Overview
Bosch Rexroth MHD-series synchronous servo motors are commonly integrated with Siemens SINAMICS S120 drives in retrofits, machine upgrades, and OEM machines that pair Rexroth motors with Siemens control architectures. The integration hinges on selecting the correct SINAMICS Sensor Module Cabinet (SMC) for the encoder inside the MHD motor. The MHD093C-035-PG1-AA in particular carries a digital servo feedback encoder (EnDat 2.1-style with 1 VPP sin/cos track plus a serial data channel), which dictates SMC20, not SMC10 or SMC30. This reference explains the encoder family, the SMC selection decision, the signal mapping, the cable situation, the drive-side commissioning parameters, the EMC requirements, and the diagnostic path when communication fails.
SINAMICS S120 Sensor Module Cabinet (SMC) Family
The Sensor Module Cabinet reads motor feedback and converts it into a DRIVE-CLiQ link to the SINAMICS Control Unit. There are four mainstream SMC variants, each tied to a specific encoder technology.
| Module | Siemens Article Number | Supported Encoder Types | Typical Use |
|---|---|---|---|
| SMC10 | 6SL3055-0AA00-5AA2 | Two-pole resolver, multi-pole resolver | Asynchronous motors or synchronous motors with resolver feedback |
| SMC20 | 6SL3055-0AA00-5BA2 / -5BA3 | EnDat 2.1 / 2.2, SSI, SSI + sin/cos 1VPP, sin/cos 1VPP with serial data, DRIVE-CLiQ encoders (passive) | Absolute encoders — most synchronous servo motors including Rexroth MHD digital servo feedback |
| SMC30 | 6SL3055-0AA00-5CA2 | TTL (RS-422), HTL (with/without inversion), sin/cos 1VPP without serial data, SSI without 1VPP | Incremental encoders, incremental-only SSI |
| SMC40 | 6SL3055-0AA00-5DAx | EnDat 2.2, purely digital serial protocols, DRIVE-CLiQ | Newer high-speed absolute encoders, functional safety |
Sensor Module Cabinet Mounted (SBM2 / BSM) variants are for direct DRIVE-CLiQ encoders (encoders that already speak DRIVE-CLiQ) and do not apply to an MHD motor whose encoder outputs raw 1 VPP + serial signals.
Bosch Rexroth MHD093C-035-PG1-AA Encoder Identification
The Rexroth MHD part-number scheme encodes the encoder type. MHD093C-035-PG1-AA breaks down as follows:
- MHD = Synchronous servo motor series (medium-inertia / Mitteldruck family)
- 093 = Frame size (93 mm flange)
- C = Length code (medium length, between B and D)
- 035 = Winding code (rated speed/torque variant)
- P = Digital servo feedback (inductive, 1 VPP sin/cos + EnDat 2.1 serial data channel)
- G1 = Encoder generation 1 (Heidenhain EQN family, 20-bit single-turn / 12-bit multi-turn)
- AA = Shaft, sealing, holding-brake option (smooth shaft, IP65, without brake)
Section 5 of the Rexroth MHD motor documentation ("Motor Encoder") lists configuration 5.2 = P as a multiturn absolute encoder. Because the encoder carries a true absolute position (no battery, no zero-routine required at startup), the SMC must support the absolute serial protocol — that is SMC20, not SMC30.
