Resolving S7-300 NetPro Global Data No Valid Connection Error

1. Problem Description

The "no valid connection" message is the single most common error reported while configuring Global Data (GD) in NetPro on STEP 7 V5.x (SIMATIC Manager). The error appears either when the user clicks the Select... button in the GD table to pick a connection partner, or when a download of the configured PC station / S7 station is rejected with a similar message. The symptom is consistent across mixed CPU pairings, for example CPU 315-2 PN/DP (6ES7315-2EH14-0AB0) and CPU 314 (6ES7314-1AG14-0AB0), and across all firmware levels that support Global Data.

Unlike a normal S7 connection, Global Data does not require a manually entered S7 connection object in NetPro first. The GD mechanism is implemented directly on top of the MPI/DP layer, which is why STEP 7 expects the connection partner to be discovered from the stations on subnet list rather than from a user-created S7 connection. When no partner is listed, the root cause is almost always one of the following:

• Partner CPU is not placed in the same MPI subnet (or PROFIBUS subnet, when applicable).
• Partner CPU is not configured as a GD participant.
• CPU type or firmware does not support Global Data on the selected interface.
• STEP 7 project has not been compiled, or only one side of the project is open.
• Wrong STEP 7 version for the CPU firmware (CPU newer than STEP 7 knows).

2. Global Data Architecture and Protocol Boundaries

Global Data is a cyclic broadcast of up to 8 packages per GD circle, with a configurable transmission rate of 1 through 255. The maximum user payload per package is 22 bytes for an S7-300 sender and 16 bytes for an S7-400 sender. Each GD circle may contain up to 15 stations when configured over MPI, and up to 32 stations over PROFIBUS (subject to bus parameters).

GD is implemented on the following bus media:

GD packages are sent without acknowledgement and without any application-layer retry. The receiving CPU updates the configured receive area only on receipt of a fresh frame. STEP 7 therefore tracks a scan time and an ST (status) bit per receive package; the status bit clears if the frame is missed within the configured scan time multiplied by the configured scan rate.

3. Hardware and Firmware Prerequisites

Confirm the actual catalog (MLFB) and firmware of every CPU that should participate in the circle. The order number printed on the front of the module must match what is installed in HW Config. Open HW Config → right-click CPU → Object Properties → Diagnostic Buffer to read the firmware version that STEP 7 sees.

The DP interface of the CPU 315-2 PN/DP is X1 (combined MPI/DP). On delivery it is set to MPI. To use GD over PROFIBUS, open HW Config → CPU 315-2 PN/DP → MPI/DP interface → Properties → Interface: PROFIBUS. Once changed, the CPU must be recompiled and downloaded; otherwise the online view of the interface will still report MPI and the GD table will be empty.

4. Software Prerequisites

STEP 7 V5.6 supports the full line of S7-300 CPUs including the latest firmware variants of CPU 315-2 PN/DP. If a CPU in the rack is newer than the installed STEP 7 version, the module will appear as "unknown CPU" in HW Config, and the GD table will refuse to list it. Install the matching Hardware Support Package (HSP) before continuing; the HSP is delivered as a self-extracting archive and is loaded into HW Config via Options → Install HW Updates.

5. Configuration Order That Prevents the Error

STEP 7 must encounter the partner station during project compilation. If the order is wrong, the GD dialog can compile without errors but no connection partner will be offered. The order below is the only order that reliably exposes a valid partner for a CPU 315-2 PN/DP / CPU 314 combination.

1. Insert both stations in the S7 project: SIMATIC 300 Station for the CPU 314, and a second SIMATIC 300 Station for the CPU 315-2 PN/DP.
2. Open HW Config on each station, insert the CPU with the correct MLFB and firmware.
3. Configure the MPI/DP interface of each CPU with the same Subnet and the same Transmission Rate (default 187.5 kbit/s). Set the MPI address of the CPU 315-2 PN/DP to 2 and the CPU 314 to 3, or whatever addresses are free on the plant MPI bus.
4. Save and compile each HW Config (Station → Save and Compile).
5. Open NetPro from SIMATIC Manager (Options → NetPro or click the Network button on the toolbar).
6. Verify in NetPro that both CPU icons sit on the same MPI subnet. If one is on a different subnet or has no subnet at all, drag it onto the MPI subnet line, or right-click the CPU and Insert New Connection to add a virtual S7 connection that anchors the CPU to the subnet.
7. Open the GD table: Options → Define Global Data.
8. Fill the GD table with the desired send/receive assignments.
9. Compile the GD table (GD Table → Compile). The Save and Compile All button (Ctrl+F9) compiles both HW Config and NetPro/GD together; this is the safer option.
10. Download to both CPUs.

