Resolving Siemens CP 1613 OPC Server S7 Communication Errors

Problem Overview

Engineers integrating a SIMATIC Rack PC IL 40 with a CP 1613 communications processor into a SIMATIC NET OPC server architecture frequently encounter two distinct failure modes: (1) the PLC-side CP 443-1 reports a connection "is being set up" while the OPC server status remains "not active" with a diagnostic buffer message that the SDB file cannot be created, and (2) once basic connectivity is restored, OPC Scout tags return BAD quality with the property "Access Path not supported by server" when a secondary ISO-on-TCP connection is added against an S7 OPC Server v6.3.0. This reference consolidates the field-proven diagnostic and recovery procedure for both scenarios, including the relevant configuration objects, Item ID syntax, and verification checks against SIMATIC NET, S7 RedConnect, and OPC Scout.

The two faults are typically mistaken for a single network or routing issue, but the underlying causes are independent: fault 1 is a configuration-deployment failure rooted in the SIMATIC NET driver and Station Configurator, while fault 2 is a server-side access-path grammar mismatch that occurs even when the underlying S7 connection is fully established. Treating them as a single problem is the most common reason a CP 1613 OPC integration stalls for days.

System Architecture and Component Identification

The reference topology is a single-tier S7 communication between a SIMATIC S7-400 station and a SIMATIC PC station. The PC station uses a CP 1613 PCIe/PCI card installed in a SIMATIC Rack PC IL 40. The OPC server on the PC side exposes S7 memory areas (Inputs, Outputs, Merkers, Data Blocks) to OPC Data Access (DA) clients such as OPC Scout, WinCC, or third-party SCADA packages.

Fault 1 — "Unable to Create SDB File" and Inactive Status

After a successful STEP 7 download to both the S7-400 station and the PC station, the engineer observes the following pattern in NetPro and the Online Station Configurator:

• CP 443-1 connection entry reports the runtime state "is being set up" (German: "wird aufgebaut") and never transitions to "established".
• OPC server application status in the Station Configurator shows "inactive" / "not active" even though the configuration download completed without an error message.
• Diagnostic buffer of the Communications Configuration (CoCo) lists the entry "Unable to create SDB file" (German: "SDB-Datei konnte nicht erzeugt werden").

The SDB (System Data Block) container is the compiled output of NetPro and contains the connection description, port assignment, and routing information for the CP 1613. Without a valid SDB, neither the PLC nor the PC station can bring the S7 connection up. The error is generated locally on the PC station when the SIMATIC NET Configuration Console attempts to materialise the SDB into the running OPC server process.

Root Causes

The Siemens knowledge base documents this class of issue in the S7-400 H / CP 1613 communication FAQ, which states that the CP 1613 enables S7 communication from a user application to an S7-400 H system on Ethernet and that an existing configuration must be re-imported after driver changes:

Siemens Support: FAQ – Communication via IE with CP 1613 and S7-400 H (PDF)

Diagnostic Buffer Decoding

The CoCo diagnostic buffer records the SDB generation result as a hexadecimal event code. The most common values are:

Fault 2 — "Access Path Not Supported by Server" and BAD Quality Tags

After the initial SDB problem is resolved (typically by re-importing the PC station and confirming the driver), the engineer may successfully establish an S7 connection but see the following when adding a second connection using ISO on TCP (RFC 1006):

• NetPro on the PLC side reports the connection as "established".
• S7 RedConnect S7 Connection Diagnostics shows "OK".
• OPC Scout, when reading tags, returns quality BAD with the tag property Access Path not supported by server.

This error is raised by the OPC DA server when the client supplies an Item ID whose access path component uses a syntax the server does not implement. With the SIMATIC NET S7 OPC Server v6.3.0, the access-path grammar is fixed and does not accept all of the same forms the S7 protocol itself supports.

OPC Item ID Syntax for S7 OPC Server v6.3.0

A valid Item ID has the following structure:

Where:

• <ConnectionName> is the S7 connection name as defined in NetPro (e.g. S7_Connection_1).
• <MemoryArea> is one of DB, M, I, Q, PIW, PQW, PI, PQ, T, C, STW, ZSW.
• <AreaNumber> applies only to DB (DB number 1..65535).
• <DataType> is one of X (bit), B (byte), W (word), D (double word), CHAR, INT, DINT, REAL, STRING.
• <Offset> is byte offset (or bit offset for X).
• <Length> is the number of items (default 1).

