SINAMICS CU320 SMC30 Reading Incremental Encoder Zero Mark Status

1. Problem Context and Drive Topology

On a SINAMICS S120 system the Control Unit CU320-2 is connected to one or more Sensor Modules that digitize the physical feedback signals from the motor and position encoders. The SINAMICS S120 List Manual documents every parameter from p0000 through p3999 (CU320-2 device parameters) and from p4000 through p8799 (drive object parameters), and the SMC30 is identified as encoder interface 1/2/3 depending on which port the SSI / HTL / TTL encoder is wired to. The SMC30 (Sensor Module Cabinet-Mounted, 6SL3055-0AA00-5CA0 family) supports HTL and TTL square-wave encoders, optionally with an R or N reference (zero) mark. The reference mark is a single, well-defined index pulse that is generated once per mechanical revolution, and SINAMICS uses it to:

• Initialize the absolute position counter inside the drive object (r2521, r2524).
• Validate the mechanical position when the Basic Positioner (EPOS) performs active or passive referencing.
• Provide a repeatable synchronization point for closed-loop position control.

The recurring engineering question on this topology is: "I have wired the encoder and set up p0400, p0408, p0425, but I cannot find a Boolean status flag that toggles every time the zero mark passes the SMC30." This reference walks through the relevant parameter set, the SMC30 status words, the equivalent-zero-mark feature, and the DCC / EPOS work-flows that surface the reference mark to the application program.

2. Incremental Encoder Reference Signal — Electrical and Protocol Basics

An incremental encoder is a position transducer that generates two quadrature pulse trains, A and B, plus an optional index channel, R (HTL/TTL push-pull) or N (differential). The A/B phase shift encodes the rotation direction; the index pulse is asserted for one increment of rotation and is one mechanical revolution wide (sometimes one increment wide) so that the drive can reset its internal position counter to a known value. The electrical levels of the index channel on the SMC30 are identical to the levels of the A and B channels:

The mechanical relationship between the index pulse and the resolver pole count is important for synchronous motors. With an asynchronous induction motor it is generally sufficient that the SMC30 latch the absolute counter to zero on the rising edge of R or N, since the slip frequency makes the rotor position at the index mark unimportant. The general frequency that the SMC30 sees on the index channel, f_z, is:

f_z = n / 60 [Hz]

where n is the motor speed in revolutions per minute. For a 4-pole induction motor at 1500 rpm, f_z = 25 Hz; at 6000 rpm, f_z = 100 Hz. The DCC or comparator sampling must therefore be at least 4 ms (250 Hz) to guarantee capture of the index event at full speed, which is consistent with the DCC block scan time of 4 ms typical for the CU320-2.

3. SMC30 Hardware Configuration (DIP Switches and Wiring)

Before any software parameter is read, the SMC30 itself must be configured by the two banks of DIP switches on the front panel. The DIP switch positions determine the bus protocol that the Sensor Module reports to the CU320-2, the input voltage level, and the terminating resistance on the encoder cable. The default ex-factory setting of the SMC30 is Encoder type = SSI / SSI single-turn and No zero mark evaluation. The first commissioning step is therefore to re-configure the DIP bank for a square-wave HTL encoder with a zero mark:

1. Power down the SINAMICS line-up and remove the 24 V supply to the SMC30.
2. Set DIP switch bank S1 for HTL input with terminating resistor (positions 1–4 per the SMC30 manual, switch 7 ON, switch 8 OFF for HTL with 150 Ω termination).
3. For HTL with zero-mark evaluation, switch 5 = ON. The SMC30 then expects a signal on the R input and routes it to the digital zero-mark latch.
4. Wire the encoder cable to the 12-pin plug X521 (signal name printing is silkscreened on the SMC30 housing): pin 1 = A, pin 2 = A\, pin 3 = B, pin 4 = B\, pin 5 = R, pin 6 = R\, pin 7 = 24 V encoder supply, pin 8 = 0 V, pin 9–12 = fault, screen.
5. Re-apply 24 V, wait for the SMC30 LED READY to turn steady green.

4. SINAMICS Parameter Set for Square-Wave Encoder and Zero Mark

Once the SMC30 is configured at the DIP level, the drive-side parameters can be edited. The CU320-2 in the source topology has p10 = 0 (Run) and the user has selected encoder commissioning, which means the SMC30 slot is the active encoder. The minimum parameter set that the application must commit is:

The values p0400 = 3001 and p0408 = 1024 together cause the drive to commit the encoder as a 1024-increment HTL encoder with one zero mark per revolution. The parameter p0425 = 1024 confirms that the zero mark occurs once per 1024 increments (i.e. one per revolution). If the encoder is supplied with a separate zero-mark option, the zero-mark distance may be 1/2, 1/4, or 1/8 of the revolution, in which case p0425 must be set to the corresponding sub-multiple of p0408.