Typical resolution values to confirm against the motor nameplate or Rexroth project planning manual (gated by Rexroth eBusiness; confirm with supplier):
| Parameter | Typical MHD "P" Encoder Value | Source |
|---|---|---|
| Incremental track | 1 VPP sin/cos, 20 or 24 µm period | Heidenhain EQN/ECN family datasheet |
| Single-turn resolution | 20 bits / rev (1,048,576 increments) | Encoder nameplate |
| Multi-turn resolution | 12 bits (4,096 revolutions) | Encoder nameplate |
| Serial protocol | EnDat 2.1 (bidirectional, ≤ 2 MHz clock) | Encoder nameplate |
| Supply | 3.6 V to 14 V DC at the encoder (5 V supplied by SMC20) | Encoder datasheet |
Other MHD encoder codes for reference
| Code | Encoder Type | Compatible SMC | |
|---|---|---|---|
| P, PG1 | EnDat 2.1 absolute, sin/cos 1VPP | SMC20 | |
| C, CG1 | Incremental sin/cos 1VPP, no serial data | SMC30 | |
| S, SG1 | Hiperface absolute, sin/cos 1VPP | SMC20 (Hiperface via SMC20 since FW 4.6) | 6SL3055-0AA00-5BA2 / -5BA3 |
| R | Resolver (multi-pole) | SMC10 |
Selecting the Correct SMC: SMC20 for the MHD Multiturn Absolute Encoder
Given the encoder is a 1 VPP sin/cos + EnDat 2.1 multiturn absolute device, SMC20 is the only SINAMICS S120 Sensor Module Cabinet that handles the full signal set in a single connector. The decision rule is protocol-driven:
- If the encoder has no serial data channel (TTL, HTL, sin/cos 1VPP only) → SMC30.
- If the encoder has a serial data channel (EnDat, SSI) and absolute position → SMC20.
- If the encoder is a resolver → SMC10.
- If the encoder is a pure digital serial channel (no analog sin/cos track) and high-speed EnDat 2.2 → SMC40.
The MHD093C-035-PG1-AA encoder combines analog sin/cos with the EnDat serial channel, which is the SMC20 sweet spot. SMC30 would receive the 1 VPP signals but ignore the absolute position, so the drive would have to home on every power-up — losing the absolute multiturn data. SMC10 cannot decode 1 VPP sin/cos at all (it expects a resolver). SMC40 can read the EnDat 2.1 protocol but typically requires EnDat 2.2 on the encoder for full benefit; it will work for the MHD application but offers no practical advantage over SMC20 here.
Signal Mapping: MHD Encoder Pinout to SMC20 Terminal Assignment
The MHD motor's digital servo feedback connector exposes the following signals (Rexroth connection cable data sheet, "Digital Servo Feedback" pinout, M23 round connector, 12-position):
| MHD Pin (M23 / M17 connector) | Signal | SMC20 Pin (X520, 15-pin D-sub) | SMC20 Terminal | Description |
|---|---|---|---|---|
| 1 | 0 V (sensor ground) | 2 | M (Encoder) | Encoder supply return |
| 2 | UG (5 V encoder supply) | 13 | P (Encoder) | 5 V/24 V supply to encoder |
| 3 | SIN+ | 5 | B+ | Sin/cos incremental track, sine |
| 4 | SIN- | 6 | B- | Sin/cos incremental track, sine inverted |
| 5 | COS+ | 3 | A+ | Sin/cos incremental track, cosine |
| 6 | COS- | 4 | A- | Sin/cos incremental track, cosine inverted |
| 7 | FS | — | — | Frame synchronization (leave floating or tie to logic high per Rexroth) |
| 8 | SCL | 9 | CLOCK+ | EnDat 2.1 clock to encoder |
| 9 | SDI | 11 | DATA+ | Serial data in (or looped to DATA+ on half-duplex encoders) |
| 10 | SDO | 12 | DATA- | Serial data out (half-duplex: loop SDO to SDI at the encoder connector) |
| 11 | — | 1 | P_Sense | Sense return for regulated supply (may be jumpered to P at terminal) |
| 12 | — | 15 | Shield | Connect to the connector housing for 360° shield termination |
Topology (Inline SVG)
Cable Selection, Shielding, and EMC
There is no Siemens off-the-shelf preassembled cable that mates an MHD motor's M23 encoder connector to an SMC20 D-sub. Rexroth sells its own power-and-feedback cable assemblies (e.g., RKG0022 or the equivalent for the MHD series) where the encoder end is the MHD connector and the drive end is open leads or a specific IndraDrive connector. To terminate on a Siemens SMC20, the user has three practical options:
- Cut and re-terminate a Rexroth RKG extension cable. Use the Rexroth-supplied MHD connector at the motor and crimp or solder a 15-pin D-sub male (e.g., Siemens 6FX2003-0SA17 backshell or 3RT2025 D-sub housing) at the SMC20 end. This is the most common field solution.