Examples of valid Item IDs:

S7:[S7_Connection_1]DB1,BYTE0
S7:[S7_Connection_1]DB10,INT2,4
S7:[S7_Connection_1]M0.0
S7:[S7_Connection_1]IB0,1
S7:[S7_Connection_1]DB5,REAL100

Examples of invalid Item IDs that produce "Access Path not supported by server" in v6.3.0:

S7:[S7_Connection_1]DB1,BYTE0:1        (v6.3.0 does not support ":length" suffix)
S7:[S7_Connection_1]DB1,BYTE0/1         (v6.3.0 does not support "/bit" notation)
S7:[S7_Connection_1]/DB1/0/0            (path-style access)
S7:[S7_Connection_1]DB1,BYTE(0)         (parenthesis not supported)
S7:DB1,BYTE0                           (missing connection name)

Corrective Procedure for BAD Quality

1. In OPC Scout, delete every tag in the failed group and re-add each tag using a single Item ID line in the syntax above. The tag configurator in older SIMATIC NET versions sometimes concatenates ranges and offsets in a way the OPC server cannot parse.
2. For bit-level access inside a DB, use the explicit DB<n>,X<byte>.<bit> form, e.g. S7:[S7_Connection_1]DB1,X0.0.
3. For ranges, repeat the Item ID per offset instead of using a length suffix. Example: add DB1,BYTE0, DB1,BYTE1, DB1,BYTE2 separately.
4. Confirm the read by right-clicking the tag in OPC Scout → Properties → Quality should read Good (192); Access Path is shown as the Item ID string.

Online Status — What Each Tool Reports

Step-by-Step Resolution Procedure

The procedure below addresses both faults in a single ordered workflow. Execute each step in sequence; do not skip the verification points.

1. Verify CP 1613 hardware: open Device Manager and confirm the adapter appears as "Siemens CP 1613" or "CP 1613 A2" with the SIMATIC NET driver, not the standard NDIS miniport.
2. Reinstall the SIMATIC NET driver if the entry is missing or wrong. After the install, restart the PC so the new driver binds cleanly to the PCI device.
3. Open the Station Configurator (Start → Siemens Automation → SIMATIC NET → Configuration Console → Station Configurator). Verify that Index 1 is "OPC Server" and Index 2 is "CP 1613", and that the slot numbers match the PC station in STEP 7 (typically slot 0 for the OPC server, slot 2 for the CP 1613).
4. Open STEP 7 / NetPro, open the PC station, and use Import Station to refresh the local configuration from the Station Configurator. This re-aligns the slot list.
5. Insert an S7 connection in NetPro on the PC station pointing to the CP 443-1 partner. Set the connection type to "S7 connection" for plain S7 communication, or "ISO on TCP connection" for T connections routed through TCP/IP. Use a TSAP in the form 03.01 (for slot 1) on the S7-400 side.
6. Compile and download the PC station. If STEP 7 reports "Target station not reachable" or the download stalls on the CP 1613, check that the Station Configurator is in "Configured mode" rather than "PG mode".
7. Download the S7-400 station (HW Config + NetPro). This populates the connection table on the CP 443-1.
8. Open the SIMATIC NET Configuration Console and watch the diagnostic buffer during download. Confirm the new entry is SDB generation completed (0x0A02), not Unable to create SDB file (0x0A11).
9. Start the S7 OPC Server service (or S7RedConnect service) via the Station Configurator. The status indicator must turn green.
10. Open S7 RedConnect S7 Connection Diagnostics and verify the connection state is "OK" / "Established". If it is "Not used", the connection is configured but no client has yet opened it.
11. Open OPC Scout and add a single test tag, e.g. S7:[S7_Connection_1]DB1,BYTE0. Confirm Quality = Good (192).
12. For the secondary ISO on TCP connection (Fault 2), repeat steps 5–11 for the new connection name and add the test tag using the v6.3.0-compliant Item ID syntax. If "Access Path not supported" persists, edit the tag, remove length suffixes and bit notation, and retry.

Verification Procedure

After the resolution steps, perform the following verification checklist before declaring the link healthy:

• Online Station Configurator shows both the OPC server and the CP 1613 in "Running" state with a green indicator.
• S7 RedConnect S7 Connection Diagnostics shows the connection in "Established" / "OK" state, with send/receive PDU counters incrementing when OPC Scout reads or writes a tag.
• OPC Scout tag properties show Quality = Good (192) and Access Path = the Item ID string entered.
• CoCo diagnostic buffer contains the most recent entry as "SDB generation completed" and no new "Unable to create SDB file" event after the last download.
• NetPro on the PLC side, opened against the online station, shows the CP 443-1 connection in "Established" state.

