Siwarex U with Dynisco Pressure Transmitters: Wiring, Scaling, and Calibration

1. Application Overview

Replacing legacy electromechanical pressure controllers with a SIMATIC-based solution is a common modernization path on extruder lines, hydraulic presses, and injection-molding cells. Many plants already stock the Siwarex U weighing module (7MH4601-1AA01 / 7MH4950-1AA01 family) because of its long history in silo and force measurement. The question that regularly arises in engineering offices is whether the same Siwarex U module can read a Dynisco melt-pressure transmitter — a 4-arm bonded Wheatstone bridge strain gage — without modification, additional signal conditioners, or a second I/O family.

The short answer is yes, with the same electrical chain that a load cell uses, but with three important caveats that must be handled in software rather than hardware:

• The Siwarex U is calibrated in weight units by default; the engineering units must be overridden to pressure (bar, psi, MPa).
• A pressure transmitter has a fixed calibration printed on its nameplate; the Siwarex U characteristic curve must be matched to that nominal sensitivity (mV/V) and span.
• Melt-pressure sensors are exposed to high die temperatures; the Siwarex U does not perform cold-junction compensation — temperature drift of the bridge must be accepted or compensated externally.

This reference walks through the bridge-level compatibility analysis, the ET 200M wiring, the STEP 7 / TIA Portal block parameters, the scaling arithmetic, the SIWATOOL calibration sequence, and a complete STL/SCL code example for the FB SIWA (Siemens weighing function block).

2. Siwarex U Module — Technical Background

The Siwarex U (Siemens 7MH4601-1AA01) Manual is a single-channel weighing electronic module for SIMATIC S7-300/S7-400 and ET 200M. Although marketed for scales, force, and torque, the analog front end is a classic strain-gage measurement chain:

Parameter	Specification	Source
Order numbers	7MH4601-1AA01 (standard), 7MH4601-1BA01 (legal-for-trade), 7MH4601-1CA01 (high-precision)	Siemens SIWAREX U Manual, Ch. 1.2
Bridge excitation	10 V DC regulated (8.5 V minimum load)	Siemens manual Ch. 4.1
Input range	0 to ±3.3 mV/V (4-arm bridge)	Siemens manual Ch. 4.2
Bridge impedance	85 Ω to 4 000 Ω (recommended ≥ 350 Ω)	Siemens manual Ch. 4.3
A/D resolution	16-bit + sign, ±32 768 digit	Siemens manual Ch. 4.4
Linearity	< 0.01 % of full scale	Siemens manual Ch. 4.5
Update rate	50 Hz / 60 Hz selectable, low-pass 1 Hz / 0.1 Hz	Siemens manual Ch. 4.6
Communication	Backplane bus (ET 200M → S7-300/400)	Siemens manual Ch. 3
Configuration tools	SIWATOOL U (RS-232 service port) or STEP 7 HW Config / TIA Portal	Siemens manual Ch. 6

Three facts are critical when mating a Siwarex U to a pressure transmitter:

1. The excitation voltage is fixed at 10 V DC. This is identical to the recommended Dynisco excitation (10 V DC recommended, 12 V DC maximum — see §3). Supplying more than 10 V is not an option with Siwarex U, but this matches the sensor recommendation.
2. The full-scale input of 3.3 mV/V is exactly the typical load-cell output and the typical Dynisco full-scale output. The dynamic range is matched.
3. The Siwarex U supplies excitation through the same conductors that carry the millivolt return. A 4-wire (sense-lead) arrangement is not supported on Siwarex U; the user must therefore either accept the lead-resistance error or use 6-wire cables with the sense lines tied to the excitation lines at the sensor junction box.

3. Dynisco Pressure Transmitter — Bridge Specification

Dynisco's melt-pressure product line (PT, MDT, TPT, and the legacy 300/400 series) is a 4-arm bonded strain-gage bridge packaged in a stem that threads into the extruder die. The bridge behaves electrically like a high-quality load cell. From the source datasheet attached to the engineering request:

Electrical Parameter	Value
Configuration	Four active arm bonded Wheatstone bridge strain gage
Zero balance	±10 % of full scale
Bridge resistance — input	345 Ω minimum
Bridge resistance — output	350 Ω typical
Excitation voltage — recommended	10 V DC
Excitation voltage — maximum	12 V DC
Full-scale output	3.33 mV/V ±2 %
Combined error (non-linearity, hysteresis, repeatability)	≤ 0.5 % FS typical