- Source a third-party pre-assembled extension. Industrial cable vendors offer custom pre-assembled encoder cables for the MHD-to-SINAMICS retrofit case. Order with the Heidenhain or Bosch Rexroth M23 connector at the motor end and a 15-pin D-sub at the SMC end.
- Use a Siemens 6FX2002-... cable plus a small adapter. Siemens' 6FX2002 preassembled cables are designed for Siemens 1FK/1FT motors and 17-pin round connectors, not for the MHD connector, so an adapter is normally required. This option is rarely cleaner than option 1.
Cable specifications
| Property | Requirement |
|---|---|
| Conductor type | Twisted-pair, individually shielded; overall shield recommended |
| Wire gauge | 24 AWG (0.25 mm²) for signal pairs, 22 AWG (0.34 mm²) for the supply pair |
| Max length SMC20 → encoder | 100 m at p0423 ≤ 1000 kHz; 50 m at p0423 = 2000 kHz |
| Differential impedance | 100 Ω ± 15 Ω for sin/cos and CLOCK/DATA pairs |
| Bend radius | ≥ 10× OD for static, ≥ 15× OD for continuous flex |
| Shield termination | 360° backshell at both ends; connect shield to D-sub housing at SMC20 side |
| Min temperature | -20 °C (static) / -10 °C (moving) |
| Insulation voltage | ≥ 100 V (signal pairs); ≥ 300 V (overall jacket) |
Twisted-pair grouping
The cable must contain at least four individually shielded twisted pairs plus a power pair:
- Pair A: SIN+ / SIN- (twist, shield, drain)
- Pair B: COS+ / COS- (twist, shield, drain)
- Pair C: SCL (CLOCK+) / SCL- (twist, shield, drain)
- Pair D: SDI (DATA+) / SDO (DATA-) (twist, shield, drain)
- Power pair: 0 V / UG (overall shield only)
A single overall braid or foil shield is acceptable when each pair is shielded. The shield drain wires from each pair should be tied to the SMC20 pin 15 (shield) and to the motor connector housing — do not connect drain to 0 V at the motor end.
EMC and routing
Route the encoder cable at least 200 mm away from VFD output power cables (motor power cable) and at least 100 mm from any 24 V power distribution. Cross motor power and encoder cables at 90° if they must intersect. The cable tray should be a continuous, grounded steel tray; do not use plastic cable ties for the encoder cable run, as the cable relies on the metal tray to drain common-mode noise.
SINAMICS S120 Commissioning Parameters
Once the SMC20 is online, the drive reads its encoder type via p0400 and the encoder identification via p0401. Below is the canonical commissioning set for the MHD093C-035-PG1-AA digital servo feedback.
| Parameter | Index | Value (Example) | Description |
|---|---|---|---|
| p0400[0] | Encoder 1 | 2042 |
Encoder type: 2042 = EnDat 2.1 multiturn, 2041 = EnDat 2.1 singleturn, 2021 = SSI multiturn |
| p0402[0] | Encoder 1 | 1 | Gear ratio motor/load (1:1 for direct drive) |
| p0404[0] | Encoder 1 | 0x...10 |
Encoder configuration bit field (rotary, sin/cos 1VPP, 5 V supply) |
| p0407[0] | Encoder 1 | — (read from encoder) | Linear measuring step (only for linear encoders; 0 for rotary) |
| p0408[0] | Encoder 1 | auto | Measuring step per revolution; SINAMICS auto-calculates from p0421 |
| p0410[0] | Encoder 1 | 0 | Inversion: 0 = not inverted, 1 = invert direction (bit 0) |
| p0421[0] | Encoder 1 | 20 | Single-turn resolution (bits/rev) — confirm against nameplate |
| p0422[0] | Encoder 1 | 12 | Multi-turn resolution (bits) — confirm against nameplate |
| p0423[0] | Encoder 1 | 2000 | EnDat/SSI clock rate in kHz: 100, 500, 1000, 2000 |
| p0425[0] | Encoder 1 | 0 | Signal type: 0 = sin/cos 1VPP, 1 = TTL, 2 = HTL |
| p0426[0] | Encoder 1 | 0 | Distance between zero marks; 0 = none for absolute |
| p0431[0] | Encoder 1 | 0 (auto) | Commutation angle offset; set by drive during first run-up |
| p0437[0] | Encoder 1 | 0x1A | Extended encoder configuration (rotary, multiturn, no reset on parameter download) |
| p0438[0] | Encoder 1 | — (read from encoder) | Serial number; populated from encoder's electronic nameplate |
| p0441[0] | Encoder 1 | 0 | Encoder commissioning / replacement: 1 = first commissioning, 2 = absolute encoder adjustment |
After setting p0400 = 2042, accept the value (RAM to ROM in Starter / TIA Portal). The drive issues a parameterized-encoder query via EnDat 2.1 and auto-fills p0401, p0402, p0421, p0422, p0431, p0403, p0438, p0430 (multi-turn offset), and p0404 from the encoder's electronic nameplate. Verify p0421 and p0422 against the motor's nameplate sticker — if they are wrong, the absolute position will shift every time the encoder passes the singleturn/multi-turn rollover.