PG/PC Interface and TSAP Configuration

The PG/PC interface assignment is a frequent hidden cause of intermittent SDB generation failures. On the Rack PC IL 40, the CP 1613 must be selected as the access point for "S7ONLINE" in the SIMATIC NET Configuration Console. If a different network adapter is selected (for example the on-board Ethernet of the IPC), STEP 7 will write SDBs into the wrong stack and the CP 1613 will not see the configuration.

Procedure: Start → Control Panel → Set PG/PC Interface → Access Point of the Application: S7ONLINE → Interface Parameter Assignment Used: CP 1613(PCI) / TCPIP.Auto.1 depending on protocol. For S7 connections, choose the CP 1613 (ISO) entry; for ISO on TCP, choose the CP 1613 (TCP/IP) entry.

TSAP (Transport Service Access Point) values for the CP 443-1 partner must follow the Siemens convention: byte 1 = connection resource (typically 03 for S7 connections), byte 2 = slot of the CPU (e.g. 01 for slot 1). On the PC side, the OPC server auto-assigns a TSAP from the configuration and the engineer does not need to set it manually, but a mismatch in the partner TSAP is a frequent cause of the connection stalling at "is being set up".

Alternate Connection Types and When to Use Them

The SIMATIC NET OPC Server v6.3.0 supports three connection types in NetPro. Choosing the wrong one is a frequent source of SDB and access-path errors.

For the secondary connection described in Fault 2, ISO on TCP is appropriate when the CP 443-1 and the CP 1613 are on different subnets and a managed switch with VLANs separates them. For a single-subnet, flat-network installation, plain S7 connection delivers lower latency and is the preferred choice.

Preventive Best Practices

• Always run the Station Configurator in "Configured mode" before downloading from STEP 7; switching modes mid-download is the most common cause of inconsistent SDB containers.
• Maintain a single STEP 7 project as the source of truth for both the S7-400 station and the PC station. Use a project-library approach so that changes to the connection propagate to both sides.
• After any SIMATIC NET version upgrade, update the CP 1613 firmware via the SIMATIC NET Configuration Console → CP 1613 → Firmware Loader. Mixed firmware and driver versions produce intermittent SDB errors.
• When using the SIMATIC NET S7 OPC Server, design the OPC namespace around single-offset Item IDs. Avoid range Item IDs in v6.3.0; the grammar is more permissive in newer SIMATIC NET releases but compatibility is not guaranteed.
• Disable Windows firewall and any non-Siemens packet filter on the CP 1613 subnet. CP 1613 implements its own Industrial Ethernet protocol stack and external filters can silently drop ISO-on-TCP packets.
• For redundant PLCs (S7-400 H) follow the additional steps in the Siemens S7-400 H / CP 1613 communication guide (Siemens Support document 16651010).
• Snapshot the working SDB container (the *.sd? files in %ProgramData%\Siemens\Automation\) before any STEP 7 download. If a download corrupts the SDB, restoring the snapshot is faster than rebuilding the configuration from scratch.
• Subscribe the CoCo diagnostic buffer to a log file (Configuration Console → Options → Trace) so that intermittent SDB-generation events are captured for post-mortem analysis.

Migration to OPC UA on S7-1200 / S7-1500

On S7-1200 and S7-1500 controllers, the classic S7 OPC Server is replaced by the integrated OPC UA server. The "Access Path not supported" condition maps to a "Wrong usage" notification in the OPC UA server's diagnostic buffer, which is documented in the SIMATIC S7-1200 manual collection. Although the protocol differs, the diagnostic-buffer interpretation methodology is identical: read the most recent event, decode the OPC UA status code, and correlate with the configured Item ID / NodeId.

For new installations targeting S7-1500 hardware, prefer OPC UA over the CP 1613 + S7 OPC Server combination. The CP 1613 remains a viable hardware option for OPC DA-only clients, but the migration path is straightforward: install the SIMATIC NET V14+ or TIA Portal V16+ PC station, replace the S7 OPC Server with the OPC UA server on the PLC, and re-author OPC tags as NodeIds in the form ns=3;s="DB1"."Tag1":

Siemens TIA Portal: OPC UA Server – Wrong Usage diagnostics

FAQ