Overview
The SINAMICS G120 CU250S-2 Control Unit is the closed-loop vector / closed-loop speed control variant of the modular G120 drive family. When paired with a PM240-2 Power Module, the CU250S-2 can evaluate an external incremental encoder and use it for speed feedback, position feedback, or as an input to the speed controller. This article covers the wiring, parameterization, and verification of a line-driver (differential) encoder — the typical HTL bipolar or TTL/RS-422 encoder found on induction motors, gear motors, and external feedback mounts.
Common encoder types supported by the CU250S-2 via the X2100 SUB-D connector:
- HTL unipolar (push-pull, single-ended)
- HTL bipolar (differential) — the focus of this guide
- TTL / RS-422 differential
- SSI absolute (gray / binary)
- EnDat 2.1 / 2.2 (with appropriate firmware)
Prerequisites
- SINAMICS G120 CU250S-2 PN or CU250S-2 DP Control Unit with firmware ≥ V4.7 (V5.1 SP1+ recommended for current G120 generation).
- PM240-2 Power Module sized to the motor.
- Encoder with HTL bipolar (differential) or TTL/RS-422 output: channels A+/A−, B+/B−, Z+/Z− + supply (VCC, GND).
- One of the following commissioning tools:
- Siemens SINAMICS Startdrive V17 / V18 (TIA Portal integrated)
- Siemens STARTER V5.6 (legacy)
- Operator panel IOP-2 or BOP-2
- Encoder data sheet (pulses per revolution, max frequency, supply voltage).
Hardware: X2100 Encoder Interface Pinout
The CU250S-2 uses a 15-pin SUB-D connector (X2100) on the front of the Control Unit. The differential wiring requires all six signal lines plus the encoder supply.
| Pin | Signal | Function |
|---|---|---|
| 1 | — | Reserved, do not connect |
| 2 | +24 V / +5 V | Encoder supply (selected via p0405) |
| 3 | GND | Encoder supply ground |
| 4 | R− | Reference pulse negative |
| 5 | R+ | Reference pulse positive (Z+) |
| 6 | A− | Channel A negative |
| 7 | A+ | Channel A positive |
| 8 | B− | Channel B negative |
| 9 | B+ | Channel B positive |
| 10 | — | Reserved / data (EnDat, SSI) |
| 11 | — | Reserved / clock (EnDat, SSI) |
| 12 | — | Reserved |
| 13 | — | Reserved |
| 14 | — | Reserved |
| 15 | — | Reserved / shield termination point |
Encoder cable requirements:
- Twisted-pair shielded cable, characteristic impedance ~120 Ω (RS-422 compatible).
- Shield bonded at the drive end through the SUB-D backshell, isolated at the encoder end unless the encoder manufacturer specifies otherwise.
- Maximum cable length: 100 m for HTL bipolar / TTL differential at ≤ 100 kHz, derated to 25–50 m at 300 kHz encoder frequency.
p0405. For encoders drawing more than 500 mA, install an external power supply and break the VCC loop in the cable.Parameter Set for Differential Encoders
All encoder-related configuration is performed under the Encoder function group in Startdrive / STARTER or via the IOP-2 menu. The parameters below are the minimum set required for a working differential (HTL bipolar or TTL) encoder on a CU250S-2.