Configuration in SINAMICS Starter / TIA Portal Startdrive
- Open the SINAMICS S120 project in SINAMICS Starter (legacy) or TIA Portal with Startdrive V15 or newer.
- Go online with the target device (Control Unit CU320-2 or CU310-2). Verify the DRIVE-CLiQ topology lists "SMC20" with order number 6SL3055-0AA00-5BAx.
- Navigate to Configuration → Drives → DO1 → Configuration → Encoder and set p0400[0] = 2042 (EnDat 2.1 multiturn).
- Click Automatic configuration from the encoder in Starter or "Load from encoder" in TIA Portal Startdrive. The drive uploads the encoder's electronic nameplate and writes p0401, p0402, p0421, p0422, p0431, p0403, p0438.
- Cross-check the auto-loaded p0421 and p0422 against the MHD motor's nameplate sticker. If the loaded value is wrong (e.g., 19 instead of 20), manually correct and RAM-to-ROM.
- Set p0423[0] = 2000 for cable lengths ≤ 50 m. For longer runs, drop to 1000 or 500 kHz.
- Set p0410[0] = 0 initially. With the motor uncoupled, command a small jog. If the actual position decreases while the motor turns positive, set p0410[0] = 1 to invert direction.
- Perform a one-time motor identification (p1910 = 1, pulsing) and encoder adjustment (p1990 = 1) for a synchronous motor. The drive measures the commutation angle offset and writes it to p0431.
- Run the speed-controller optimization (p1960 = 1) before putting the machine into production.
- Set the SINAMICS reference speed and maximum current in p2000 and p640 to match the MHD motor's rated data on the type plate.
Power Supply, Fusing, and 24 V Distribution
The SMC20 takes its 24 V supply either from the internal 24 V bus of the SINAMICS Line Module or from an external 24 V SITOP / Phoenix Contact power supply, routed through a 5 A (max 10 A) slow-blow fuse per the SINAMICS S120 Equipment Manual. A UPS on the 24 V supply is recommended to ride through short brownouts — the MHD encoder needs continuous power to retain the multi-turn count. For a 30 s UPS, use a SITOP UPS1600 with a 3.4 Ah battery module.
| Supply | Range | Connector on SMC20 | Notes |
|---|---|---|---|
| +24 V electronics | 20.4 V to 28.8 V DC, 0.4 A typ. | X524 (terminal) | Mandatory for DRIVE-CLiQ |
| +5 V encoder | 5.0 V ± 5%, 350 mA typ. | X520 pin 13 | Driven by SMC20 from 24 V supply |
Ground the 0 V of the 24 V supply at the cabinet PE bar. The shield of the encoder cable must not carry load current; the shield drain wires from each pair go to the D-sub housing at the SMC20 end, not to the 24 V 0 V terminal.