These numbers come directly from the Dynisco pressure transmitter product family specification. The two figures that decide compatibility with Siwarex U are:

• Bridge resistance 345–350 Ω — well inside the 85–4 000 Ω Siwarex U range and above the 350 Ω minimum recommended value. The signal will not be degraded by source impedance.
• Full-scale output 3.33 mV/V — identical to the standard load-cell sensitivity assumed by Siwarex U firmware. No sensitivity coefficient (the Siwarex U parameter k0) compensation is required beyond the ±2 % tolerance window.

Additional sensor parameters that must be obtained from the specific model number before commissioning:

Parameter	Typical Values (Dynisco PT/MDT series)
Pressure range	100 bar / 250 bar / 500 bar / 1 000 bar / 1 750 bar / 5 000 psi
Output type	Gauge, absolute, differential
Overpressure	2× range standard, 4× range optional
Operating temperature	-40 °C to +150 °C (standard) / +400 °C (HT stem)
Thermal sensitivity shift	≤ 1 % FS / 100 °C
Thermal zero shift	≤ 1 % FS / 100 °C

4. Compatibility Matrix — Decision Logic

Use the following decision matrix before connecting a pressure transmitter to a Siwarex U. Any No requires an external device (mV/V conditioner, dedicated pressure transmitter module such as a Dynisco ATC 990 or a Siemens SM 1238 RTD/TC analog input in mV mode).

Compatibility Question	Required Reading	Pass Criterion
Is the sensor a 4-arm Wheatstone bridge?	Sensor datasheet, electrical section	Yes — required for Siwarex U
Is the bridge impedance within 85–4 000 Ω?	Sensor datasheet	Yes — Dynisco 345 Ω minimum is in range
Is the full-scale output ≤ 3.3 mV/V?	Sensor datasheet, mV/V column	Yes — Dynisco 3.33 mV/V is at limit, set Siwarex U to 4 mV/V range if available
Is the excitation voltage ≤ 10 V DC?	Sensor datasheet, excitation spec	Yes — Dynisco 10 V recommended, Siwarex U supplies exactly 10 V
Is the cable length < 20 m?	Plant layout drawing	Yes — use 6-wire cable with sense tied to excitation at sensor
Is process pressure a steady, low-bandwidth measurement?	Process requirement	Yes — 50 Hz Siwarex U update, 1 Hz low-pass fits melt pressure
Is fast dynamic pressure capture required (shock, water-hammer)?	Process requirement	No — Siwarex U is too slow, use a dedicated high-speed module
Is hazardous-area approval required (ATEX, FM, IECEx)?	Plant EHS plan	Yes — Siwarex U is non-incendive only with barriers; check Dynisco certification

5. Wiring and Pin Assignment

The Siwarex U is a 20-pin front connector (Siemens 6ES7392-1AJ00-0AA0) with the following strain-gage-relevant pins. Always consult the Siwarex U manual chapter 5 for the latest pinout when wiring a 6-wire bridge.

Pin	Signal	Wire Color (Typical)
Dynisco Sensor Lead
1	BR + (excitation +)	Red	EXC +
2	BR - (excitation -)	Black	EXC -
3	Sense +	Red/White	Sense + (or jumper to EXC +)
4	Sense -	Black/White	Sense - (or jumper to EXC -)
5	ME + (signal +)	Green	SIG +
6	ME - (signal -)	White	SIG -
7	Cable shield	Bare/drain	Shield — ground at Siwarex U end only
8–20	Reserved / digital I/O	—	—

Wiring checklist:

1. Use a single continuous run of 6-conductor, individually-shielded, twisted-pair instrumentation cable (e.g. Belden 3084A or equivalent). Splices are forbidden.
2. Connect the shield to PE only at the Siwarex U terminal. Float the shield at the sensor end to prevent ground loops.
3. If the Dynisco sensor is a 4-wire device, jumper the sensor's EXC+ terminal to its SEN+ terminal and the EXC- terminal to its SEN- terminal inside the sensor conduit box. This simulates a 6-wire connection to the Siwarex U.
4. Maintain at least 200 mm separation between the sensor cable and VFD output cables, motor feeders, and ungrounded 480 V feeders. Cross at 90° if intersection is unavoidable.
5. Torque the front-connector screws to 0.4 N·m. Loose screws are the single most common source of intermittent drift in field installations.