| Parameter | Description | HTL Bipolar | TTL / RS-422 | Notes |
|---|---|---|---|---|
p0400[0] |
Encoder type selection | 2 (HTL bipolar) — see p0400 list | 1 (TTL bipolar) | Choose based on encoder output stage |
p0404[0] |
Encoder configuration | 0b0001 0001 0001 1000 0000 0000 0000 0000 | Same word, different bits | Bit fields: bit0 = signal monitoring, bit12 = output level |
p0405[0] |
Encoder signal type / supply | 0 = 24 V HTL bipolar | 1 = 5 V TTL bipolar | Selects encoder supply voltage |
p0407[0] |
Encoder linear/rotary | 0 = rotary | 0 = rotary | Use linear for linear scales |
p0408[0] |
Pulses per revolution | From nameplate (e.g. 1024, 2048, 4096) | Same | Critical for speed accuracy |
p0410[0] |
Encoder inversion | 0 = not inverted, 1 = inverted | Same | Flip if motor runs opposite to command direction |
p0420[0] |
Encoder connection | 0 = X2100 | 0 = X2100 | Other values: reserved / DRIVE-CLiQ via SMC |
p0421[0] |
Absolute encoder configuration | 0 = incremental | 0 = incremental | Set 1 / 2 only for SSI/EnDat absolute |
p0422[0] |
Absolute encoder resolution (singleturn bits) | 0 for incremental | 0 for incremental | For SSI/EnDat only |
p0425[0] |
Encoder rotation direction | 0 = same, 1 = opposite | Same | Or use p0410 for signal inversion |
p0430[0] |
Encoder error tolerance | Default 200 ms | Default | Adjust for noisy environments |
p0431[0] |
Encoder cable resistance | 0 = use default | 0 | For long cables > 50 m |
p0432[0] |
Number of pole pairs of resolver (if used) | N/A | N/A | Not used for HTL/TTL |
p0400 values for “HTL bipolar” and “TTL bipolar” depend on firmware. On CU250S-2 V5.x: p0400 = 4 selects HTL bipolar (differential) and p0400 = 1 selects TTL bipolar (RS-422). Always confirm via the parameter dropdown in Startdrive or the SINAMICS G120 Parameter Manual (GH6).Step-by-Step Configuration in SINAMICS Startdrive (TIA Portal)
- Open the project and navigate to the G120 drive device. Double-click “Commissioning” → “Drive functions”.
- Run the Drive commissioning wizard first: enter motor nameplate data, motor type, control mode (use “Closed-loop speed control with encoder” — this is the CU250S-2’s differentiator vs. the CU240E-2).
- When the wizard reaches “Encoder selection”, choose HTL bipolar or TTL bipolar depending on the physical encoder.
- Set Pulses per revolution to the value on the encoder nameplate. Typical values: 100, 360, 500, 512, 1000, 1024, 2048, 4096, 8192.
- Set the encoder supply voltage (24 V or 5 V) to match the encoder. This is parameterized in
p0405. - Configure signal inversion (
p0410) only after the first run; default is “not inverted”. - Click “Identify motor data” with the motor uncoupled, then perform a encoder test: Startdrive menu → Commissioning → Diagnostics → “Encoder test”. The drive rotates the motor through a small angle and compares the encoder count to the motor pole count.
- Save to RAM/ROM (button “Copy RAM to ROM”).
Step-by-Step Configuration in STARTER (legacy)
- Open STARTER and connect to the drive. Click “Configure DDS”.
- In the navigation tree, expand the drive → Configuration → Encoder.
- Select Encoder 1 (the only encoder port on CU250S-2 via X2100).
- Choose encoder type and enter the values described in the table above.
- Right-click the drive → “Commissioning” → “Motor identification”, then “Encoder test”.
- RAM → ROM copy via toolbar.
Configuration via IOP-2 or BOP-2 Operator Panel
For commissioning without a PC, the IOP-2 supports guided encoder setup:
- Menu → Commissioning → Motor → Control mode: select “Closed-loop speed with encoder”.
- Menu → Drive functions → Encoder: choose type, supply, PPR.
- Menu → Diagnostics → Encoder: view live actual speed (r61.0), encoder signal status, and the Encoder parameter set.
- Save with the green OK button (writes to non-volatile).
Drive Control Mode & Encoder Assignment
The CU250S-2 must be set to a control mode that consumes encoder feedback. The standard selections are:
| Control Mode | Encoder Required |
p1300 Setting |
|---|---|---|
| V/f open-loop | No | 0, 1, 2, 3, 5 |
| Sensorless vector (SLVC) | No | 20 |
| Vector control with encoder (VC) | Yes | 21 |
| Closed-loop speed with encoder | Yes | 21 or 23 |
For speed accuracy < 0.1 % of nominal and zero-speed torque, p1300 = 21 (vector control with encoder) is the typical choice. Set this before the first run with encoder feedback.