Verification, LEDs, and Diagnostic Outputs
SMC20 LED legend
| LED | Color / State | Meaning |
|---|---|---|
| RDY (Ready) | Green steady | SMC20 is operational, encoder connected, valid data |
| RDY | Green flashing | DRIVE-CLiQ communication is initializing |
| RDY | Red steady | Encoder error — check wiring, p0421/p0422, cable shield |
| RDY | Red flashing | DRIVE-CLiQ error — check loop-through to CU |
| RDY | Off | No 24 V supply at the SMC20 or DRIVE-CLiQ disconnected |
Real-time encoder health is also visible in the drive via r0451 (encoder status word 1) and r0453 (encoder status word 2). Look for bits 0, 1, 2 (signal levels OK), bits 8–10 (1VPP tracking), and bits 11–13 (EnDat protocol status). r0462 and r0463 report the actual position and the CRC residual of the EnDat data word — a non-zero CRC residual flags intermittent data corruption.
Commissioning test sequence
- Power-up test. Apply 24 V to the SMC20, watch for green RDY LED. Within ~2 s the drive should report "Encoder parameterized."
- Position readback test. Turn the motor shaft by hand 10 turns. The drive's r0462 should change by exactly 10 × (2p0421) increments. If r0462 jumps or rolls over, the multi-turn bits (p0422) are mis-configured.
- Direction test. Jog the motor 90° electrical. r0461 (mechanical position) should advance. If it retreats, set p0410[0] = 1.
- CRC test. Read r0453 bit 15 ("EnDat CRC error") in the drive trace. Drive 100 m or more and look for glitches; a clean cable should have a CRC error rate of zero per hour.
- Absolute position test. Power off, wait 30 s, power on. r0462 should be identical before and after. If it changes by 2p0421 increments (one full turn), the data line polarity is wrong — check DATA+ and DATA− wiring.
- Commutation test. With p1990 = 1, command a small positive move (e.g., 90° electrical). The drive should report p0431 as a non-zero value (typically between 0 and 360°). If p0431 = 0 or 360 after the move, the incremental sin/cos track is wired backwards (swap SIN+/SIN− or COS+/COS− at the connector).
Fault Code Matrix for SMC20 with Rexroth MHD Encoders
| Fault Code | Meaning | Likely Cause | Action |
|---|---|---|---|
| F31100 | Position encoder fault | No encoder connected, broken wire, UG < 4.5 V | Verify UG (5 V) at the encoder; check X520 pin 13 |
| F31110 | Serial communication fault | EnDat 2.1 / SSI not responding, wrong clock rate | Lower p0423 from 2000 to 1000 or 500 kHz |
| F31111 | Encoder signal fault (incremental) | Sin/cos track noisy or amplitude out of spec | Check SIN/COS shield, look for r0451 bits 0–2 = 0 |
| F31115 | Coarse position fault | Encoder powered off briefly or p0421 wrong | Re-enter p0421 from nameplate; check UG stability |
| F31117 | Inversion error / direction mismatch | A/B swapped, DATA+ and DATA− swapped | Set p0410 = 1 to invert; check DATA+ / DATA− polarity |
| F31121 | Multiturn overflow | Encoder turned more than 2p0422 revolutions | Verify p0422 = 12 (or per nameplate), not 8 |
| F31122 | Multiturn underflow | Same as above; reversed count | See F31121 |
| F31130 | Encoder parameterization fault | p0400 mismatch with p0425 / p0421 | Re-set p0400 = 2042 and re-load from encoder |
| F31131 | Parameterization error after warm restart | Saved project p0401 does not match new encoder | RAM-to-ROM after a successful "load from encoder" |
| A32911 | Warning: encoder warning | CRC residual non-zero, signal margin low | Check cable, terminations, EMC environment |
For a complete fault list, see the SINAMICS S120/S150 List Manual entry for fault codes F3x1xx and A3x9xx.
Field-Proven Notes, Edge Cases, and SMC40 Migration
- Half-duplex vs full-duplex EnDat. Most MHD digital servo feedback encoders use EnDat 2.1 half-duplex (SDI and SDO are tied together at the encoder). The SMC20 handles this internally — the drive does not need to know — but the cable still needs both wires, with SDI and SDO tied at the MHD connector.
- Battery-free absolute. The MHD encoder retains absolute position with a capacitive or rotational counter; it does not need a battery. The SMC20 supplies 5 V continuously, so a UPS on the 24 V SMC20 supply is recommended to keep the encoder warm during brownouts.
