Overview
The SINAMICS CU310 DP Control Unit is the standalone controller variant of the SINAMICS S120 drive family, designed for single-axis applications where the drive is mounted separately from a higher-level SIMOTION or SIMATIC controller. Encoder feedback is connected through the X23 Sub-D interface on the front panel of the Control Unit. X23 accepts HTL or TTL encoders and is the canonical interface for incremental encoders such as the SEW ES7C, Heidenhain ROD 426, Sick DFS60, and similar 5 V / 24 V devices.
One of the most frequently asked questions in the field concerns the encoder power supply on X23: which pins carry +5 V DC or +24 V DC, where the 0 V (ground) return is located, and how the voltage level is actually selected at the firmware level. Siemens documentation labels the supply pin "P_Encoder 5 V / 24 V", which is ambiguous because the label does not indicate which voltage is actually present on the pin at any given moment. The voltage is selectable, and the selection is governed by parameter p404.21.
This reference consolidates the pinout, the configuration mechanics, the wiring practice for a TTL encoder, and the typical field pitfalls (most notably the situation where a 24 V configuration still produces 5 V on the pins).
CU310 DP Hardware Identification
The CU310 DP has the following ordering designations (typical):
- 6SL3040-0LA00-0AA1 — CU310 DP Control Unit (PROFIBUS variant)
- 6SL3040-0LA01-0AA1 — CU310 DP Control Unit (PROFIBUS, later hardware release)
- 6SL3040-0MA00-0AA1 — CU310 PN Control Unit (PROFINET variant, X23 still present)
The CU310 DP must not be confused with the CU305 (article number 6SL3040-0JA00-0AA0/1), which is the SINAMICS S110 standalone controller. The two units share similar front-panel aesthetics but differ in firmware, parameter set, and supported encoder types. X23 on the CU305 has a different pinout and a different p404 sub-parameter. Verify the exact type using the nameplate and the firmware readout of r0018 and p0010 = 0.
X23 Pinout (TTL/HTL Encoder Interface)
X23 is a 9-pin female Sub-D connector. The pinout as documented in the SINAMICS S120 Control Units Manual (LH1) is:
| Pin | Signal | Function | Notes |
|---|---|---|---|
| 1 | A | Encoder track A (TTL/HTL) | Differential pair with pin 2 |
| 2 | A* (A inverted) | Encoder track A inverted | Differential pair with pin 1 |
| 3 | B | Encoder track B (TTL/HTL) | Differential pair with pin 4 |
| 4 | B* (B inverted) | Encoder track B inverted | Differential pair with pin 3 |
| 5 | P_Encoder 5 V / 24 V | Encoder power supply (+) | Configurable, see p404.21 |
| 6 | — | Reserved / not connected | Do not wire |
| 7 | M_Encoder (0 V) | Encoder ground | Reference for pins 5 and 8 |
| 8 | R | Reference / zero pulse (TTL/HTL) | Differential pair with pin 9 |
| 9 | R* (R inverted) | Reference / zero pulse inverted | Differential pair with pin 8 |
Power supply pins summary:
- Pin 5 — Encoder power supply positive terminal (P_Encoder). The voltage present here is either +5 V DC or +24 V DC depending on the firmware configuration.
- Pin 7 — Encoder power supply return / 0 V (M_Encoder). This is the reference for both pin 5 and any encoder shield referenced through the connector shell.
The pinout table printed in some CU310 manuals lists the encoder supply as appearing on both pin 4 and pin 5 in older releases. This is a documentation inconsistency: in the current hardware, the supply is on pin 5 only, with pin 4 reserved for the B* signal. Some third-party cable drawings still reference the dual-pin arrangement; always verify against the latest SINAMICS S120 manual edition before cutting cables.
