SINAMICS V90 with S7-1500: Reading Current via Telegram 750

Problem Statement

Engineers integrating a SINAMICS V90 PN servo drive with a SIMATIC S7-1500 controller frequently hit a structural wall: the drive ships with a fixed set of PROFIdrive telegrams, only one of which can be assigned to the primary slot used by the SINA_SPEED function block. Telegram 1 (standard positioning, 32-bit speed setpoint) handles motion control. Telegram 3 (8-bit speed setpoint with 4 PKW words for parameter access) was historically used to scrape parameter values such as motor current and alarm state, but it is incompatible with SINA_SPEED because SINA_SPEED requires the standard 32-bit setpoint layout of Telegram 1 and the cyclic data of Telegram 3 collides with its I/O map.

The follow-on symptom is reproducible: the moment the user switches the V90 slot from Telegram 1 to Telegram 3 in TIA Portal, SINA_SPEED enters a fault state (ErrorID non-zero, StatusWord bit 6 = drive inhibited), even though the drive itself remains online. Conversely, running SINA_PARA alone with Telegram 1 lets you read parameters but cannot control the motor because SINA_PARA does not provide a control word or speed setpoint channel.

The correct architecture for the SINAMICS V90 is to keep Telegram 1 on slot 0 for SINA_SPEED, add Telegram 750 as a supplementary slot for torque feedback, and run SINA_PARA in parallel on the acyclic record channel for any non-cyclic parameters (motor temperature, fault history, encoder diagnostics).

SINAMICS V90 Telegram Catalogue and Constraints

The V90 PN firmware (V1.x and V2.x branches) supports the telegrams listed below. Unlike SINAMICS S120 or G120, the V90 does not expose a free BICO interconnection editor, so users cannot assemble a custom telegram containing current actual values. This constraint drives the architecture decision.

The Telegram 750 layout, per the V90 PROFIdrive supplement, is one process-data word outbound from drive to controller containing the smoothed torque actual value (r0079) referenced to p2003 reference torque. Because torque and current are proportional in the motor's field-weakening-free operating range (below base speed), r0079 gives a usable proxy for motor current when multiplied by the torque constant kt from the motor data sheet.

Why Telegram 3 Breaks SINA_SPEED Control

The SINA_SPEED function block (FB 285 in the Siemens DriveLib) maps directly onto the PROFIdrive Standard Telegram 1 frame layout. The block expects:

• Input word 0: Control word 1 (STW1), bits per PROFIdrive state machine
• Input word 1: Speed setpoint (NSOLL) as 32-bit signed, normalized to p2000
• Output word 0: Status word 1 (ZSW1)
• Output word 1: Speed actual value (NIST), 32-bit signed

Telegram 3 replaces the 32-bit NSOLL with a 16-bit NSOLL and prepends four PKW words (PKE, IND, PWE1, PWE2). When TIA Portal hands Telegram 3 to SINA_SPEED, the function block's internal decoder reads the first PKW word as STW1, decodes garbage, and never sets the OFF1/OFF2/OFF3 enable bits correctly. The drive stays in Switching On Inhibited (ZSW1 bit 6 = 1), and the block reports ErrorID = 16#0001 (PROFIdrive state machine fault).

The same logic applies to Telegram 102 and 105: any telegram that grows the PKW region or shifts the PZD region breaks SINA_SPEED because the block has no parameter for a flexible base offset.

Running SINA_SPEED and SINA_PARA in Parallel

Acyclic parameter access (the PROFIdrive DP-V1 record-data channel) is independent of the cyclic PZD channel. The PROFINET controller in the S7-1500 multiplexes acyclic requests onto the same physical connection without disturbing the cyclic I/O update. The following architecture is valid and production-proven:

Implementation rules:

1. Call SINA_SPEED in a cyclic OB (OB1 or OB30-OB38 depending on your motion cycle).
2. Call SINA_PARA in a slower OB (e.g., OB35 at 100 ms) to poll r0031 (actual current smoothed), r0035 (motor temperature), or r0945 (fault code array). Trigger one read per cycle; do not stack multiple in-flight requests.
3. Do not call SINA_SPEED while a SINA_PARA request is pending on the same logical connection; the Siemens DriveLib handles this internally, but excessive acyclic traffic can push the watchdog.

SINA_PARA Call Skeleton

// SCL example - acyclic read of smoothed actual current (r0031)
#SINA_PARA_Instance(REQ := #bReq,                  // pulse to start a single read
                    ID   := 16#0040,               // HW identifier of Telegram 1 (PROFINET slot 0)
                    Axis := 1,                      // axis number, 1-based
                    PNU  := 31,                     // parameter number (r0031)
                    SubIndex := 0,
                    Format := 0,                    // 0=word, 2=double word
                    Error := #wErr,
                    ErrorID := #dwErrID,
                    Value := #dwCurrentPercent);    // returns value, scaling per r0031

Telegram 750: Supplementary Torque Feedback

Telegram 750 is a one-word read-only supplementary telegram containing the smoothed torque actual value. It is added as a second slot under the V90 PN device in the PROFINET topology. The drive pushes the value every PROFINET IRT cycle (typically 1 ms), giving you a true real-time torque/current proxy.

Scaling and Conversion

The raw word in the Telegram 750 output region is a 16-bit signed integer scaled to p2003 (reference torque). Convert with:

Torque_Nm   = Telegram750_word * p2003 / 27648
Current_A   = Torque_Nm / kt     // kt from motor nameplate, Nm/A

For example, a V90 1FL6062 (6 Nm rated, 2.5 Nm/A torque constant) with p2003 = 6.0 Nm produces a raw word of 27648 at full rated torque. If the word reads 13824, the motor is delivering 3 Nm, which corresponds to roughly 1.2 A phase current.