- Functional Safety. SMC20 with EnDat 2.1 supports SIL 2 / PL d on the drive side if the drive firmware is V5.x or newer and the encoder is rated accordingly. For SIL 3 / PL e, an EnDat 2.2 encoder on SMC40 is the correct path.
- IndraDrive retrofit. If the MHD motor was previously driven by a Rexroth IndraDrive C or M, the same SMC20 works. The old Rexroth encoder cable's color code (RKG0022 style) is white-blue, white-green, etc. — these are the same physical signals as the Siemens cable color code from 6FX2002.
- Spare part. Order 6SL3055-0AA00-5BA2 (the SMC20 module) as a hot-swap spare. The module is hot-swappable on DRIVE-CLiQ — pull it out, the drive goes into standby, push the new one in, the drive auto-detects the topology.
- Firmware compatibility. SMC20 supports firmware V2.4 and newer on the SINAMICS S120. For SMC20 with p0423 = 2000 kHz, firmware V4.6 or newer is required on the drive; older firmware may limit to 1000 kHz.
Standards and Compliance Reference
The SINAMICS S120 drive family is designed to comply with:
- IEC 61800-3 (Adjustable speed electrical power drive systems — EMC requirements)
- IEC 61800-5-1 (Safety requirements for electrical, thermal, and energy)
- EN 50178 / EN 62477-1 (Electronic equipment for use in power installations)
- IEC 61508 (Functional safety), up to SIL 3 when paired with the appropriate Safety Integrated firmware
Encoder cables should be shielded to comply with the drive's CE EMC declaration; field installers must follow the SINAMICS S120 Equipment Manual's cable routing rules to retain compliance.
For the SMC20 datasheet and ordering data, see the SINAMICS S120/S150 product support page. For MHD motor encoder specifications, see the Bosch Rexroth MHD project planning manual (gated; request via Rexroth eBusiness or local distributor).
Which SMC should I use with a Rexroth MHD093C-035-PG1-AA?
Use the SMC20 (Siemens part number 6SL3055-0AA00-5BA2 or 5BA3). The "P" encoder code designates a 1 VPP sin/cos track with an EnDat 2.1 absolute serial channel, which is the SMC20's primary use case. SMC30 ignores the absolute position, SMC10 cannot read 1 VPP at all, and SMC40 is overkill for EnDat 2.1.
How do I set p0400 for the MHD digital servo feedback encoder?
Set p0400[0] = 2042 for EnDat 2.1 multiturn absolute, or 2041 for singleturn. After the value is written and saved, use Starter or TIA Portal Startdrive to "load from encoder" so the drive auto-fills p0421, p0422, p0431, and p0403 from the encoder's electronic nameplate.
What is the maximum cable length between an SMC20 and the MHD motor's encoder?
100 m at EnDat clock rates ≤ 1 MHz (p0423 ≤ 1000 kHz), or 50 m at 2 MHz clock (p0423 = 2000 kHz). Use 100 Ω twisted-pair shielded cable with 360° backshell terminations at both ends, and keep the UG supply voltage at the encoder ≥ 4.75 V under load.
Why is my SMC20 flashing red after a long cable run?
Red flashing on the RDY LED after a long cable almost always means the EnDat data word is corrupted by reflections or by voltage drop on the 5 V supply. Reduce p0423 to 1000 kHz, check shield termination, and confirm that the voltage at the encoder's UG pin is ≥ 4.75 V under load. If the cable is fixed-length, consider adding a sense line and a thicker supply pair.
Do I need a battery on the MHD encoder when using SMC20?
No. The MHD "P" digital servo feedback is a battery-less absolute encoder; the SMC20 supplies 5 V continuously and the absolute position is retained in the encoder's mechanical gear counter. A UPS on the 24 V SMC20 supply is recommended to ride through short power outages.
Can I use SMC40 instead of SMC20 for an MHD EnDat 2.1 encoder?
Yes, SMC40 will read EnDat 2.1, but the additional cost is not justified unless the application requires EnDat 2.2 functional safety (SIL 3 / PL e). For the standard MHD093C-035-PG1-AA digital servo feedback, SMC20 is the correct and most cost-effective choice.