Voltage Selection — Parameter p404.21
The voltage on pin 5 is selected by parameter p404.21. This is a sub-index of the encoder parameter structure for the X23 interface.
| p404.21 value | Voltage on X23 Pin 5 | Typical use |
|---|---|---|
| 0 | +5 V DC (TTL) | TTL incremental encoders (e.g. SEW ES7C, ROD 426) |
| 1 | +24 V DC (HTL) | HTL encoders (e.g. SICK DFS60 in HTL mode, Hübner HOG) |
Equivalent behaviour is sometimes achieved by changing p0404.0 (encoder configuration effective for the encoder interface on X23). When p0404 selects a 5 V TTL encoder type, p404.21 follows. When p0404 selects a 24 V HTL encoder type, p404.21 follows. The relationship is therefore:
p404.21 = 0 ⟺ TTL encoder type configured (p0404.0 = 1)
p404.21 = 1 ⟺ HTL encoder type configured (p0404.0 = 2)In other words, the encoder type chosen in the drive configuration (Commissioning → Encoder → Encoder type) automatically sets the supply voltage. Setting p404.21 manually is normally not required if the encoder type has been correctly configured.
Configuring a TTL Encoder (SEW ES7C Example)
The SEW ES7C is a 5 V TTL sin/cos incremental encoder used on SEW Movimot, Movidrives, and similar gear-motor/servo combinations. The relevant configuration sequence in STARTER (or TIA Portal with Startdrive) is:
- Connect the commissioning PC to the CU310 DP via PROFIBUS or the service interface (X22 / Ethernet service).
- Open the project containing the CU310, or detect the drive online (Target device → Online → Accessible nodes).
- Navigate to Commissioning → Encoder (X23) (also reachable via parameter p0400, p0404, p0408, p0420).
- Set the encoder type to TTL encoder (5 V). In STARTER this is the dropdown "Encoder type (X23)".
- Confirm the line count (for ES7C: typically 1024, 2048, or 4096 pulses per revolution — verify against the SEW nameplate and set p0408 accordingly).
- Apply the configuration with RAM to ROM (copy RAM to ROM, icon: floppy disk) so the change is non-volatile.
- Set p0010 = 0 to exit commissioning mode.
- Power-cycle the Control Unit (24 V off → on) to ensure the encoder supply level is re-evaluated.
After power-up, measure the voltage at X23 pin 5 against pin 7 with a multimeter. Expect +5 V DC ± 5 % under no load. The same measurement must be performed before any encoder cable is connected to avoid loading the supply.
Wiring the SEW ES7C to X23
Use a shielded, twisted-pair cable with individual pairs for A/A*, B/B*, and R/R*. Standard practice:
- Pin 1 (A) → SEW ES7C pin A (or S+ depending on cable)
- Pin 2 (A*) → SEW ES7C pin A*
- Pin 3 (B) → SEW ES7C pin B
- Pin 4 (B*) → SEW ES7C pin B*
- Pin 5 (P_Encoder) → SEW ES7C pin +UB (typically red wire)
- Pin 7 (M_Encoder) → SEW ES7C pin 0 V (typically blue wire)
- Pin 8 (R) → SEW ES7C pin R (zero pulse)
- Pin 9 (R*) → SEW ES7C pin R*
- Connector shell → cable shield, 360° bonded at the drive end only
Pin 6 is left unconnected. Cable length must not exceed the encoder manufacturer's specification (typically 50 m for TTL, 100 m for HTL, both at reduced maximum frequency). Beyond 30 m, route the cable in a grounded steel conduit or at minimum maintain a 200 mm separation from VFD output power cables.