For alarm logic on a rubber-roller pressure application, the typical workflow is:

1. Establish a baseline torque reading at known good roller condition.
2. Compute TorqueThreshold = 0.7 * TorqueBaseline (adjustable).
3. If TorqueActual < TorqueThreshold for > 2 s, raise operator alarm.
4. Add hysteresis (e.g., reset threshold at 0.85 * baseline) to avoid chatter.

TIA Portal Integration: GSD vs. HSP

There are two paths to place the V90 in the TIA Portal device view. Telegram 750 is supported by both, but the configuration steps differ slightly.

HSP Path (TIA V16, recommended for V90 V2 firmware)

1. Open Options → Support Packages → Install; select the SINAMICS V90 PN HSP for your TIA version.
2. Add the V90 PN device from the hardware catalog under Siemens AG → Drives → SINAMICS V90 PN.
3. Drag the V90 into the PROFINET network; assign the IP and device name.
4. Open the device view; slot 0 will already contain the V90 head module.
5. Click on the empty slot 1; from the catalog add Telegram 750, supplementary torque actual value.
6. Compile the configuration and download to the CPU.

GSD Path (fallback for older TIA or V90 V1 firmware)

1. Download the current GSDML from the Siemens support portal (search for the V90 PN entry under PROFINET GSD files).
2. Import via Options → Manage General Station Description Files.
3. Insert the V90 PN as a generic PROFINET IO device.
4. In the slot list, set slot 0 to Telegram 1 and slot 1 to Telegram 750.
5. Assign the IP, name, and download.

Reading Telegram 750 with the GETIO Function Block

The GETIO (distributed I/O read) FB is the simplest way to read the one-word Telegram 750 input region without disrupting SINA_SPEED. You do not need SINA_PARA for this signal because Telegram 750 is cyclic, not acyclic.

// SCL example - read Telegram 750 raw word
#GETIO_Instance(REQ     := TRUE,                    // continuous read
                ID      := %QWxx:T750SlotHWID,      // HW ID of Telegram 750 slot
                LADDR   := #wT750InputAddress,      // input start address (e.g., IW 256)
                LEN     := 2,                       // one word = 2 bytes
                STATUS  := #wStatus,
                ERROR   := #bError);
// Result appears at %IW256 on the CPU side

Once the raw word is in a standard input address, your SCL/ST program scales it to engineering units:

#TorquePct := INT_TO_REAL(%IW256) / 27648.0 * 100.0;
#TorqueNm  := #TorquePct / 100.0 * #p2003;
#MotorCurrent := #TorqueNm / #kt;

If you prefer symbolic I/O, leave the GETIO call out and reference the Telegram 750 input directly via the symbolic name assigned in the device view (e.g., "V90PN_1_Slot1_T750".TorqueActual).

Application Context: Rubber Roller Pressure Monitoring

A common application driving this question is monitoring the contact pressure of a rubber roller against a driven drum. The torque required to keep the drum at constant speed is a direct function of the friction coefficient between roller and drum. As the rubber ages or loses surface tack, friction drops, motor torque drops, and current drops proportionally.

Recommended Alarm Logic

// Track baseline at machine commissioning
IF #bLearnMode THEN
    #TorqueBaseline := #TorqueBaseline * 0.9 + #TorqueNm * 0.1;  // low-pass
END_IF;

// Alarm detection with debounce and hysteresis
#bBelowThreshold := (#TorqueNm < #TorqueBaseline * 0.7);
IF #bBelowThreshold THEN
    #iDebounce := #iDebounce + 1;
    IF #iDebounce > 200 THEN                          // 2 s at 100 ms
        #bRollerAlarm := TRUE;
    END_IF;
ELSE
    #iDebounce := 0;
    IF #TorqueNm > #TorqueBaseline * 0.85 THEN       // hysteresis
        #bRollerAlarm := FALSE;
    END_IF;
END_IF;

For finer resolution, also alarm on the rate of change of torque: a sudden > 20% drop within one second usually indicates roller contamination, not gradual wear.

Implementation Checklist

1. Confirm V90 PN firmware version (V1.x vs V2.x) before selecting HSP or GSD.
2. Configure Telegram 1 on slot 0, Telegram 750 on slot 1 in TIA Portal device view.
3. Wire SINA_SPEED to the Telegram 1 HW identifier; verify control word bits toggle correctly (DriveLib example project provides a watcher).
4. Use GETIO or direct symbolic read to pull the Telegram 750 word.
5. Scale to Nm using p2003; convert to A using motor-specific kt.
6. For non-cyclic parameters (r0031, r0035, r0945), use SINA_PARA in OB35 at 100 ms minimum interval.
7. Establish baseline torque at known-good roller condition during commissioning.
8. Add debounce and hysteresis to the alarm logic.

Verification and Commissioning Checks

Troubleshooting Matrix

Notes on Related Telegrams and Future Firmware

Siemens periodically extends V90 telegram support via firmware updates. Confirm the firmware version on the drive nameplate and review the release notes for any new supplementary telegrams (for example, future revisions may add an encoder-actual-value supplementary telegram analogous to 750). The current V2 firmware supports Telegram 750 as documented above; Telegram 352 remains unsupported and should not be configured.

For users who prefer entirely acyclic access (no Telegram 750), the workable ceiling is roughly 20-50 ms update rate using SINA_PARA with hand-shaken REQ/DONE logic. This is adequate for slow thermal monitoring (r0035, r0036) but marginal for fast load alarms. For roller-pressure monitoring with a debounced alarm, SINA_PARA alone is sufficient; for closed-loop reaction within one controller cycle, Telegram 750 is the correct path.