DRIVE-CLiQ Clock Frequency: SINAMICS S120 Encoder Cycle Timing

Overview

DRIVE-CLiQ is the proprietary point-to-point serial backbone that connects the SINAMICS S120 Control Unit (CU320-2, CU310-2, or SIMOTION D), the Line Module, the Motor Modules, the Sensor Modules, and the Terminal Modules. It is not a process-data fieldbus in the classical sense; it is a deterministic, isochronous backplane that is locked to the current controller cycle of the drive line-up. Because of that lock, the DRIVE-CLiQ clock frequency on any given drive object equals the current controller sampling time, which in turn equals the speed controller cycle. This single timing relationship controls how often a position or speed value from an EnDat 2.2, SSI, incremental, or Hall-effect encoder can be sampled, evaluated, and propagated to the controller.

For a 30,000-line linear encoder whose readout head has a -3 dB cutoff of 350 kHz, the engineering question is rarely "can DRIVE-CLiQ keep up with the encoder" but rather "which Sensor Module Cabinet (SMC) supports both my encoder protocol and the current controller cycle I need for the application." The encoder's own line count and maximum input frequency impose one ceiling; the SMC's hardware revision and firmware impose another; the drive line-up's current controller cycle imposes a third. All three must be reconciled.

DRIVE-CLiQ Clock Fundamentals

DRIVE-CLiQ operates at 100 Mbit/s on a fixed cable topology. Each node receives a synchronous time stamp from the Control Unit so that all Motor Modules sample their phase currents on the same edge. The current controller cycle T_i is configured globally for the drive object and is replicated on the DRIVE-CLiQ ring. The standard values offered in STARTER, Startdrive, and the SINAMICS web server are:

Because DRIVE-CLiQ cycle = current controller cycle = speed controller cycle, choosing 31.25 µs effectively gives 32,000 current samples per second per axis. Sensor Modules therefore must be capable of completing an encoder evaluation inside that window. The DRIVE-CLiQ interface itself is fast enough; the bottleneck is the Sensor Module's FPGA, the protocol decoder, and the safety logic on safety-integrated variants.

Each DRIVE-CLiQ port also supplies the connected sensor with 24 V DC and a regulated 5 V/8 V encoder supply, with up to 450 mA available per port for encoders and measuring systems. Power budget on a single CU320-2 is therefore finite when many Sensor Modules and Motor Modules are connected.

Cycle-Clock Relationships

Three timing domains coexist in a SINAMICS S120 drive object and they must be kept separate in the engineering mind:

1. Encoder interface clock (f_enc) – the maximum input frequency the Sensor Module can decode. For an SMC20 this is typically 500 kHz for incremental EnDat 2.1/2.2 signals; for an SMC30 it is 1 MHz for incremental TTL/HTL; for an SMC40 the same 500 kHz figure applies for EnDat 2.2 and SSI absolute encoders.
2. DRIVE-CLiQ cycle (T_DQ = T_i) – the period at which the Sensor Module returns a fresh measured value to the Control Unit. This is always equal to the current controller cycle.
3. Application/speed controller cycle (T_n) – usually an integer multiple of T_i. Typical T_n = 4 × T_i, so at T_i = 125 µs the speed controller runs at 500 µs.

A common misreading is to treat the encoder's internal EnDat 2.2 clock (up to 16 MHz on modern Heidenhain LIP, LIC, or LIDA systems) as if it were the DRIVE-CLiQ clock. It is not. EnDat 2.2's high clock is only used to serialise position data inside the encoder ASIC and to transport additional information such as diagnostic words, temperature, and the safety CRC for SIL 2/SIL 3 encoders. Once the Sensor Module has decoded the position word, the value sits in the module's register until the next DRIVE-CLiQ tick, which is when it is forwarded to the Control Unit.

Sensor Module Cabinet (SMC) Variants

Three Sensor Module Cabinet families are commonly deployed with SINAMICS S120. Their timing behaviour differs significantly and is the primary selection criterion.

SMC20 – Incremental and Absolute Encoders (EnDat, SSI)

The SMC20 is the workhorse for Heidenhain EnDat 2.1/2.2 absolute encoders and SSI absolute encoders. The standard article number supports a current controller cycle down to 125 µs. A specific higher-performance variant, 6SL3055-0AA00-5BA3, is required when the drive line-up runs at T_i = 31.25 µs or T_i = 62.5 µs. Without this hardware revision, attempting to configure 31.25 µs current control against an SMC20 produces fault F01317 (Hardware fault, sensor module) or F08501 (COMM failure with sensor module) during commissioning. The -5BA3 module doubles the internal logic clock and contains the firmware revision that closes the encoder-evaluation timing margin at 32 kHz current control.