Common Field Issue: "Configured 24 V but Pin 5 reads 5 V"
This symptom appears regularly. Causes, ranked by frequency observed in field service reports:
| Rank | Cause | Diagnostic | Resolution |
|---|---|---|---|
| 1 | Change to p404.21 / p0404 not saved to ROM | Read p404.21 online after a power cycle | RAM → ROM, then 24 V power-cycle |
| 2 | Drive still in commissioning mode (p0010 ≠ 0) | Check p0010 online | Set p0010 = 0, acknowledge faults, run |
| 3 | Wrong encoder interface selected (X21 vs X22 vs X23) | Check p0400 (encoder selection) | Set p0400 = 1 (X23) or the correct source |
| 4 | Encoder cable short on the supply pair | Measure resistance between pin 5 and pin 7 with cable disconnected | Replace cable |
| 5 | CU310 DP hardware in 5 V default mode (early FW) | Check firmware version (r0018) | Update firmware to ≥ 4.7 for reliable 24 V switching |
| 6 | Measured under no-load while drive is in commissioning offline | Re-measure in RUN with enable | Apply enable, then measure |
| 7 | DMM impedance loading a high-Z supply | Use a DMM with ≥ 10 MΩ impedance | Use proper meter |
The dominant cause is the absence of a RAM to ROM transfer followed by a 24 V power-cycle. The encoder supply selector is evaluated at Control Unit boot. Without a power-cycle, the change is held only in RAM and is not propagated to the encoder interface logic.
Verification Procedure After Configuration
- Power off the CU310 DP, wait 30 s for DC-bus and logic capacitors to discharge.
- Verify p404.21 online in STARTER / TIA Startdrive, confirm the saved ROM value matches the intended level.
- Reconnect, apply 24 V control voltage. Wait until the CU310 DP boots (RDY LED green).
- With the encoder cable disconnected, measure pin 5 → pin 7. Expected: +5.0 V ± 5 % or +24.0 V ± 10 %.
- Reconnect the encoder cable, measure again under no-load. Expect a drop of < 0.3 V at 5 V or < 1.0 V at 24 V.
- Enable the drive (setpoint 0, enable). Verify the encoder count increments in r0061 or the corresponding trace.
- Rotate the encoder shaft slowly by hand. Each pulse should produce a clean square wave on A/A* (and B/B* with 90° quadrature). Observe with a 2-channel oscilloscope, not a DMM.
Troubleshooting Matrix — X23 Encoder Supply Symptoms
| Symptom | Likely cause | Parameter / hardware check | Action |
|---|---|---|---|
| 0 V on pin 5 | Encoder supply not enabled, drive in commissioning, or HW fault | p0010, p0400, p0404, p404.21, F3xxx fault buffer | Exit commissioning, reconfigure, replace CU310 if F30004 / F30005 persists |
| 5 V despite 24 V configured | Change not saved / not power-cycled | p404.21 read online, ROM content | RAM to ROM, 24 V off/on |
| Voltage present but encoder not counting | Wrong channel, A/B swapped, wiring fault | r0061 trace, oscilloscope on pins 1, 2, 3, 4 | Verify wiring against table above; check p0408 line count |
| Intermittent encoder loss (F3xxx) | Cable shielding, VFD-induced noise, connector pin contact | Shake test, replace Sub-D backshell | Re-terminate shield, separate from power cable |
| Encoder counts backwards | A/B phase inversion | p0410 = 1 (invert direction) | Set p0410 or swap A↔A*, B↔B* |
| F31900 / F08501 | Encoder parameter invalid | Check p0400, p0404, p0408, p0420 consistency | Reset to factory defaults, reconfigure encoder |
| Encoder OK at 0 RPM, fails at high RPM | Maximum input frequency exceeded | f_max = line count × RPM / 60; check against X23 limit (500 kHz) | Reduce mechanical speed or use higher line count |
CU310 DP vs CU320 — Encoder Selection Logic
The CU310 DP has only one encoder interface on X23. The CU320 (multi-axis Control Unit, 6SL3040-0MA00-0AA1 family) has X21, X22, and X23 for three encoders per active line module. The configuration principle is identical (p404.xx parameter structure, p0400 source selector), but on the CU320 the user must additionally route the encoder to the drive object via p0400 on the Servo DO. Always confirm which Control Unit is installed by reading the nameplate (look for "DP" vs "PN" and the full MLFB).
