Resolving 'CPU Could Not Be Linked Online' When Monitoring Multiple S7-400 Controllers in CFC

In SIMATIC PCS 7 V7.1, the error 'CPU could not be linked online' or prompts to 'Register CPU for testing' when attempting to monitor CFC charts across different S7-400 controllers simultaneously occurs because the CFC Test Manager by default limits active online testing registration to a single CPU per editor session, or because the PG/PC Interface has exhausted available S7 communication credits. To resolve this, open separate SIMATIC Manager / CFC instances for each automation station, configure the 'Register CPU for Testing' dialog to switch targets cleanly, or increase available connection resources in NetPro.

The CFC Test Manager Registration Architecture

In large-scale SIMATIC PCS 7 multiprojects containing multiple Automation Stations (e.g., three redundant S7-417-5H controllers connected across an Industrial Ethernet Plant Bus), commissioning engineers frequently open CFC charts from different CPUs simultaneously.

When an engineer activates Test Mode (the glasses icon) on a chart in CPU 1, monitoring starts smoothly. However, when they attempt to activate Test Mode on a second chart belonging to CPU 2, SIMATIC Manager reports:

The CPU could not be linked online.
Another CPU is already registered for testing.

Or prompts:

Register CPU for testing:
Do you want to disconnect CPU 1 and register CPU 2?

This behavior is rooted in the internal architecture of the STEP 7 / CFC Test Manager engine.

Why Multi-CPU Simultaneous Test Mode Fails in V7.1

To understand this limitation, evaluate how the Engineering Station handles online connections:

1. Single-Target Test Session Locking

In classic PCS 7 versions (including V7.0, V7.1, and early V8.0), a single running instance of the Continuous Function Chart (cfc.exe) application operates under a single active test session context.
* When you click the Test Mode icon, the editor binds the local test engine to the specific S7 Program folder and CPU network address of that chart.
* Attempting to monitor a chart in another CPU within the same CFC shell forces a session collision: the test engine cannot multiplex different CPU target addresses simultaneously within a single document interface.

2. S7 Communication Credit Limits

Every online test connection consumes an S7 Connection Resource (Credit) on the communications processor (e.g., CP 1613 / CP 1623) and on the target CPU 417-5H. If the Engineering Station's access point (S7ONLINE) is restricted or if the target CPU has exhausted its reserved PG/OP connection resources, subsequent online registration attempts are blocked with communication errors.

Comparison Table - Online Monitoring Modes and Resource Consumption

The following table compares different methods for monitoring multiple controllers from a single Engineering Station.

Monitoring ApproachSimultaneous Multi-CPU SupportS7 Connection Resource LoadStabilityTypical Use CaseSingle CFC Editor InstanceNo (Requires Target Switching)Low (1 PG connection at a time)HighRoutine single-loop commissioningSeparate SIMATIC Manager WindowsYes (Full Multi-CPU)Moderate (1 PG connection per CPU)HighMulti-controller interlocking testsProcess Object View (PO View)Yes (Cross-CPU Monitoring)Managed dynamicallyModerateBulk signal commissioningWinCC OS Runtime FaceplatesYes (Full Multi-AS Integration)Managed via S7-REDCONNECTHighestPlant-wide operational monitoring

Step-by-Step Multi-CPU Online Monitoring Procedure

To monitor charts across different CPUs simultaneously without connection conflicts, execute the following verified procedure:

Method A: Launch Independent SIMATIC Manager Instances:
* Open the first instance of SIMATIC Manager from the Windows Start menu.
* Open the Multiproject and navigate to CPU 1 -> Charts. Open the target chart and click Test Mode (glasses).
* Launch a second independent instance of SIMATIC Manager from the Start menu (do not use File -> Open within the same window).
* In the second instance, navigate to CPU 2 -> Charts. Open the second chart and click Test Mode.
* Because each CFC editor runs in an isolated process space with its own test context, both charts monitor online simultaneously.
Method B: Clean Target Switching via Test Manager:
* If working within a single window:
* When switching from CPU 1 to CPU 2, click the Test Mode icon to deactivate monitoring on CPU 1.
* Select Debug -> Register CPU for Testing.
* Select CPU 2 from the list and click Register.
* Open the chart in CPU 2 and activate Test Mode. The connection succeeds instantly without communication fault warnings.
Verify S7 Connection Allocation in NetPro:
* Open NetPro.
* Double-click the CPU 417-5H to open its properties.
* Under the Communication tab, verify that PG communication has at least 2 or 3 reserved connection credits.
* Ensure the total allocated connections do not exceed the CPU limit.

Verification

Confirm that multi-CPU online monitoring functions properly:

Verify Simultaneous Value Updating:
* Position the two CFC chart windows side by side.
* Confirm that live dynamic values on both charts update in real time with green status banners.
Verify Module Diagnostic Buffer Access:
* In SIMATIC Manager, select CPU 1 and press Ctrl+D. Confirm module information opens.
* Select CPU 2 and press Ctrl+D. Confirm module information opens concurrently without connection dropped warnings.
Verify PG/PC Interface Diagnostics:
* Open Set PG/PC Interface.
* Click Diagnostics -> Test. Confirm connection to all Industrial Ethernet nodes reports OK.

Limitations and Operational Precautions

PG Connection Exhaustion: Opening multiple online monitoring sessions across several CPUs consumes PG connection resources on the controller. Always disconnect Test Mode when commissioning is finished to free controller communication bandwidth.
Network Bandwidth Considerations: Running live monitoring across multiple large charts increases Plant Bus Ethernet broadcast traffic. Avoid running dozens of simultaneous test charts on slow 10 Mbps or heavily loaded networks.
PCS 7 Version Enhancement: Modern PCS 7 versions (V8.1, V8.2, V9.0+) feature improved multi-CPU session handling, reducing the frequency of CPU registration prompts.

Engineer FAQ

Can I monitor a CPU 410-5H and a CPU 417-5H at the same time?

Yes. By running separate SIMATIC Manager / CFC processes, you can monitor heterogeneous controller models simultaneously from the same Engineering Station.

Why does CFC prompt 'Do you want to disconnect CPU 1'?

The single-process CFC Test Manager can only lock its internal memory pointer to one CPU symbol database at a time. The prompt gives you the choice to release CPU 1 and switch testing focus to CPU 2.

Does S7-PLCSIM support multi-CPU testing?

S7-PLCSIM V5.4 can run multiple simulated CPU instances simultaneously (PLCSIM 1, PLCSIM 2). Each instance requires its own separate CFC session and virtual network binding.

Where can I see how many PG connections are currently active on the CPU?

Open SIMATIC Manager, select the CPU, press Ctrl+D to open Module Information, and navigate to the Communication tab. It displays active PG, OP, and S7 connections.