PROFINET IRT Switch for SIMOTION D455-2 DP/PN: Topology Guide

Application Context: SIMOTION D455-2 DP/PN Driving Distributed SINAMICS Axes

The SIMOTION D455-2 DP/PN motion controller is typically deployed with multiple SINAMICS S120 CU320-2 PN Control Units to drive a large number of servo axes across an automated machine or production line. The decision of whether to add an external SCALANCE X204IRT switch, daisy-chain the CU320-2 PN units on the integrated PROFINET switch, or migrate to SINAMICS CX32-2 controller extensions is governed by three engineering constraints:

1. Isochronous cycle time required by the motion task (camming, gearing, cam disc synchronization).
2. Number of PROFINET IO devices and the topology rules of the SIMOTION D455-2 X150 integrated switch.
3. Mechanical positioning accuracy of the driven axis and the resulting IRT send-clock budget.

This reference consolidates the topology rules, the IRT cycle-time impact of every switched hop, and the part numbers of the only Siemens switches that support IRT Class 3 cut-through forwarding in this context.

PROFINET IRT Fundamentals for SIMOTION Motion Control

PROFINET IRT (Isochronous Real Time) is required whenever SIMOTION, SINAMICS S120, and the higher-level controller exchange setpoints and actual values with deterministic latency. Standard PROFINET RT and non-managed Ethernet switches cannot be used on the IRT portion of the network.

The SIMOTION D455-2 DP/PN supports IRT in Class 3 mode. The send clock (T_send) can be configured from 250 µs upward in steps of 31.25 µs. Every PROFINET device and every IRT-capable switch in the sync domain reserves a slot in the time-aware shaper. Adding an IRT switch does not speed up the bus cycle time; it adds to the jitter budget and reserves bandwidth. This is the most common misunderstanding when scaling a SIMOTION line.

Built-in Switch Resources: X150 on D455-2 and CU320-2 PN

Both the SIMOTION D455-2 DP/PN and the SINAMICS S120 CU320-2 PN carry an integrated PROFINET switch. The PROFINET IO interface is brought out at port X150 on both devices.

Because both devices have integrated switches, a line topology can be built without any external SCALANCE. The D455-2 acts as the line head with three outgoing lines; each CU320-2 PN terminates one port and forwards the next.

SCALANCE X204IRT and XF204IRT: Approved IRT Switches

When a star topology, network segmentation, or additional diagnostic visibility is required, Siemens specifies only two SCALANCE family devices for the IRT portion of a SIMOTION network. Both are members of the SCALANCE X-200IRT line managed from the Siemens Industry Mall SCALANCE catalog.

Key technical features of the X204IRT family that matter for the SIMOTION use case:

• Cut-through switching (on-the-fly frame forwarding) keeps the IRT switch delay to < 5 µs per hop.
• Sync domain participation — the X204IRT can be configured as a sync master or sync slave in the PROFINET sync domain.
• IRT Top and IRT Flex support — the switch is transparent to both IRT topologies used by SIMOTION SCOUT / TIA Portal.
• Web-based management (WBM) and SIMATIC STEP 7 / TIA Portal integration for diagnostics, port statistics, and topology discovery (LLDP).
• PROFINET conformance class C — the highest PROFINET switch conformance class for motion networks.

Topology Limits Configured in HW Config / TIA Portal

The PROFINET configuration tool (HW Config in SIMOTION SCOUT, or the TIA Portal device configuration) enforces several limits for a sync domain rooted at the SIMOTION D455-2 X150. The maximum configuration that the tool will accept on a single D455-2 is:

• 3 IRT lines originating from the X150 ports (P1, P2, P3) of the D455-2.
• 7 SINAMICS CU320-2 PN on each line.
• 6 axes on each CU320-2 PN.
• Send clock down to 250 µs.