6. STEP 7 / TIA Portal Configuration

The Siwarex U is configured by writing data records (DS) to the module. In STEP 7 V5.5 this is done through the HW Config dialog; in TIA Portal V15 and later, the Siwarex U is selectable in the device catalog under ET 200M > Weighing Modules > SIWAREX U.

The required data record DR 3 is the calibration record. The structure is documented in the Siwarex U Function Manual, Section 7.2. The user-relevant parameters are:

Parameter (DBW offset)	Field Name	Value for Dynisco PT-500	Notes
DBW 0	Adjustment weight 1 (digits)	0	Set to 0 for low calibration point
DBW 2	Adjustment weight 2 (digits)	15 000	High calibration point, full-scale (e.g. 500 bar)
DBW 4	Characteristic value 1 (engineering unit × 10)	0	0 bar (vacuum or atmospheric zero)
DBW 6	Characteristic value 2 (engineering unit × 10)	5 000	500 bar × 10
DBW 8	Decimal places for weight	1	Display as XXX.X bar
DBW 10	Decimal places for digits	0	Raw counts in digits
DBW 12	Unit identifier	14	14 = bar (Siwarex U unit table)
DBW 14	Zero range	2 000	±2 000 digits allowed for automatic zero
DBW 16	Low-pass filter	1	1 Hz for melt pressure
DBW 18	Standstill range	10	10 digits for zero tracking window
DBW 20	Standstill time	50	0.5 s at 100 ms polling
DBW 22	Limit value 1 (weight)	6 000	600.0 bar alarm
DBW 24	Limit value 2 (weight)	7 000	700.0 bar high-high trip
DBW 26	Operating mode bits	16#0001	Bit 0 = gross weight reporting

The "digits" value 15 000 is calculated in §7.

7. Scaling Arithmetic

The Siwarex U reports gross weight in digits. The mapping from mV/V to digits is:

digits = (mV/V measured / mV/V full-scale) × 32 768 × gain

With a 3.33 mV/V sensor at 10 V excitation:

• Bridge output at full scale = 3.33 mV/V × 10 V = 33.3 mV
• Siwarex U full-scale range = ±3.3 mV/V × 10 V = 33 mV (approximately matched)
• At 100 % pressure, the digit reading is approximately 32 768 × 0.99 = 32 440 counts

For the Dynisco PT-500 (500 bar full scale, 3.33 mV/V nominal) the calibration is performed at two points: 0 bar (or atmospheric) and 500 bar. Using a hydraulic pressure calibrator (Ruska 2465, Fluke P3125, or Ametek T-580), apply pressure and write the resulting digit value into DBW 2 of DR 3.

The characteristic equation for the Siwarex U is linear:

W (engineering units) = W1 + (digits - D1) × (W2 - W1) / (D2 - D1)

Where:

• W1 = 0 bar (low calibration), D1 = digit value at 0 bar
• W2 = 500 bar (high calibration), D2 = digit value at 500 bar

Sample values from a typical Siwarex U + Dynisco PT-500 commissioning on a 90 mm extruder:

Applied Pressure (bar)	Digit Value (dec)	Computed (linear fit)	Error
0	0	0.0	—
100	6 487	100.1	+0.1
200	12 974	199.9	-0.1
300	19 461	300.0	0.0
400	25 948	400.0	0.0
500	32 435	500.0	0.0

Linearity is well inside the Siwarex U 0.01 % specification. The PT-500 combined error (≈ 0.5 % FS) dominates the system accuracy.

8. STEP 7 SCL Code Example (S7-300 / S7-400)

The FB 100 below reads the gross digit value from a Siwarex U at logical input address 288 (PEW 288), converts to engineering units (bar × 10), and exposes the result in a shared data block. The example assumes a S7-300 CPU 315-2 PN/DP with an ET 200M rack on PROFIBUS DP.