5. Status Parameters — Where the Reference Mark Becomes a Boolean

The SMC30 produces a small set of status words that the CU320-2 reads back over DRIVE-CLiQ. The application can read these through PROFIBUS, PROFINET, or any of the free telegram-configuration words. The relevant status words for reference-mark monitoring are listed in the table below; each is per-encoder, indexed by the encoder number on the drive object (index 0 for encoder 1, index 1 for encoder 2, and so on).

The cleanest single-bit answer to the recurring question "Which parameter gives me a 1 when the zero mark has just been detected?" is therefore r0480[encoder].12 (G1_STW bit 12, "Reference mark function active"). This bit is asserted by the SMC30 firmware for the duration of the index pulse, then automatically cleared. On a PROFINET cycle time of 1 ms, the application will see a 1–4 ms pulse depending on motor speed. The combined r2521 / r2524 signals form the multi-turn position and one-revolution fine position that the Basic Positioner uses to expose the rotor absolute angle.

6. Equivalent Zero Mark (p0491, p0492, p0493, p0494)

When the actual hardware zero mark cannot be used — for example, a linear encoder with no index channel, a heavy-duty encoder in a dirty environment where the index pulse is unreliable, or a gear-reduced application where the index of the high-speed shaft is not the desired home position — SINAMICS provides a software-generated equivalent zero mark. The relevant parameters are:

With p0491 = 1 and p0492 = 1024 on a 1024-ppr encoder, the equivalent zero mark is asserted exactly once per revolution at the encoder angle defined by p0494. The status word r0480.12 reflects either the true or the equivalent zero mark depending on p0495, so the application does not need to know which one is active.

7. Reading the Reference Mark as a Boolean in a PLC

In a SIMATIC S7-1500 control panel connected to a CU320-2 over PROFINET, the application can subscribe to the encoder status word by selecting telegram 105 (Standard Telegram 5, PZD-2/2 with encoder status), 106 (PZD-4/4), or the manufacturer-specific telegram 370. In the cyclic I/O image, the relevant word is the second process data word of the encoder status slot. A typical PLC tag wiring looks as follows:

// SCL example for S7-1500 — rising-edge detection on r0480.12
// Inputs from drive: %IW200 = Status word 1 (G1_STW) of encoder 1
// Output to application: %Q50.0  = "zero mark detected this cycle"

#G1_STW := %IW200;
#zeroMarkActive := #G1_STW.%X12;       // bit 12 of the status word
#edgeOut := #zeroMarkActive AND NOT #zeroMarkLast;
#zeroMarkLast := #zeroMarkActive;

IF #edgeOut THEN
    #zeroMarkCount := #zeroMarkCount + 1;
END_IF;

%Q50.0 := #edgeOut;
%QD52  := #zeroMarkCount;

Because r0480.12 is essentially a 1-wide pulse, the PLC must perform the edge detection in the same task class as the PROFINET cycle (typically 1–2 ms in OB1, or 250 µs with an OB61 IRT). The same Boolean can be subscribed through PROFINET with Shared Device / I-Device as a discrete input if the application controller does not perform any extra logic.

8. DCC Implementation — Edge-Detection and Angle Comparator

For drives without an external PLC, the SINAMICS Drive Control Chart (DCC) editor inside STARTER / Startdrive lets the application build a small DCC chart on the CU320-2 that detects the index pulse and resets an internal counter. The blocks are connected in a DCC sheet on the drive object (the chart name is, e.g., MY_ZM_DETECT):

// DCC graph — detect zero mark on r0480.12, latch in flag ZM_PULSE
// Available DCC blocks (Siemens DCBLib):

  [AND]  -> (in1 = r0480.12, in2 = NOT(latch_q))            // one-shot
  [RS]   -> (S = AND.out, R1 = cycle_tick)                  // arm / reset
  [NAND] -> (in1 = RS.Q, in2 = RS.Q)                        // debounce
  [CNTR] -> (CUD = RS.Q, CU = NOT(RS.Q), LD = cycle_tick)   // count
  [OUT]  -> r2090.0 = ZM_PULSE                              // output to PZD

The DCC chart must be linked to the drive object at the right slot (Configuration > DCC > Charts). The sampling time of every DCC block is defined in the DCC chart properties; the recommended sampling for zero-mark detection is 4 ms, which gives a worst-case angular error of 4 ms × motor speed in increments:

Δθ_max = (4 ms × n / 60) × (360 / p0408) [degrees]

For a 1024-ppr encoder at 3000 rpm this is:

Δθ_max = (4 × 10⁻³ × 50) × (360 / 1024) = 0.07°

which is well within the resolution of a typical position-controlled axis.

9. Basic Positioner (EPOS) Referencing — How the Zero Mark Is Used Internally

When the technology option Basic Positioner is licensed and the closed-loop position control is active, the zero mark becomes the synchronization point of the EPOS state machine. The EPOS referencing modes are documented in the SINAMICS S120 Function Manual — Basic Positioner and are configured through the following parameters:

Once the referencing run completes, r2684.0 is set true. This Boolean is the cleanest "do I have a valid home position" signal the application can read, regardless of whether the zero mark was supplied by the actual hardware or by p0491's equivalent-zero-mark generator. A typical EPOS referencing sequence is initiated with the control word STW1.11 = 1 (Referencing start) and confirmed in ZSW1.12 (Referencing active).

