Troubleshooting Using SINAMICS CU320-2 as a PROFINET IRT Switch

Overview

When commissioning a machine built around a SIMATIC S7-1500 controller and a multi-axis SINAMICS S120 drive line-up, the PROFINET IRT (Isochronous Real-Time) backbone normally terminates at a dedicated IRT switch such as a SCALANCE XC-200, XC-300, or XR-300. If the IRT switch has not been delivered in time for the integration test, each SINAMICS CU320-2 control unit can substitute as a forwarding node because it integrates a managed 2-port PROFINET switch that supports IRT Class 2 (High Flexibility) and IRT Class 3 (High Performance) traffic. This reference documents the topology, the engineering constraints, the TIA Portal configuration, and the verification steps required to run a temporary network with the CU320-2 acting as the IRT forwarder before the SCALANCE switch is wired in. The same procedure is also valid for permanent installations where a cabinet footprint must be minimised and a small star can be realised with the drive-internal switch fabric.

IRT Communication Fundamentals

PROFINET IRT is the time-deterministic variant of PROFINET and is mandatory for closed-loop control applications where jitter must stay below 1 µs. The protocol is defined in IEC 61158 and IEC 61784 and is realised by reserving a fixed time slice inside each send clock for IRT frames, while non-real-time frames use the remaining bandwidth. Three IRT capability classes exist:

An IRT domain requires a sync master, normally the PROFINET interface of the S7-1500 CPU, and one or more sync slaves. The CU320-2 with PROFINET (article number 6SL3040-1MA01-0AA0) is a sync slave and can be cascaded so that downstream CU320-2 units receive the synchronisation stream through the upstream device's integrated switch.

SINAMICS CU320-2 PROFINET Capabilities

The CU320-2 PN control unit provides two PROFINET ports, X150 P1 and X150 P2, that together form a 2-port cut-through switch. Both ports support 100 Mbit/s full duplex, autocrossover, and autonegotiation. The switch is PROFINET-conformant and forwards IRT frames in cut-through mode inside the synchronised phase, which keeps the per-hop latency at approximately 3 µs. The relevant product variants are:

The CU320-2 PN is documented in the SINAMICS S120 Function Manual, entry ID 59748931. The device supports up to 254 PROFINET IO devices when acting as IO controller, but in the cascaded topology it acts only as IO device, and the maximum number of cascaded nodes is bounded by the send clock and the sync-master capability of the S7-1500 CPU. As a rule of thumb, a CPU 1515-2 PN supports up to 32 IRT devices at 500 µs send clock, while a CPU 1518-4 PN/DP supports up to 128 IRT devices at 250 µs send clock.

Topology Architecture Comparison

The two valid topologies for the integration test are shown in the diagram below. Topology A is the production architecture; topology B is the temporary architecture with one CU320-2 forwarding IRT frames to a second line.

Both topologies are valid for IRT Class 2 and Class 3. Topology B is a pure line because the CU320-2 only has two PROFINET ports, so a third branch is impossible without a real switch. If the line-up has more than 5 drives, a star cannot be realised with drive-internal switching and Topology A must be adopted before the FAT (Factory Acceptance Test).

Prerequisites and Engineering Data

Before changing the topology, gather the following data from the existing project:

• PROFINET device name and IP of every CU320-2 (default name sinamics-s120-cu320-2, default IP 192.168.0.10 + offset).
• Firmware version of each CU320-2 (read with SINAMICS Starter or TIA Portal online → Diagnostics → Module Information).
• Send clock currently used in the PROFINET IO system (typical: 500 µs, 1 ms, 2 ms, or 4 ms).
• Reduction ratio of the IRT applications (typical 1, 2, 4, 8, 16, 32, 64).
• Sync domain ID and role (sync master / sync slave) of the S7-1500 PROFINET interface.
• Topology editor port interconnections from the TIA Portal project.

The hardware support package (HSP) for SINAMICS S120 must be installed in TIA Portal. For firmware V5.x and V6.x use HSP SINAMICS_S120_V60_HSP.zip; for V4.8 use HSP SINAMICS_S120_V48_HSP.zip. The HSPs are delivered via the TIA Portal hardware catalog under "Other field devices → PROFINET IO → Drives → Siemens AG → SINAMICS".