For the 30,000-line linear encoder in the source scenario, an EnDat 2.2 LIC 4000 series head running at 350 kHz cutoff is well within the SMC20 envelope. The -5BA3 variant is only mandatory if the drive is configured for 31.25 µs current control; for 125 µs servo control the -BA2 is sufficient.

SMC30 – Incremental Encoders (TTL, HTL, SSI)

The SMC30 handles TTL (RS-422) and HTL incremental encoders, plus SSI absolute encoders up to 1 MHz input frequency. It is used when the feedback device is not a Heidenhain-EnDat light source but a third-party TTL/HTL linear scale, resolver-to-digital converter, or SSI device. The SMC30 supports T_i = 125 µs minimum; the 31.25 µs servo profile is not officially released for the SMC30. For high-resolution linear encoders with 1 Vpp (sin/cos) interface, the SMC20 must be used, not the SMC30.

SMC40 – EnDat 2.2 with Extended Safety

The SMC40 is the safety-integrated Sensor Module for EnDat 2.2 encoders that carry the EnDat Safety protocol (SIL 2 / SIL 3 / Category 3 / PL d/e). It supports T_i = 31.25 µs and includes a second independent encoder channel and a Safe Encoder Evaluation (SEE) logic that adds roughly 4 µs latency to the encoder evaluation. For drives requiring PROFIsafe over PROFINET and safety functions such as SLS, SOS, or SDI on the encoder, the SMC40 is mandatory; SMC20 and SMC30 cannot validate EnDat Safety messages.

External Encoders via SME20/SME25

For motors without an integrated DRIVE-CLiQ interface, the SME20 (EnDat/SSI) and SME25 (incremental) Sensor Module External connect to DRIVE-CLiQ at the motor side. They carry the same timing characteristics as the cabinet-mounted SMC20 and SMC30, so the same 31.25 µs constraint applies via the 6SL3055-0AA00-5BA3 module or equivalent SME20-5BA3 hardware.

Encoder Protocol Compatibility Matrix

Current Controller Cycle Limits with Multiple Motor Modules

When several Motor Modules share a DRIVE-CLiQ line, the achievable current controller cycle is bounded by the slowest, most-demanding axis. For vector-controlled (VC) Motor Modules, every additional module on the line adds 125 µs to the current sampling time. This is documented for the SIMOCRANE and SIMOTION D line-ups but applies generally to SINAMICS S120 vector control on a single Control Unit. With three vector modules, the current controller cycle cannot be reduced below 250 µs because the DP cycle (bus) and the internal pipelining cannot keep up. Servo (SC) modules do not incur the same 125 µs penalty and remain at the configured T_i independent of the axis count, but the 32 kHz servo profile still requires the -5BA3 hardware revision on the Sensor Module side.

Sizing the Encoder for a Given DRIVE-CLiQ Cycle

Three inequalities must hold simultaneously:

1. Line-count vs velocity: f_enc = (v × resolution) / 1000 ≤ f_enc,max [Hz]
   where v is the maximum linear velocity in m/s and resolution is the line period in nm (for sine/cosine encoders) or line count in lines/mm (for incremental). For a 30,000-line per-meter linear encoder at 10 m/s: f_enc = 30,000 × 10 = 300 kHz. At 12 m/s: f_enc = 360 kHz, just above the 350 kHz -3 dB cutoff. The head cutoff is the real physical ceiling.
2. SMC capability: f_enc ≤ f_SMC,max (500 kHz for SMC20/40, 1 MHz for SMC30)
3. DRIVE-CLiQ sample rate vs motion smoothness: Δx per T_i = v × T_i must remain below the required position-loop quantisation. At 10 m/s and T_i = 125 µs: Δx = 10 × 125e-6 = 1.25 mm. At 31.25 µs: Δx = 0.3125 mm. If the application is a 0.1 µm positioning loop, the cycle must be ≤ 10 µs, which is not achievable in SINAMICS S120. In that case, the answer is to oversample within the SMC (EnDat 2.2 incremental subdivides the 1 Vpp signal) and accept that the position controller is fed every 125 µs but with sub-µm resolution.

