S120 CU320-2 Drive-CLiQ Topology: DO Counting and Limits

Overview: SINAMICS S120 CU320-2 Multi-Axis Architecture

The SINAMICS S120 drive system is built around the CU320-2 Control Unit, a centralized controller that operates Motor Modules in Booksize, Chassis, and Blocksize formats. The CU320-2 supports PM240-2 Power Modules directly via CUA31 / CUA32 Control Unit Adapters, and PM340 Power Modules through the same CUA path. The control unit coordinates up to a fixed number of drive objects (DOs) over a single Drive-CLiQ ring, and the system enforces hard upper limits on the count and on the mix of control modes.

For multi-axis modules, every individual axis is counted as a discrete drive object. A Double Motor Module (Booksize) therefore consumes two of the available DO slots on the CU320-2, not one. This counting rule is the most frequent source of configuration errors and is the primary reason an apparently small cabinet can exceed the capacity of a single control unit.

Planning an S120 cabinet therefore requires a DO budget that is calculated before any wiring begins, because the question "do I need one CU, two CUs, or a performance extension CF card?" cannot be answered without first knowing the total DO count and the control mode of every Motor Module.

Drive Object Concept in SINAMICS S120

A drive object is any logical node that the CU320-2 must process in its drive-side firmware. The DO types that contribute to the topology limit include:

• Motor Modules (Single and Double, Booksize / Chassis / Blocksize) – each axis is a DO
• Power Modules with CUA (PM240-2, PM340) – the power module is a DO
• Sensor Modules (SMC10, SMC20, SMC30, SMC40) – each encoder on the Drive-CLiQ ring is a DO
• Terminal Modules (TM31, TM41, TM54F, TM120, TM150) – each TM is a DO even when it carries no motor
• Communication Boards (CBE20 / CBE25 Ethernet, CBP2 PROFIBus) – each counts as a DO
• Onboard I/O of the CU – the digital/analog I/O on the CU320-2 itself is a DO
• Control Unit (CU320-2) – one DO for the controller itself

CU320-2 Hard Topology Limits

Siemens defines three independent ceilings on the number of DOs that can run on a single CU320-2 at the same time. The active ceiling is the lowest of the three that applies to the chosen control mode.

These limits apply to a standard CU320-2 with the default firmware card. The performance extension CF card does not raise the DO ceiling; it raises the computational throughput (shorter current controller cycle, higher number of supported DOs in servo) and is sized by the firmware function block, not the topology rule.

Drive-CLiQ Wiring Rules

Drive-CLiQ is a proprietary Siemens real-time serial ring that carries encoder feedback, setpoints, diagnostics, and safety telegrams between the CU320-2 and every connected node. The rules that govern a legal topology are summarized in the SINAMICS S120 Function Manual (07/2016 edition), Chapter 13.3 (Drive Object Definition) and 13.10.2 (Drive-CLiQ Rules):

1. Maximum nodes per CU320-2 port chain – 14 Drive-CLiQ nodes per DRIVE-CLiQ port on the CU320-2 (the CU has two ports, so 28 nodes is the absolute upper ring size in special topologies).
2. Maximum DO count – bound by the SERVO / VECTOR / U/f ceilings above, not by the port count.
3. Star vs. daisy chain – both are supported. Daisy chain (linear) is preferred because it preserves the ring redundancy of the DRIVE-CLiQ topology; a break anywhere drops the affected segment.
4. TM54F placement – must be the first node after the CU320-2 in the safety chain; its DO is reserved at commissioning.
5. SMC30 placement – sensor modules can be placed on the motor's secondary port, allowing dual-encoder feedback without consuming a CU port.
6. CUA31 / CUA32 – a Blocksize PM240-2 Power Module with CUA adapter appears in the topology as one Power Module DO plus the CUA's own DO. The CUA must sit between the CU and the power module on Drive-CLiQ.

Counting DOs for a 5 × PM240-2 + 5 × SMC30 Application

For the typical cabinet sketched in the question, the drive-side object list is fixed and the only variable is the control mode of the PM240-2 power modules.

Even in the minimum configuration (5 PM240-2 + 5 CUA31 + 5 SMC30 + 1 CU), the topology already holds 16 drive objects. The control mode of the PM240-2 axes then decides whether this fits on a single CU320-2:

Configuration Scenarios: Single CU, CF Card, or Two CUs

Scenario A – Single CU320-2, No Performance Extension

This is viable only when all five PM240-2 axes run in open-loop U/f control and the total DO count (including CUAs, SMCs, and the CU itself) does not exceed 12. In the standard 5 × PM240-2 + 5 × SMC30 + 5 × CUA31 + 1 × CU configuration (16 DOs) this scenario fails the topology check even in U/f mode. Reducing the design – for example, removing the SMC30 on the four axes that share an encoder with the CU – can bring the count down to 12 and make Scenario A legal. The current controller cycle remains at the default 125 µs; the basic regulator bandwidth is sufficient for fans, pumps, and conveyor U/f applications.