Firmware Considerations
Earlier SINAMICS S120 firmware versions (≤ 4.5) exhibited instability when switching between 5 V and 24 V on the same connector without a power-cycle. Firmware 4.7 SP3 and later added hot-swappable voltage selection with safety validation. The current public release is SINAMICS S120 V5.2 SP3 (or equivalent), which fully supports the configuration flow described above. Update procedure is the standard STARTER → Online → Drive → Firmware update, or the TIA Portal / Startdrive equivalent.
109766706 for SINAMICS S120 firmware and 109769175 for SINAMICS S120 CU310 DP manuals. Always verify the firmware SHA against the SIOS-published checksum before applying in production.
Parameter Quick Reference
| Parameter | Function | Typical value (TTL) | Typical value (HTL) |
|---|---|---|---|
| p0010 | Commissioning filter | 0 (run) or 4/5 (encoder) | 0 (run) or 4/5 (encoder) |
| p0400 | Encoder selection | 1 (X23) | 1 (X23) |
| p0404 | Encoder configuration effective | 1 (TTL) | 2 (HTL) |
| p0408 | Encoder line count | 1024 / 2048 / 4096 | 500 / 1024 / 2048 |
| p0410 | Invert direction | 0 or 1 | 0 or 1 |
| p0420 | Encoder shaft | 0 (motor shaft) | 0 (motor shaft) |
| p404.21 | Encoder supply voltage | 0 (+5 V) | 1 (+24 V) |
| r0061 | Speed actual (rpm) | Trace monitor | Trace monitor |
Safety and Wiring Practice
- Encoder cable shields must be 360° bonded to the Sub-D backshell at both ends if the encoder manufacturer permits; otherwise, bond only at the drive end to avoid ground loops.
- Route the encoder cable at least 200 mm from any VFD output cable carrying motor currents. Cross perpendicular if the routing is unavoidable.
- Never connect the encoder cable shield to protective earth (PE) at a third point. Multiple ground paths create HF noise currents on the shield that couple into the signal pairs.
- Tighten the Sub-D connector screws to 0.4 Nm to maintain shell contact and shield continuity.
- Verify the encoder housing is isolated from the motor frame, or use an insulated mounting kit, to avoid bearing currents.
FAQ
Which pins on the CU310 DP X23 carry +5 V and 0 V for a TTL encoder?
Pin 5 is the encoder power supply positive terminal (P_Encoder) and pin 7 is the encoder ground (M_Encoder). With a TTL encoder configured, pin 5 delivers +5 V DC ± 5 % referenced to pin 7.
How do I change the X23 encoder supply from 5 V to 24 V (or vice versa)?
Open the drive online in STARTER or TIA Portal Startdrive, navigate to the X23 encoder configuration, select the desired encoder type (TTL = 5 V, HTL = 24 V), execute RAM to ROM, then power-cycle the 24 V control voltage to the CU310 DP. The voltage on pin 5 follows the encoder type selected.
Do I need to connect both pin 4 and pin 5 of X23 to the encoder?
No. Pin 4 is the B* signal (encoder track B inverted), not a power pin. The encoder supply is only on pin 5 (+ supply) and pin 7 (0 V). Wiring pin 4 to the encoder power may damage the encoder or the Control Unit.
I configured 24 V but the multimeter still shows 5 V on pin 5. Why?
The most common cause is that the change has not been saved to ROM, or that the drive has not been power-cycled after the change. Execute RAM to ROM, then remove and reapply the 24 V control supply. Other causes include p0010 still being in commissioning mode (set p0010 = 0) or the wrong encoder interface selected in p0400.
What is the maximum cable length for a TTL encoder on X23?
Siemens specifies up to 50 m for a 5 V TTL encoder with proper shielded twisted-pair cable, dependent on line count and maximum speed (maximum input frequency 500 kHz). For a SEW ES7C at 1024 pulses per revolution, this corresponds to approximately 29 000 RPM mechanical, derated by cable capacitance in practice.