For a 30,000-line encoder the intrinsic resolution of 1 Vpp interpolation inside the SMC20 is 1/4096 of a line period, giving a position quantisation of 1 / 30,000 / 4096 ≈ 8.1 nm. That value is preserved regardless of whether the drive is at 8 kHz or 32 kHz current control. Increasing the DRIVE-CLiQ clock only reduces the latency between samples; it does not increase the position resolution.

Encoder Cable, Voltage Drop, and Signal Integrity

The DRIVE-CLiQ 24 V encoder supply delivers up to 450 mA at 24 V. For long EnDat 2.2 cables (Heidenhain specifies up to 100 m for pure EnDat, 40 m for safety variants), the voltage drop on the supply core must be checked. Use:

ΔU = (2 × L × I) / (κ × A × 56) for the loop resistance, with κ = 58 S·m/mm² for copper. If ΔU exceeds 1.5 V at the head, the encoder's internal DC-DC converter may drop out of regulation and produce CRC errors that show up on the SMC as fault F3x117 (encoder signal level) and F08501 (DRIVE-CLiQ COMM). Lower the cable cross-section, reduce the cable length, or use a 5 V regulated tap from a local power supply, but never parallel-feed the encoder supply.

Commissioning Procedure

1. Wire the encoder head to the SMC20/SMC30/SMC40 using the Siemens pre-assembled DRIVE-CLiQ cable for the cabinet side and a Heidenhain or third-party signal cable for the encoder side. The DRIVE-CLiQ cable is mandatory between Sensor Module and Control Unit; do not splice.
2. Power up the Control Unit and connect via STARTER, Startdrive, TIA Portal, or the SINAMICS web server (HTTPS on CU320-2 PN).
3. In the drive object configuration, set p0115[0] = 31.25 µs, 62.5 µs, 125 µs, 250 µs, or 500 µs for the current controller cycle. STARTER/Startdrive will warn if any Sensor Module in the topology is below its minimum T_i.
4. Configure the encoder in p0400 (encoder type) and p0420 (encoder configuration). The encoder must be selected from the type list so that the SMC loads the correct protocol decoder. If the encoder is not in the list, use p0400 = 9999 (user-defined) and supply p0421, p0422, p0423, p0424 (resolution, fine resolution, multiturn bits, singleturn bits).
5. Run the automatic encoder adjustment: p1990 = 1. The drive moves the motor (if safety permits) by a defined angle and learns the offset, commutation, and direction.
6. Verify in r0061 (actual speed, sensorless) vs r0063 (actual speed, encoder) and check r0465 for encoder diagnostics (signal level A/B, fine resolution, CRC error counter).

Verification Checklist

Troubleshooting Matrix

Field-Edge Cases

• Direct-drive linear motor with high pole-pair count – if the pole-pair pitch divided by the encoder line period is non-integer, mechanical beat frequencies can fall inside the current controller bandwidth and excite resonances. Run the drive at 8 kHz and 4 kHz and check vibration with an accelerometer; the cycle choice is a vibration-tuning tool, not just a loop-bandwidth tool.
• Spindle with 16 kHz current control and gear ratio – the speed controller cycle T_n is typically 4 × T_i. If the spindle encoder is on the motor and the load encoder is on the spindle, the controller runs on the motor encoder at 31.25 µs but observes the spindle through a software-calculated ratio. Position lag across the gear must be compensated in p0341 (gear ratio) and p0342 (gear direction).
• Long DRIVE-CLiQ cable between cabinets – the maximum DRIVE-CLiQ cable length is 100 m with the Siemens pre-assembled cable, including the loop from the Control Unit to the Sensor Module. Star topology is required: every Sensor Module connects back to the Control Unit, never in a daisy chain. Daisy chains cause topology-detection faults and uneven propagation delays.
• EnDat 2.2 encoder with 16 MHz internal clock – the high internal clock is irrelevant on the SINAMICS side. Do not interpret the encoder's 16 MHz rating as a "16 MHz DRIVE-CLiQ"; the actual propagation is at 32 kHz current controller cycle with sub-µs latency inside the SMC.

Standards and Safety

Sensor Modules with safety functions are certified to EN 61800-5-2 (adjustable speed electrical power drive systems – functional safety) for SIL 2 / SIL 3 depending on configuration. The EnDat Safety protocol is defined by Heidenhain and integrates with PROFIsafe over PROFINET on the SINUMERIK/SIMOTION side and PROFIsafe on the SIMATIC ET 200SP F-CPU side. Refer to the SINAMICS S120 Function Manual "Safety Integrated" for the exact list of supported functions per Sensor Module revision.