the SCL back-end:

"In the 'General' tab, specify the block type properties before starting compilation (FB number, symbolic name, name (header), family, author, version) and specify the target system on which the block will be used. This information (S7-300/S7-400) is relevant for the startup blocks, since the SCL compiler needs to generate a different code for each target system."

For the S7-300 path the SCL compiler emits loadable FBs and inserts the corresponding FB14/FB15 instances in the S7 program. For the S7-400 path the compiler emits SFB14/SFB15 calls and expects the system blocks to exist in the CPU's OS image. If the target system setting drifts to S7-400 inside a CFC chart bound to an S7-300 CPU, the catalog mismatch described above occurs.

6. PCS7 vs STEP 7 Project Type

The reported case explicitly notes that a PCS7-type project does not communicate over the PN port, while a STEP 7-type project does. The two project types differ in:

If the project origin is PCS 7, the CFC chart may inherit PCS 7 block bindings that target the S7-400 system block catalog. For a CPU 315-2 PN/DP deployed outside a PCS 7 plant, the cleanest fix is to either (a) create a STEP 7 project, copy the CFC charts in, and re-resolve the block types to the CPU_300 family, or (b) verify the PCS 7 catalog is the correct, licensed one for that specific PCS 7 version and CPU 315-2 PN/DP FW.

7. Step-by-Step Resolution Procedure

7.1 Confirm the CPU and Firmware

1. Open HW Config. Identify the MLFB and FW of the CPU 315-2 PN/DP. Typical FW V3.x supports S7 communication over PN.
2. Right-click the CPU → Object Properties → Diagnostic. Confirm the integrated PROFINET interface is configured with an IP address and subnet.

7.2 Replace SFB14/SFB15 References with FB14/FB15

1. In SIMATIC Manager, open the S7 program → Blocks.
2. Delete any SFB14, SFB15, SFB8, SFB9 references that exist in the offline program.
3. Open Standard Library → Communication Blocks → CPU_300.
4. Drag FB14 GET, FB15 PUT (and FB8 USEND, FB9 URCV if used) into the offline Blocks folder. The blocks must reside in the offline S7 program so the loader can write them to the CPU.
5. If using multi-instance DBs, allocate a separate instance DB per call (e.g., DB14 for GET, DB15 for PUT). Assign with LAD/STL/FBD → Instance DB.

7.3 Fix CFC Target-System Bit

1. Open every CFC chart in the S7 program.
2. Right-click the GET/PUT block instance → Object Properties → General.
3. Set Target System = S7-300. Apply.
4. Recompile the chart. The SCL back-end should now emit FB14/FB15 references and an instance DB.

7.4 Bind the NetPro Connection

1. Open NetPro. Insert a new S7 connection between the two CPU 315-2 PN/DP nodes.
2. Note the local Connection ID (default 1, may differ).
3. Wire the ID input of the FB14/FB15 instance to that exact integer.
4. Compile and download NetPro to both CPUs.

7.5 Compile and Download

1. In SIMATIC Manager, mark the S7 program → Blocks → Compile and Download Objects.
2. During compile, watch the output window. Any remaining SFBxx not in the OS error is now eliminated.
3. Download the entire program to the CPU 315-2 PN/DP. The download pushes FB14/FB15, instance DBs, and the NetPro system data blocks (SDB).

8. Verification

1. Go online with the CPU. Open Blocks → System Blocks in the online view. Confirm FB14 and FB15 are present in the online block folder.
2. Open the FB14 instance DB in monitor/modify. Force REQ := TRUE. The DONE, ERROR, and STATUS outputs update on the next OB1 cycle.
3. Decode the STATUS word. Common values for FB14/FB15 on S7-300:

4. Use the partner CPU's Monitor/Modify view to confirm the data has been written/read. A successful PUT writes to the partner's specified data area; a successful GET reads from it.
5. Cycle the partner CPU through STOP/RUN to confirm the S7 connection re-establishes. STATUS should return to 0000 within a few OB1 cycles.

9. Error Matrix and Common Pitfalls

10. Alternative Communication Approaches

If the GET/PUT path remains problematic on a CPU 315-2 PN/DP, three officially supported alternatives exist:

1. S7 Communication via BSEND / BRCV (FB12 / FB13 on S7-300): Block-oriented, larger data volumes, same PN port. Useful for variable-length data.
2. Open Communication over TCP / ISO-on-TCP (FB63 / FB64 / FB65 / FB66 / FB67 / FB68 on S7-300, TCON, TSEND, TRCV, TDISCON, TUSEND, TURCV): The user manages the connection. Use this if the partner is non-Siemens or non-PROFINET-aware.
3. PROFINET IO Data Record read/write (RDREC / WRREC, SFB52 / SFB53 on S7-400; FB52 / FB53 on S7-300): Useful for acyclic IO data exchange with PN IO devices.

Each of these blocks also has S7-300 vs S7-400 naming differences. Apply the same rule: the S7-300 catalog uses loadable FBs; the S7-400 catalog uses system SFBs.

11. Diagnostic Tools and Online Monitoring

Beyond STATUS decoding, the following STEP 7 / PCS 7 tools accelerate root-cause identification on the CPU 315-2 PN/DP PN port:

• PLC → Diagnostics/Setting → Diagnostic Buffer: shows the timestamped event log, including SFB/FB calls that produced an error and the source line.
• PLC → Accessible Nodes: confirms the CPU 315-2 PN/DP is reachable on the PN subnet.
• NetPro → Connection Diagnostics: shows S7 connection state (established / not established / aborted) for every configured S7 connection.
• SIMATIC Manager → Options → Set PG/PC Interface: ensure the PG/PC is on the same subnet as the CPU 315-2 PN/DP. For direct PN attachment, set the PG/PC interface to TCP/IP → Intel(R) ... with the appropriate NIC.
• Wireshark or Siemens PRONETA for low-level PN frame inspection during commissioning.

12. Field Commissioning Notes

When bringing up S7 communication on the integrated PN port of the CPU 315-2 PN/DP, follow this order to avoid re-work:

1. Configure both CPUs in HW Config with valid IP addresses on the same subnet.
2. Download HW Config to both CPUs and cycle power if requested.
3. Configure the S7 connection in NetPro. Verify the connection appears in the online SDB.
4. Drag FB14/FB15 from the Standard Library into the offline Blocks folder. Do not use SFB14/SFB15 from the S7-400 catalog.
5. Create the instance DBs (e.g., DB14 for GET, DB15 for PUT). Wire ID to the NetPro connection ID.
6. In CFC, set every block instance target system to S7-300 before compile.
7. Compile, then download the entire Blocks folder. Verify FB14/FB15 are listed in the online block folder.
8. Set REQ := TRUE on the instance DB and monitor STATUS.

13. Frequently Asked Questions