Fixing Slow Online Monitoring on Siemens IM 151-7 CPU ET200S

Problem Statement

When a programming device (PG) attempts to monitor program variables on a Siemens IM 151-7 CPU (order number 6ES7 151-7AA11-0AB0) inside an ET 200S station, the connection establishes successfully but every read/write operation is excessively slow. The CPU reports no diagnostic (SF/BF) errors, the OB1 cycle time is healthy (3 ms in the reported case), and the PC/PG interface test passes, yet the STEP 7 online view (VAT, monitor/modify, reference data) still responds at sub-second tick rates.

This symptom typically indicates a communication-routing bottleneck between the PC Adapter USB A2 and the ET 200S CPU rather than a CPU overload. Because the IM 151-7 CPU is a compact PLC acting as a PROFIBUS slave, every online request is bridged across the bus segment before it reaches the application code.

Affected Hardware and Network Topology

The reported configuration has three PROFIBUS segments and three CPU-like devices, which is the most common cause of routing-related online slowness:

The PC Adapter USB A2 normally plugs into the MPI/DP port of the IM 151-7 CPU. The two onboard DP interfaces of the IM 151-7 CPU are independent PROFIBUS segments; only one is bridged to the MPI/DP connector. If the PC Adapter is plugged into the wrong physical connector, every request has to be routed through the DP master expansion module before it reaches the application, which multiplies online-monitoring latency.

For full hardware reference data, see the SIMATIC ET 200S distributed I/O system manual on the Siemens Industry Online Support portal.

Root Cause Analysis

Online monitoring of program variables is implemented as an S7 monitor/modify service. Each variable refresh consumes a small portion of the OB1 cycle in a special "communication" window. The IM 151-7 CPU shares its scan cycle between user program execution and PG/OP communication using the parameter Scan cycle load from communication (German: Auslastung der Zykluszeit durch Kommunikation). The default value is 20 %, meaning only one fifth of the OB1 is reserved for PG services. When the user opens a large VAT or refreshes a Process Diagnostics view, the queue grows and the PG sees multi-second delays.

Three independent bottlenecks usually combine:

1. Communication cycle share too low – the default 20 % is insufficient for an ET 200S CPU running online services while the application keeps requesting DP acyclic data.
2. PC Adapter connected to the wrong DP segment – the engineering station must reach the IM 151-7 CPU on the segment that contains the CPU's own MPI/DP port, not on the segment that hosts the SICK PLC and HMI.
3. Mixed baud rates – the MPI/DP port runs at 1.5 Mbps, the lower DP bus at 500 kbps. Any routing requires the slower bus, and each packet must wait for the PROFIBUS token to rotate on both segments.

Diagnostic Procedure

Run the following checks in order. Each step rules out a single layer of the stack before you change any parameter.

1. Validate the Physical Connection Point

• Locate the IM 151-7 CPU in the ET 200S rack. The unit has two Sub-D connectors on the front labelled X1 (MPI/DP) and X2 (DP).
• Verify that the PC Adapter USB A2 is plugged into X1, not X2. The MPI/DP port is the only one that supports PG functions on the IM 151-7 CPU; the X2 port is reserved for PROFIBUS DP master/slave operation through the insertable DP master module.
• Power-cycle the ET 200S station. A swapped connector can leave residual routing tables in the PC Adapter driver.

2. Run the PC/PG Interface Test

Open Start » SIMATIC » STEP 7 » Set PG/PC Interface, select PC Adapter (MPI/DP) » PC Adapter USB A2, then click Test. The dialog should report OK and show a transmission rate of 1.5 Mbps. If the test reports Fault or a lower rate, the driver or cable is the problem, not the IM 151-7 CPU.

3. Inspect the CPU Diagnostic Buffer

Online » Accessible Nodes » right-click the IM 151-7 CPU » CPU Messages » Diagnostic Buffer. Look for entries with ID 0x4542 (communication overload), 0x4521 (DP bus fault), or 0x40C2 (cycle time overrun). A clean buffer combined with a 3 ms cycle time confirms the application code is healthy.

4. Measure the PROFIBUS Bus Timing

Use the integrated bus diagnostics of the PC Adapter or a PROFIBUS tester (e.g. Softing PROFIusb, Indu-Sol PROFINET-INspektor) to verify:

If the token rotation time exceeds the configured Target Rotation Time (Ttr) on either segment, the bus is overloaded and STEP 7 will queue PG requests until the token returns. See the ET 200S Commissioning Manual for the procedure to read Ttr from STEP 7 HW Config.

5. Check the Cycle Time Distribution

Open Online » Diagnostics » Module Information on the IM 151-7 CPU and switch to the Performance tab. The values should look like the table below. If the communication share is consistently hitting 100 % during online sessions, the cycle share is the bottleneck.