These numbers describe the tool-accepted maximum. The integrated switch on the D455-2 and the device switches on the CU320-2 PN are IRT-capable, so the topology itself is realizable in the project. The 250 µs figure should be treated as the theoretical minimum send clock for a fully populated three-line configuration. In practice, projects rarely run below 500 µs because:

1. The jitter budget consumed by the chain of 7 device switches reduces margin.
2. The bandwidth consumed by the IRT bandwidth-reservation mechanism grows with the number of slots.
3. The diagnostic traffic of LLDP and DCP adds to the reserved time slice.

Switch Hop Impact on IRT Cycle Time

Every device in the sync domain reserves a time slot in the IRT cycle. The cut-through delay of an X204IRT is published at < 5 µs per hop; the integrated device switches on the D455-2 and CU320-2 PN are similar. The accumulated delay budget for a line of 7 CU320-2 PN units is therefore approximately:

t_hop_total = n_devices × t_cut_through
t_hop_total = 7 × 5 µs = 35 µs

This 35 µs is part of the send-clock budget and must be subtracted from the available processing window. As a rule of thumb, target the send clock at least 4× the accumulated switch hop delay to leave room for the controller ↔ drive telegram:

T_send_min = 4 × t_hop_total + t_telegram
For n = 7: T_send_min = 4 × 35 µs + 50 µs = 190 µs
Round to next valid T_send: 250 µs

When an external SCALANCE X204IRT is inserted, the same logic applies. One X204IRT adds 5 µs; ten X204IRT switches in series would add 50 µs and the project tool will refuse the configuration below 500 µs.

Line Topology Without an External Switch

For up to 7 CU320-2 PN per line and 3 lines from the D455-2, no SCALANCE is required. The wiring is direct CAT5e / CAT6 PROFINET copper between X150 ports. The sequence is:

Figure 1 — Line topology on the SIMOTION D455-2 X150 integrated switch. Three lines of CU320-2 PN, no external SCALANCE required.

Advantages of the no-external-switch approach:

• Zero additional cut-through delay.
• Lower cabinet cost — no SCALANCE module, no extra 24 V supply.
• Fewer configuration objects in HW Config / TIA Portal.
• Reduced failure surface — a SCALANCE failure cannot break the IRT line.

Disadvantages and constraints:

• No central point of network diagnostics — statistics must be read from each CU320-2 PN web server.
• A cable break on the line splits the downstream CU320-2 PN units from the controller.
• Maximum of 7 CU320-2 PN per line; for more, an external X204IRT is required.
• Maximum PROFINET copper segment length is 100 m (per IEEE 802.3 / IEC 61784-5-3); for longer distances, use SCALANCE X204-2 fiber variants in the non-IRT portion.

When to Add an External SCALANCE X204IRT

Insert an external X204IRT (6GK5204-0BA00-2BA3) or XF204IRT (6GK5204-0BA00-2BF2) in any of the following cases:

1. More than 7 CU320-2 PN on a single line — the line must be split into two segments, each terminated at the SCALANCE.
2. Star topology with multiple D455-2 controllers on a shared IRT sync domain.
3. Network segmentation between the motion subnet and the diagnostic / MES subnet — the X204IRT can apply VLAN tagging to keep the IRT frames isolated.
4. Centralized PROFINET diagnostics — the X204IRT's WBM and SNMP integration provide LLDP neighbor, port error counters, and sync status in a single pane.
5. Fiber uplinks between cabinets — the X204IRT family includes 100 Mbps SC fiber variants in the catalog line; the standard 6GK5204-0BA00-2BA3 is copper-only.

For a project of 20 CU320-2 PN units and 90 axes, the recommended topology is two or three D455-2 controllers in a star with the X204IRT at the center of each. Alternatively, if the 7-CU limit is exceeded, use one X204IRT per line to break the daisy-chain into two 7-CU lines.

SINAMICS CX32-2 Controller Extension as Alternative to CU320-2 PN

The SINAMICS CX32-2 is a slim controller extension that couples to the CU320-2 PN (or CU320-2 DP) over DRIVE-CLiQ rather than PROFINET. It adds two drive axes without consuming a PROFINET slot. Compared to a full CU320-2 PN, the CX32-2 has:

Internal axes on the D455-2: The D455-2 can control up to 36 drive axes directly over its internal PROFIBUS / PROFINET interface without any external CU. For applications that fit within 36 axes, the D455-2 is the single point of motion control and no IRT line is required at all — the integrated D455-2 axis interface handles the isochronous cycle internally.