6. Step-by-Step: Building the GD Table Correctly

The following sequence is the minimum reproducible procedure on a PC with STEP 7 V5.6 SP1, a CPU 314 (FW 3.3) and a CPU 315-2 PN/DP (FW 3.3) connected via MPI at 187.5 kbit/s.

1. In the SIMATIC Manager project tree, select the S7 Program or the SIMATIC 300 Station object, then Options → Define Global Data. The empty GD table opens.
2. Click the first row under CPU (the leftmost column). The CPU that owns the current S7 program is preselected.
3. Click the second column, Send / Receive. The dialog GD Elements: Choose CPU opens. All CPUs of the same project that are on the same MPI subnet are listed here.
4. Pick the partner CPU. If the partner CPU is greyed out, the project is not compiled, the partner is on another subnet, or its CPU does not have GD configured in its object properties.
5. Click the row label, R (Receive) or S (Send), and toggle the direction. For a receive, click in the Address column and enter a DB or bit-memory address, e.g. DB1.DBD0 for 4 bytes or M0.0 BYTE 1 for one byte. STEP 7 will accept absolute addresses and symbol names from the symbol table.
6. Add the corresponding row in the partner CPU (second station in the table). The send address of CPU A and the receive address of CPU B are independent; they are not paired by STEP 7.
7. Set the Transmission Rate for the circle: 1 through 255. Value 1 is "every cycle", higher values are skip counters. A safe default for control loops is 8; for status broadcasts, 16 or 32 is fine.
8. Set the Scan Rate for receive packages (1 through 255). The receiver clears the status bit if no new frame has been received within Scan Rate × Transmission Rate × OB1 cycle time. A scan rate of 3 is a good starting value.
9. Click GD Table → Compile (or the toolbar icon). The Status Bar should read No errors.
10. Download the configured data: PLC → Download to Target System → All Stations on Subnet. The download dialog lists every station that is reachable. Confirm with the password if the CPUs are write-protected.

6.1. Configuration Parameters at a Glance

7. Why the Error Is Reported as "No Valid Connection"

GD does not use the S7 connection object, so the Select... dialog in the GD table does not list existing S7 Connections. It walks the NetPro subnet topology and offers any CPU that satisfies three conditions:

1. It is part of the same STEP 7 project and not greyed-out (i.e., not a CPU that is unconfigured or off-subnet).
2. Its interface object has Global Data enabled. This is on by default for every S7-300 CPU, but can be turned off in HW Config → CPU → Properties → Global Data tab.
3. It has at least one free GD receive or send slot in the current circle.

When Select... shows "no valid connection", condition 1 is normally the one that fails. STEP 7's phrasing is a translation of the German original Keine gültige Verbindung, which dates back to STEP 7 V3 and predates the unified Select dialog of STEP 7 V5. The wording was kept for compatibility; the actual condition is "no CPU on this subnet has a free slot".

8. Root Cause Matrix

9. Verification Procedure

After the configuration is downloaded, the engineer should validate the GD circle both online in STEP 7 and from the user program. The minimum verification set is the following.

1. In the GD table, click View → Scan Rates. Each row has a small indicator: a green check mark means data has been received within the configured scan rate, a red cross means the receiver has not seen a fresh frame. A red cross at the receiver with a green check at the sender indicates one-way bus trouble (cable, termination, or address).
2. Open the Module Information on the receiver CPU (PLC → Module Information). The Communication tab lists the number of GD errors. A non-zero value with green checks in the GD table indicates that the receiver was offline during some scans; the error count is the only persistent record.
3. From the user program, read the status bit that STEP 7 generates for each receive package. The bit is part of the GD_STATUS data block. A simple OB1 segment is enough:

   A   DB100.DBX 0.0   // status bit of GD package 1
   =   M 10.0          // status of incoming GD
   JC  ok
   S   M 11.0          // flag: GD failed
   BEU
   ok: R M 11.0

4. Read the last received byte stream from the configured receive area and compare to the data the sender is generating. A difference in length or in structure means a mismatch in package definition; the GD protocol does not carry any header that would align the data for you.
5. Stop the receiver CPU. The sender must continue to broadcast, and the error counter must increment. This confirms that the sender is broadcasting and not simply echoing back. Bring the receiver back online; the green check mark should return within Scan Rate × Transmission Rate OB1 cycles.