Resolution Steps

Apply the following changes in the order given. Each one addresses a different layer of the stack, so re-test after every change.

Step A – Increase the Communication Share of the Scan Cycle

1. In STEP 7, open HW Config and double-click the IM 151-7 CPU.
2. Switch to the Cycle / Clock Memory tab.
3. Locate the field Scan cycle load from communication (range 10 % – 50 %).
4. Raise the value from 20 % to 50 % if the application allows. A 50 % share is the maximum supported by the IM 151-7 CPU and is the recommended setting whenever permanent online monitoring or large VAT tables are required.
5. Save, recompile, and download the hardware configuration to the IM 151-7 CPU.

Trade-off: a higher communication share means the user program runs slower under load. For a 3 ms OB1 with a 50 % share the program receives approximately 1.5 ms; this is still acceptable for the typical ET 200S cycle profile.

Step B – Move the PC Adapter to the Correct Connector

Plug the PC Adapter USB A2 into the X1 (MPI/DP) port of the IM 151-7 CPU. If the engineering station must remain connected to the lower 500 kbps bus, add a CP 5711 or use an IE/PB Link to bridge Ethernet to PROFIBUS instead of relying on the USB adapter to relay the request.

Step C – Reduce PROFIBUS Load on the Lower Segment

• Confirm that the SICK PLC and the HMI use the minimum required DP cycle (avoid 1 ms polling on 500 kbps segments).
• Disable acyclic DP-V1 services on the lower bus if the application does not require them.
• Verify that the bus terminator is enabled on both ends of every segment and disabled on intermediate stations.

Step D – Switch to a Faster PG Interface

For permanent online monitoring, replace the PC Adapter USB A2 with one of the following:

Higher baud rates shrink the time required to push a VAT update across the bus. The PC Adapter USB A2 product page lists the supported baud rates of 19.2 kbps through 1.5 Mbps only.

Step E – Refresh the PG Connection

After the parameter change, the new share is only active for connections opened after the new online session. In STEP 7, go to Online » Accessible Nodes, select the IM 151-7 CPU, right-click and choose Disconnect, then double-click again to re-establish the PG connection with the new share active.

Verification

Confirm the fix with the following four checks:

1. Open a VAT with 20 variables and refresh (Variable » Update). The first refresh must complete in less than 500 ms on a healthy configuration.
2. Open Module Information » Performance on the IM 151-7 CPU. The communication share used should now stay below 30 % during continuous refresh.
3. Toggle a bit from the VAT and verify the change appears in the OB1 scan within one OB1 cycle (3 – 5 ms).
4. Disconnect and re-connect the PC Adapter USB A2 several times. The connection should re-establish in 2 – 3 s with the new parameter set.

Preventive Measures

• Document the PC Adapter port in the project drawing. Add a label to the IM 151-7 CPU that says "PG: X1 (MPI/DP) only".
• Lock the baud rate in HW Config for both PROFIBUS segments. Auto-detect causes the segments to negotiate to the slowest device after a bus fault.
• Enable bus diagnostics on the CPU 318-2 master and the insertable DP master module. The diagnostic buffer will record a permanent bus fault before the PG performance degrades.
• Use Ethernet-based online services when the application allows. A CP 343-1 / CP 443-1 in the higher-level CPU and a PROFINET-to-PROFIBUS link from the PG side typically deliver sub-100 ms online response.
• Avoid mixing acyclic DP-V1 traffic with online monitoring on the same 500 kbps segment. Acyclic read/write services consume 2 – 4 token rotations per call, which can stall PG jobs for several hundred milliseconds.

Troubleshooting Matrix

Reference Parameters for the IM 151-7 CPU

For the full parameter list, see the SIMATIC ET 200S IM 151-7 CPU manual collection in the Siemens Industry Online Support portal.

Edge Cases and Field Notes

• If the IM 151-7 CPU is configured as a DP slave only, the application code lives on the higher-level CPU 318-2 master. In that case, online monitoring must target the CPU 318-2, not the IM 151-7 CPU, and the IM 151-7 CPU is reachable only as a passive DP slave.
• If the engineering PC is running STEP 7 inside a virtual machine, the USB pass-through of the PC Adapter USB A2 adds 50 – 200 ms latency per PG request. Connect the adapter to a physical USB port of the host and dedicate the USB controller to the VM (no USB sharing).
• If the user changes the MPI/DP address of the IM 151-7 CPU to a value above 31, the PG cannot discover it through standard MPI scan. Use the PG/PC interface direct connection and enter the address manually.
• If the insertable DP master module is also configured as a master, two active stations share the bus token. The token rotation time on the 500 kbps segment doubles; raise the share to 50 % on the IM 151-7 CPU and reduce the number of slaves on the segment to compensate.