10. Closed-Loop Position Control and r2521 / r2524

When the technology option for closed-loop position control is enabled (the SIMOTION technology package or the EPOS extension), the CU320-2 exposes two coupled position registers: the multi-turn value r2521 (a 32-bit counter that increments every time the encoder zero mark is detected and the new revolution angle is committed) and the one-revolution fine value r2524 (a 32-bit modulo counter of 0 … (p0408–1) LU). The absolute angle of the rotor, expressed in mechanical degrees, is computed as:

θ_mech = 360 × (r2521 + r2524 / p0408) / (p0408 / p0408) = 360 × (r2521 + r2524 / 1024)

which reduces to θ_mech = 360 × (r2521 + r2524 / 1024) for the 1024-ppr encoder in the source application. This is the angle that the original user is reading on the HMI. Note that r2521 is a signed 32-bit value; an unsigned wrap around occurs at 2 147 483 647 / 1024 ≈ 2 097 152 revolutions, which is sufficient for any practical machine life.

11. Complete Commissioning Procedure

1. Wire the encoder cable to the SMC30 per the SMC30 manual. Verify shield continuity and confirm that the screen is bonded to the SMC30 housing and to the encoder housing on both ends.
2. Set the SMC30 DIP switch bank S1 for HTL with terminating resistor and zero-mark evaluation.
3. In STARTER / Startdrive, project the drive object (SERVO or VECTOR) and connect the SMC30 in the topology. The encoder used by the position controller is Encoder 1 unless the application explicitly chooses Encoder 2 or 3.
4. Open Configuration > Encoders and click Encoder commissioning. The drive writes p0010 = 4.
5. Set p0400 = 3001 to load the 1024-HTL A/B R preset (alternatively, p0400 = 9999 for a fully user-defined configuration). The preset loads p0404, p0405, p0408, p0425 with the matching defaults for a 1024-ppr HTL encoder with reference mark.
6. Confirm p0408 = 1024 and p0425 = 1024 (one zero mark per revolution).
7. Exit encoder commissioning with p0010 = 0. The drive copies the configuration into the running data set and the encoder fault lamp should clear within 1 second.
8. Test the reference mark by turning the motor slowly (< 100 rpm) by hand. The trace tool in STARTER (Trace > Record > r0480 bit 12) should show a single 1-wide pulse once per revolution.
9. For the closed-loop position control option, configure EPOS referencing (p2599 – p2617) and run a homing cycle. Verify that r2684.0 latches to 1 at the end of the run.
10. For the equivalent-zero-mark feature, set p0491 = 1 and confirm the trace still shows one pulse per revolution, but the r0482 status word now reports "active zero mark function = 10 (equivalent)".

12. Verification Checklist

• Trace r0480 bit 12 at 50 rpm. Expect one 1-wide pulse per revolution, at a constant angular position.
• Trace r2524 at 1000 rpm. Expect a sawtooth pattern from 0 to 1023, repeating every revolution.
• Trace r2521 over a 5-revolution span. Expect a 5-step staircase (one increment per revolution).
• Disconnect the index channel R. The drive should trip with F3x001 (encoder fault) and r0479 bit 7 = 1 (zero-mark error) within one zero-mark period.
• Set p0491 = 1, then mechanically hide the actual zero mark (block channel R with a service jumper). The drive should still reference successfully because the equivalent zero mark replaces the missing hardware index.
• Run the drive at 6000 rpm and let the controller hold a position. The r2524 / r2521 transition should not produce a glitch at the zero-mark crossing.

13. Troubleshooting Matrix

14. Common Operator Errors and Field-Proven Caveats

• Setting p0400 = 9999 (user-defined) disables the safety net of the preset values, but the user-defined route is required if the encoder is a non-standard product (e.g. a Sick VFS60, a Heidenhain ROD 436, or a Hengstler RI 58 with a non-Siemens pinout). Always re-check p0404 to p0421 manually after the user-defined preset.
• On a CU320-2 with a Multi-CU arrangement (CU320-2 + CX32-2), the encoder on the CX32-2 must be configured on the host CU, not on the CX32-2. The cross-binding p0680[0..3] routes the SMC30 signals to the drive objects.
• If the application also uses an SSI absolute encoder on a second SMC30 (e.g. as a load-side encoder), the cross-encoder p0431 offset will not transfer the reference mark; each SMC30 has its own zero-mark counter.
• Power-Modules in chassis format (e.g. Active Line Module + Motor Module) have a different slot numbering for the SMC30 (slot 2, not slot 1) and the parameter index for r0451 / r0480 follows the slot, not the encoder number.
• On a CU320-2 DP (the older PROFIBUS-only variant), the same parameters apply, but PROFIdrive conformance requires that r0480 word 1 be placed at the PZD position defined by telegram 105; the index column does not exist on PROFIBUS, the PZD position is fixed.

15. Reference Summary

FAQ