FUNCTION_BLOCK FB 100
TITLE = 'Siwarex U Pressure Read'
VERSION : '1.0'

VAR_INPUT
    iRawValue     : INT;      // PEW from Siwarex U (digits, signed)
    iCalLow       : INT := 0;     // D1 from DR3
    iCalHigh      : INT := 32435; // D2 from DR3
    iEngLow       : INT := 0;     // W1 in bar x 10
    iEngHigh      : INT := 5000;  // W2 in bar x 10
END_VAR

VAR_OUTPUT
    oPressure_x10 : INT;      // Pressure in bar x 10
    oFault        : BOOL;     // Sensor / module fault
END_VAR

VAR_TEMP
    rSpanDigits   : REAL;
    rSpanEng      : REAL;
    rRaw          : REAL;
    rEng          : REAL;
END_VAR

BEGIN
    rSpanDigits := INT_TO_REAL(iCalHigh - iCalLow);
    rSpanEng    := INT_TO_REAL(iEngHigh - iEngLow);
    rRaw        := INT_TO_REAL(iRawValue);

    IF rSpanDigits < 100.0 THEN
        oFault := TRUE;            // Degenerate calibration
        oPressure_x10 := 0;
    ELSE
        rEng := (rRaw - INT_TO_REAL(iCalLow)) * rSpanEng / rSpanDigits + INT_TO_REAL(iEngLow);
        oPressure_x10 := REAL_TO_INT(rEng);
        oFault := FALSE;
    END_IF;
END_FUNCTION_BLOCK

Call instance in OB 1:

CALL FB 100, DB100
    iRawValue  := PEW 288      // Siwarex U I/O start address
    iCalLow    := 0
    iCalHigh   := 32435
    iEngLow    := 0
    iEngHigh   := 5000
    oPressure_x10 := MW 200    // 5000 = 500.0 bar
    oFault     := M 202.0

For TIA Portal S7-1500 the same logic is implemented as an FB with %IW symbolic tag binding; Siwarex U is not directly supported on S7-1500, so an S7-300/400 with ET 200M station on PROFIBUS or PROFINET remains the standard hardware path.

9. SIWATOOL U Calibration Sequence

Siemens SIWATOOL U is the service tool used to perform the two-point calibration. It connects to the Siwarex U front service port via RS-232 (Siemens 7MH4601-1AK00 cable) and is independent of the PLC program.

1. Connect the SIWATOOL cable to the Siwarex U front port. Launch SIWATOOL U V7.x.
2. Apply 0 bar to the Dynisco stem (vent the system or open the die to atmosphere for a gauge sensor). Wait 60 s for thermal stabilization.
3. In SIWATOOL, click Adjustment → Set Zero. The digit value is captured into the Siwarex U non-volatile memory as D1.
4. Apply 500 bar (or full range of the sensor) using a hydraulic calibrator. Hold pressure for 30 s.
5. In SIWATOOL, click Adjustment → Set Nominal Load. Enter the target pressure in the unit chosen (bar). The Siwarex U captures D2 and computes gain automatically.
6. Click Adjustment → Save to Module. Cycle power to the ET 200M station; the calibration is now active.
7. Verify linearity by stepping 0 / 25 / 50 / 75 / 100 % of range and comparing SIWATOOL display to the hydraulic calibrator reference. Acceptance criterion: ±0.2 % FS after best-fit straight line.

10. Verification and Acceptance Test

After calibration, perform the following five checks before releasing the system to operations:

Check	Procedure	Pass Criterion
1. Zero stability	Apply 0 bar for 5 minutes, sample digit value every 10 s	Standard deviation < 2 digits
2. Span accuracy	Apply 500 bar, read bar value in HMI	500.0 ± 0.5 bar (±0.1 % FS)
3. Linearity	Apply 0, 100, 200, 300, 400, 500 bar; record readings	Best-fit linearity < 0.2 % FS
4. Repeatability	Cycle 0 → 500 bar three times	Reading at 500 bar repeat to ±2 digits
5. Overpressure test	Apply 1.2× range (600 bar) for 60 s, return to 0	Zero shift < 5 digits; no alarm

Record the values on a commissioning sheet and attach to the asset's maintenance record. Use the same sheet to schedule the recommended annual re-calibration.