Step-by-Step: TIA Portal Configuration

1. Open the TIA Portal project that contains the S7-1500 station and the SINAMICS S120 devices. Switch to the Devices & Networks view.
2. Select the SCALANCE switch entry and disconnect its port P1 from the S7-1500 PROFINET port. Keep the device in the project; it is required for the final topology.
3. Connect the S7-1500 PROFINET port X1 P1 directly to port P1 of the first CU320-2 (the head of line 1). Use the Topology editor and pick "Add PROFINET port interconnection". The local port name appears as Port 1 on the CU320-2 PN.
4. Connect port P2 of the first CU320-2 to port P1 of the second CU320-2 (line 1 second node). Repeat for every drive in line 1.
5. Connect port P2 of the second CU320-2 of line 1 to port P1 of the first CU320-2 of line 2 if a second line is required. The cascading CU320-2 forwards the IRT stream in cut-through mode and remains a sync slave to the S7-1500.
6. Open the properties of the PROFINET interface of the S7-1500 CPU and verify the Real-Time settings: Send clock = 500 µs, Reduction ratio = 1, Watchdog time = 3 × send clock, IRT class = Class 3 if isochronous application; otherwise Class 2.
7. For each CU320-2, open Properties → PROFINET interface → Real-Time settings and set the device role to IO device, IRT mode = IRT top (mandatory for sync forwarding), and Synchronisation role = Sync slave.
8. Compile the project. In the topology check the message "Topology is consistent" must appear; otherwise open the Topology editor and verify the partner port numbers.
9. Download the project to the S7-1500. Restart the drives in sequence (line 1 head first, then cascaded drives) so the sync domain builds in the correct order.

Send Clock and Reduction Ratio Tuning

The send clock defines the basic cycle of the IRT phase, while the reduction ratio determines how many send clocks fit into one application cycle. Both parameters must be set consistently across the sync domain. The following table lists valid combinations for the S7-1500 and CU320-2 combination:

When the network grows beyond 16 nodes, increase the send clock to 1 ms or 2 ms before the reduction ratio; raising the reduction ratio does not help scalability. The CU320-2 with firmware V5.2 SP3 and later supports a send clock of 250 µs; V4.8 only supports 500 µs minimum.

SINETPLAN Validation Workflow

SINETPLAN is a Siemens network planning tool that loads a TIA Portal export, models the wired topology, and simulates update time, jitter, and bandwidth utilisation. The V2.0 trial mentioned in the original engineering note is still distributed via the Siemens Industry Online Support, entry ID 109762388. The validation workflow is:

1. Export the project from TIA Portal via "Project → Export → SINETPLAN (.xml)".
2. Open SINETPLAN V2.0 and import the XML file. The tool will draw the topology and colour the IRT lines in red, RT lines in green.
3. Open the Simulation tab and set the PROFINET update time for the S7-1500 IO system to the configured send clock.
4. Run the bandwidth check. SINETPLAN will report a warning if the IRT phase exceeds 50 % of the send clock. A red exclamation icon means the network is overbooked and the send clock must be increased.
5. Compare two project snapshots: one with the SCALANCE switch and one with the cascading CU320-2. The tool will list the additional per-hop latency of approximately 1.8 µs introduced by the drive-internal switch.
6. Save the SINETPLAN report as a PDF and attach it to the commissioning log.

For permanent installations a SINETPLAN report is recommended as part of the FAT documentation because the simulation exposes bottle-necks that are not visible in TIA Portal.

Performance Implications

The integrated 2-port switch in the CU320-2 PN adds approximately 1.8 µs of cut-through latency per hop. In a temporary line of five drives the total synchronisation latency is therefore 5 × 1.8 µs = 9 µs, which is well below the 1 µs jitter limit per hop. In a star with a SCALANCE XC-300, the per-hop latency is typically 0.9 µs and the store-and-forward jitter is bounded to ± 0.3 µs. The two values are not directly additive: cut-through latency is deterministic for IRT frames, while store-and-forward jitter is the more relevant metric for RT traffic that shares the line. The practical conclusions are:

• For pure IRT Class 3 networks with isochronous application, the difference between SCALANCE and CU320-2 cascading is negligible because the IRT phase is scheduled to the slowest device.
• For mixed IRT Class 2 networks with significant RT load (HMI, ET 200, third-party IO), the SCALANCE switch is preferred because the larger frame buffer absorbs bursts.
• The maximum number of cascaded CU320-2 units is governed by the cumulative sync jitter, which must stay below ± 5 µs across the whole chain. With a 1.8 µs per-hop latency this allows a chain length of up to 5 devices at 500 µs send clock.
• The CU320-2 does not support port-priority or VLAN tagging for IRT frames. If the network requires QoS separation between IRT and RT, a SCALANCE must be inserted in front of the CU320-2 cluster.

Verification and Commissioning Checklist

1. Check that all CU320-2 devices show a green PROFINET link LED on both X150 ports and that the Sync LED (SF/BF) is off. A steady yellow SF LED indicates a topology error; a flashing BF LED indicates loss of sync.
2. In TIA Portal online, open the Diagnostics view of the S7-1500 PROFINET interface and confirm the message "Sync domain 1 active, 5 sync slaves connected" (or the appropriate count).
3. Open Starter / Startdrive online and verify the isochronous time stamp of every drive. The stamp should be stable at the configured value (e.g., 500 µs ± 1 µs).
4. Trigger a controlled stop and re-start of the line. Verify that all drives come back to the synchronised state within 3 send clocks (1.5 ms at 500 µs).
5. Run a 24-hour soak test with the maximum axis velocity and confirm zero synchronisation faults in the diagnostic buffer.
6. Replace the temporary topology with the SCALANCE switch, re-run the same checklist, and compare the per-axis jitter values. A delta of less than 0.5 µs confirms that the temporary CU320-2 cascading was within spec.

Troubleshooting Matrix

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