Scenario B – Single CU320-2 + Performance Extension CF Card (CF Slot 2)

The performance extension card, order number 6SL3054-0FB01-1BA0 (and successor part numbers depending on firmware version), does not increase the DO ceiling. It enables shorter cycle times (e.g., current controller 62.5 µs instead of 125 µs, speed controller 125 µs instead of 250 µs) and supports the licensed advanced functions. It is the right answer when the application is computationally too fast for the default card but the topology count is already within the ceiling. In the 16-DO example above, the CF card alone does not bring the topology into compliance.

Scenario C – Two CU320-2 Control Units

This is the only topology that satisfies the SINAMICS rules for a 5 × PM240-2 (in SERVO or VECTOR) + 5 × SMC30 design. The DOs are partitioned across the two CUs. A typical split is 3 axes on CU #1 and 2 axes on CU #2, with the SMC30 of each axis on the same CU as its PM240-2. Cross-CU coordination – for example, a master speed reference shared between cabinets – is achieved over PROFINET, PROFIBus, or the SINAMICS link. The two CUs each have their own CF card, and the CUA31 of each PM240-2 stays on the same CU as the power module it controls.

Performance and Cycle-Time Considerations

The topology ceiling and the cycle-time budget are independent parameters, but the response time of the drive system is a direct function of how many DOs are on the same CU. As the DO count rises, the firmware scheduler spends more time on each axis and the regulator jitter increases. Practical guidelines for a 125 µs current controller clock:

For a 5-axis SERVO design, running all 5 DOs in SERVO on one CU leaves only 1 free slot (ceiling = 6) and the CPU is loaded to roughly 80 – 90 percent of capacity. This is a fragile design; the recommended engineering practice is to keep SERVO loading at 50 – 70 percent to leave headroom for diagnostics, free function blocks, and future scope growth.

Step-by-Step Sizing Procedure

1. List every node that will sit on the Drive-CLiQ ring: PM240-2, CUA31, SMC30, TM54F, TM31, CBE20, the CU itself.
2. Assign the control mode of every Motor Module / Power Module (SERVO, VECTOR, HLA, U/f).
3. Count the DOs per node and group by control mode.
4. Compare to the ceiling: SERVO/HLA ≤ 6, VECTOR ≤ 6, U/f ≤ 12, mixed SERVO+VECTOR ≤ 6.
5. Add headroom: subtract 1 to 2 DOs from the ceiling for the controller, the CBE, the TM54F, and for free function blocks.
6. Decide on the controller count: if the count fits with headroom, a single CU is sufficient. If it does not, partition the design into two CUs.
7. Decide on the firmware card: if cycle time or advanced function blocks are needed, license the performance extension. The card does not change the DO ceiling.
8. Document the topology in Starter / Scout / Startdrive and run an offline consistency check (see Verification below).
9. Cross-check the part numbers of the CU320-2 (e.g., 6SL3040-1MA01-0AA0 for the DP variant, 6SL3040-1MA00-0AA0 for the PN variant) and the CF card against the SINAMICS S120 Catalog D31.

Verification Checklist

• DO count in STARTER / Startdrive: navigate to Drive > Topology > DO List and confirm the total is below the active ceiling for the chosen control mode.
• Firmware consistency: all CU320-2 firmware versions on the CF card must match the project version (e.g., 5.2 SP3 HF3). Mismatches trigger alarm F01000 at download.
• Drive-CLiQ wiring test: with all nodes powered, check the SINAMICS diagnostic buffer for F08501 (Drive-CLiQ communication error) and resolve any port assignment mismatches.
• Safety wiring: if a TM54F is present, confirm it is the first node after the CU on the Drive-CLiQ ring and that the PROFIsafe address is set on the DIP switch.
• Encoder feedback test: each SMC30 must show green status in the topology view; an orange state means an encoder wiring or parameter error.
• Cyclic load test: ramp each axis to 100 percent speed and observe the CU320-2 CPU utilization in Diagnostics > System Utilization. Sustained load above 90 percent indicates the design is at the limit of the topology.

Troubleshooting Matrix

Engineering Notes and Field-Proven Caveats

• CF card slot 1 vs slot 2: the lower slot (slot 1) holds the base firmware, slot 2 holds the performance extension license. Swapping the cards can put the CU into a fault state on boot; always mark the slots and use the Siemens spare-part program to keep revisions straight.
• CUA31 vs CUA32: the CUA32 is the recommended replacement for the CUA31 and supports HTL/TTL encoders natively. The CUA31 supports HTL only. Both count as one DO each.
• Mixing Booksize and Blocksize: the CU320-2 can drive Booksize (Single / Double Motor Modules) and Blocksize (PM240-2 via CUA) on the same ring. The DO ceiling is global, not per format.
• SINAMICS link: a second CU320-2 in the same cabinet can be cross-coupled via the SINAMICS link for high-speed data exchange without consuming PROFINET bandwidth. This is the recommended interconnect for the dual-CU variant of the 5 × PM240-2 design.
• Firmware version lock: Siemens releases a compatibility matrix for every firmware version. Before commissioning, confirm the CU320-2 firmware, the SMC30 firmware, the TM54F firmware, and the Starter / Startdrive project version are all on the same compatibility band.