Cycle Time Sizing from Mechanical Accuracy Requirements

The customer's specification drives the IRT send clock. With the example of "0.5 mm accuracy at 1 mm per 0.2 ms (i.e. 5 m/s or 300 m/min) full speed," the time budget is:

t_budget = 0.5 mm / (1 mm / 0.2 ms) = 0.1 ms = 100 µs

To resolve 100 µs of motion, the IRT cycle must be at least one order of magnitude faster, so target a 250 µs send clock. At 500 m/min (~8.33 m/s), 0.5 mm takes 60 µs, so the cycle must be ≤ 60 µs to keep the sample-and-hold error below the accuracy target. This is the hard physical floor and is the reason PROFINET IRT cannot reach 500 m/min with 0.5 mm accuracy on a 1 ms cycle. At 1 ms send clock, the sample-and-hold error alone is 8.3 mm and the system accuracy is dominated by interpolation, not cycle time.

Conclusion: At 500 m/min with 0.5 mm accuracy, no PROFINET IRT cycle will work in isolation. The mechanical loop, drive tuning, and interpolation between motion samples dominate the result. At 120 m/min or below, a 250 µs IRT send clock is feasible and provides the required motion resolution.

Commissioning and Verification Procedure

1. Open the SIMOTION SCOUT / TIA Portal project and load the device configuration. Verify the D455-2 DP/PN is added as a PROFINET IO controller with the IRT option enabled in the interface properties. The TIA Portal configuration workflow for SIMOTION D455-2 with PROFINET IO drives is described in the SIMOTION controller with PROFINET IO drive configuration guide.
2. Add the CU320-2 PN devices to the PROFINET topology. The maximum is 7 CU320-2 PN per D455-2 X150 line and 6 axes per CU. Drag them into the line in the topology editor.
3. Configure the sync domain: in the PROFINET interface properties of the D455-2, set the role to Sync Master and the send clock to the calculated value (typically 500 µs or 1 ms for first commissioning). All CU320-2 PN devices default to Sync Slave.
4. Insert SCALANCE X204IRT (6GK5204-0BA00-2BA3) only when the topology requires it. Set the X204IRT role to Sync Slave; do not designate it as Sync Master when the D455-2 is already master.
5. Compile and download the HW Config. The tool will report topology errors (over-length lines, missing sync assignments) in the messages pane before going online.
6. Go online and read the diagnostic buffer of the D455-2. Verify that each CU320-2 PN reports Sync OK and that the IRT slot is reported as Active. Open the X204IRT WBM (default IP 192.168.x.x) and check the LLDP neighbor list and port error counters.
7. Run a motion test with a single axis at low speed. Verify the setpoint / actual value trace in the SIMOTION trace tool. The jitter of the actual-value sampling should be < 10% of the send clock.
8. Run a multi-axis cam test. The electronic cam should not show any visible step on the oscilloscope; if a step is present, the sync domain is misconfigured or a switch is consuming the budget.

Troubleshooting Matrix

Recommendations Summary

• For 1–7 CU320-2 PN and ≤ 36 axes total, use the D455-2 X150 integrated switch with direct daisy-chain. No SCALANCE is required.
• For 8–20 CU320-2 PN or more than one D455-2 in a shared sync domain, add one SCALANCE X204IRT (6GK5204-0BA00-2BA3) per line, configured as Sync Slave.
• For pure axis expansion, use the SINAMICS CX32-2 over DRIVE-CLiQ instead of a full CU320-2 PN to free PROFINET slots and reduce cost.
• Always use SCALANCE X204IRT or XF204IRT on the IRT portion. The SCALANCE X005 (unmanaged) is not IRT-capable.
• Size the send clock to the mechanical speed: 250 µs is the practical floor for a populated line, 500 µs to 1 ms is the typical operating range.
• Verify the topology, sync role, and LLDP neighbors in the TIA Portal before commissioning the motion task.