10. Online Diagnostics with STEP 7

Beyond the GD table itself, STEP 7 exposes the live bus state. The most useful diagnostic actions in the order they should be attempted are:

• Accessible Nodes on the PG/PC interface. This enumerates every station on the currently selected MPI/PROFINET subnet. If the partner CPU is not listed here, the bus itself is the problem, not STEP 7.
• PLC → Module Information → Memory → Communication. The number of GD send/receive errors and the current state of the GD protocol are visible here. A persistent error count on the receiver that grows on every scan means the partner is not in the circle.
• Module Information → Diagnostic Buffer. Look for entries with Event ID 0x39xx and 0x3Axx (GD-specific events on S7-300). These are documented in the SIMATIC S7-300 CPU 31xC and CPU 31x: Diagnostic Buffer entry.
• On a CPU 315-2 PN/DP, the SF LED is lit only if a permanent error is present. A flashing SF LED at 0.5 Hz with a single configured GD circle typically means a partner CPU has been removed from the bus. The diagnostic buffer will record this with Event ID 0x3931 ("GD package failure") and the address of the missing station.

11. Migration Considerations: TIA Portal and S7-1500

If the project is being migrated to TIA Portal, or if a S7-1500 is replacing the S7-300, the Global Data concept does not carry over. The migration options are:

• S7-1500 to S7-300: S7-1500 does not support GD. Replace the GD circle with S7 connection PUT/GET blocks in the S7-300 and an S7 client in the S7-1500.
• Both S7-300 in TIA Portal: TIA Portal does not support GD directly, but the legacy S7-300 station can be re-imported and the GD table kept in the SIMATIC Manager project, side-by-side with the TIA Portal project. This dual-tool approach is officially supported as long as the TIA Portal side does not touch the S7-300 stations.
• Both S7-300 in classic STEP 7 V5.6: No change. Stay with the current toolchain.

12. Edge Cases and Field-Proven Caveats

1. GD across two MPI segments bridged by an S7-400. Not supported. The router forwards S7 connections but terminates GD.
2. GD when one CPU is in RUN and the other is in STOP. The sender will continue to broadcast; the receiver, when restarted, will not accept the configuration until the sender has been downloaded at least once.
3. GD and S7 connection coexistence on the same MPI bus. Supported. The S7 connection does not consume GD resources. The GD protocol uses different SAPs (Service Access Points) and does not interfere with S7 communication.
4. GD on a CPU in a non-local rack (extension rack). The receive area must be in the process image or in a DB that resides in the load memory of the receiver. The compiler accepts M, I, Q, DB, and PI/PQ areas. The S7-300 restricts M to the first 256 bytes and DBs to 1 through 255 (size up to 16 KB on most CPU 31x variants).
5. GD with timestamped data. The GD frame does not carry a timestamp. The user program must add a timestamp using the receiver's local time if cross-station order matters.
6. Hot-swapping a CPU. After replacing a CPU 314 with a CPU 314 of the same MLFB but newer firmware, the new CPU does not pick up the GD circle automatically. The configuration must be re-downloaded. Always mark a CPU replacement as a "trigger for full download" in the maintenance procedure.
7. GD with redundant MPI cables. Not supported. If the plant requires redundancy, the architecture must change to PROFINET with S7-1500 and an S7 connection with redundancy protocol (S7-REDCONNECT), or to PROFINET with a CP 1628/CP 443-1 Advanced.

13. Commissioning Checklist

14. Related Siemens Documentation

• STEP 7 V5.6 - Function Manual: Global Data — the canonical reference for the GD table editor and parameter ranges.
• S7-300 CPU 31xC and CPU 31x: Installation — hardware installation rules, MPI/DP interface pinout, bus termination.
• Diagnostic Buffer: S7-300 CPU 31xC and CPU 31x — event IDs for GD errors.
• SIMATIC NET: PROFIBUS Network Manual — bus parameters, repeater rules, transmission rate selection.

15. Frequently Asked Questions