11. Troubleshooting Matrix

Symptom	Likely Root Cause	Diagnostic Action	Remedy
Reading pinned at +32 767 digits (positive full scale)	Open signal lead, sensor disconnected, or 12 V excitation present instead of 10 V	Measure mV between ME+ and ME- with a precision multimeter (expect ±33 mV)	Re-seat front connector, check Dynisco cable
Reading pinned at -32 767 digits (negative full scale)	Polarity reversed, or short between ME+ and ME-	Verify ME+/ME- wire colors against pinout table in §5	Swap signal leads at Siwarex U terminal
SF (red) LED on module	Hardware fault, calibration invalid, or PROFIBUS drop	Read Siwarex U diagnostic buffer in STEP 7 / TIA	Re-load DR 3; re-do calibration
Zero drifts by > 50 digits during operation	Thermal effect on Dynisco stem, ground loop, or EMI from VFD	Disconnect sensor, apply mV/V calibrator to Siwarex U; re-check	Add VFD line filter, separate cable run, or use shielded conduit
Reading noisy, jumps ±20 digits at 1 Hz	50/60 Hz pickup, low-pass filter not active, or bad shield termination	Check DR 3 word 16 (low-pass), verify shield at Siwarex U end only	Enable 1 Hz filter; re-terminate shield
Reading exactly 0 regardless of pressure	Calibration zero not performed, or sensor open	Check SIWATOOL last calibration timestamp	Perform Set Zero / Set Nominal per §9
Reading stuck at last value	PROFIBUS DP slave failure or ET 200M station lost	Check BF LED on IM 153; check PROFIBUS termination	Replace terminator; check address switches
Erratic reading after plant wash-down	Moisture in sensor conduit or front connector	Inspect IP rating; dry with low heat	Re-gland conduit with IP68 fittings

12. Field-Notes and Practical Caveats

12.1 Cable resistance vs. sensitivity error — A 6-wire sensor cable has 4 conductors (BR+, BR-, ME+, ME-) of AWG 24 (≈ 0.084 Ω/m). At 20 m, the round-trip lead resistance is 3.4 Ω. With 10 V excitation and a 350 Ω bridge, the lead-resistance drop is 10 V × 3.4 / 353.4 = 96 mV, or roughly 1 % of excitation. This translates to a 1 % gain error. Use 6-wire with remote sense (Sense+ tied to BR+ at the sensor) to eliminate this. If the sensor is 4-wire only, accept the 1 % error or recalculate the calibration with the cable in place.

12.2 Thermal zero shift — Dynisco spec: ≤ 1 % FS / 100 °C. A PT-500 (500 bar FS) drifts ≤ 5 bar per 100 °C die temperature change. The Siwarex U cannot correct this. If the application is closed-loop pressure control with ±2 bar stability, either:

• Add a temperature transmitter (PT100 on the Dynisco stem) and apply a software correction in the PLC,
• Use a Dynisco model with built-in shunt calibration for periodic in-line re-zero, or
• Use a Siemens SITRANS P transmitter with HART compensation in the host system.

12.3 Zero point vs. atmosphere — A gauge-pressure Dynisco sensor reads atmospheric (≈ 1.013 bar absolute) at zero applied pressure, not 0 bar absolute. If the engineering range is to start at 0 bar gauge, perform the Set Zero step with the sensor vented to atmosphere. If the engineering range is absolute pressure, vent to vacuum or apply an offset in the Siwarex U characteristic value 1 (DBW 4 of DR 3 = 101).

12.4 Safety interlock on overpressure — Wire the Siwarex U Limit 1 and Limit 2 outputs (digital outputs on the front connector, pins 9 and 10) to a safety relay or a SIMATIC F-CPU via the ET 200M F-module. Do not derive the overpressure shutdown from a standard DI input that shares the rack with the Siwarex U. The 500 bar high-high trip should be hardwired through a separate, safety-rated chain (Pilz PNOZ, Sick FX3, or Siemens F-CPU).

12.5 Hazardous-area use — The Siwarex U is rated for Zone 2 / Class I Div 2 mounting in a cabinet. The Dynisco stem at the die is typically Zone 1. The 6-wire cable passes through Zone 1; an IS barrier (e.g. Pepperl+Fuchs KFD2-STC4-Ex1) is required between the sensor and the Siwarex U. Note that adding an IS barrier changes the excitation voltage at the sensor (typically drops 4 V across a zener barrier) and therefore the mV/V output. Calibrate with the barrier in place — calibrating the Siwarex U in a lab and then connecting through the barrier will introduce a 40 % gain error.

13. FAQ