Verification: Live Signal Checks
After parameterization, confirm the encoder is wired correctly and the drive is reading valid counts.
| Parameter / Action | What to Check |
|---|---|
r0061[0] |
Actual speed — should change smoothly with motor rotation, sign matches command |
r0090 / r0799
|
Encoder status word — bit 0 should be 1 (encoder OK) |
r0480 |
Encoder status word (bit field) — see drive manual GH6 |
r0060 |
Speed setpoint — cross-check against command |
| Turn motor slowly by hand (disconnected mains, motor side only) | Counts in r0482 should advance smoothly without jitter |
| Oscilloscope on A+/A−, B+/B− | 90° quadrature, clean edges, no ringing > 50 % Vcc |
Quick directional check: spin the motor shaft manually with command disabled. r0061 should increment in the same direction as the motor turns. If it counts down when the motor turns clockwise, set p0410 = 1 to invert the encoder channels.
Troubleshooting: Faults & Alarms
| Code | Cause | Likely Fix |
|---|---|---|
| F3x050 (x = encoder index) | Encoder wire break or signal error | Check A/B/Z pairs for continuity, shield bonding, cable < 100 m |
| F3x051 | Encoder power supply fault | Verify p0405, encoder current < 500 mA |
| F3x052 | Encoder signal level below threshold | Verify TTL vs HTL selection, check for cable degradation |
| F3x053 | Encoder undervoltage | Check VCC at the encoder end, not just the drive end |
| F3x054 | Wrong encoder parameter set | Verify p0400, p0408 match nameplate |
| A3x050 | Encoder warning — signal degraded | Inspect cable routing, increase EMC separation from VFD output |
| F7900 / F7901 | Speed controller disabled due to encoder fault | Resolve root encoder fault, then acknowledge |
Where x = 1 for encoder 1 on X2100 (CU250S-2). On PM240-2 hardware versions with the “F3x” alarm system, the encoder number appears as the second digit.
p0492 to a non-zero value (default “lock after 100 ms”) to allow motor identification without encoder feedback. Once the encoder is validated, restore the original value.Frequently Asked Questions
What is the correct p0400 value for HTL differential (bipolar) encoders on the CU250S-2?
On CU250S-2 firmware V5.x, set p0400 = 4 for HTL bipolar (line-driver) encoders. For TTL/RS-422 differential encoders, use p0400 = 1. Always confirm against the Startdrive dropdown for your specific firmware version — the SINAMICS G120 Parameter Manual (GH6) lists every p0400 option.
How do I set the encoder supply voltage (5 V vs 24 V) on the CU250S-2?
Use parameter p0405. p0405 = 0 supplies 24 V (typical for HTL encoders), p0405 = 1 supplies 5 V (typical for TTL encoders). Maximum load is 500 mA per encoder supply rail.
My motor runs in the opposite direction of the speed command. How do I flip the encoder feedback?
Set p0410[0] = 1 to invert the encoder channels A/B without rewiring. Alternatively, swap A+/A− or B+/B− at the X2100 SUB-D connector. Do not swap both pairs simultaneously — this will cause direction ambiguity and F3x052.
Why does the drive show F3x050 right after I power up the encoder?
Most commonly caused by a missing shield termination, broken wire, or selecting the wrong encoder type in p0400. Use an oscilloscope on A+/A− to verify both legs toggle. Re-seat the SUB-D connector and verify pin 3 (GND) is bonded to the encoder supply ground.
Can I run a vector control mode without an encoder on the CU250S-2?
Yes — set p1300 = 20 for sensorless vector control (SLVC). The CU250S-2 falls back to SLVC if p0400 is set to 0 (no encoder). However, SLVC has limited zero-speed torque and slow response; use encoder-based vector (p1300 = 21) for low-speed / high-dynamic applications.