Replacing SITRANS P DSIII PROFIBUS Electronics with 4-20mA

Overview

Siemens SITRANS P DSIII transmitters in the 7MF4xxx family are field-proven pressure and differential-pressure instruments used for gauge, absolute, and differential measurement in gases, vapors, and liquids. The same housing and sensor module are shared across communication variants; only the front-mounted electronics module (MLFB series 7MF4997) and the terminal board change between PROFIBUS PA, HART, and FOUNDATION Fieldbus. A common revamp task is to swap a PROFIBUS electronics insert for a 4-20 mA / HART insert when the host PLC is an S7 system without a PROFIBUS PA segment, or when the customer standardises on analogue loops.

This guide documents the conversion of three specific DSIII configurations:

• 7MF4997-1DK – PROFIBUS PA electronics currently fitted in a DSIII pressure transmitter.
• 7MF4997-1DN – related electronics variant on the same sensor module.
• 7MF4434-1BA02-1AF07 – DSIII differential-pressure transmitter used as a flow element, currently on PROFIBUS PA.

It also covers the cross-reference MLFB for the replacement 4-20 mA / HART electronics, the safe removal and refit procedure (chapters 7.4.2 and 7.4.4 of the DSIII manual), the terminal-board re-wiring, and the S7 STEP 7 / TIA Portal hardware configuration changes required after the swap. A migration note to the current SITRANS P320 / P420 family is included at the end for projects that prefer a full transmitter replacement over an electronics retrofit.

SITRANS P DSIII Electronics Family and MLFB Structure

The DSIII transmitter is a two-board instrument: the sensor module (strain-gauge or piezo-resistive cell plus analogue front end) and the electronics module (digital signal processor, communication, and display driver). The same sensor accepts any of the following electronics inserts:

Two physical differences determine which electronics can be dropped into a given housing:

1. Terminal board – PROFIBUS/FF boards have a 4-pin connector for the bus; HART boards have a 2-pin (+/-) screw terminal for the 4-20 mA loop. The boards are not pin-compatible.
2. Display connector – All variants use the same 4-pin keyed header on the electronics, so the existing display can be retained.

Identifying the Installed PROFIBUS Variant

Before ordering replacement electronics, read the rating plate on the electronics module (visible after removing the front cover) and the nameplate on the housing. Cross-check the 11-digit order code against the table below to confirm you are about to remove a PROFIBUS module and not, for example, a HART module that was mis-cabled at installation.

Required Replacement MLFBs

For each of the three configurations in the original revamp request, the following Siemens catalog numbers apply. The two DSIII pressure-transmitter variants and the one differential-pressure (flow) variant are listed side by side so the same spare-parts kit can be ordered for the whole replacement batch.

The base-code change on the 7MF4434 (suffix -1AB07 vs -1AF07) is what re-classifies the unit on the rating plate from PROFIBUS to HART. The 7MF4434 sensor body is unchanged; only the electronics insert and the terminal board are physically replaced. The Siemens catalog structure means you order the complete replacement transmitter as a new MLFB and the spare-parts electronics for the units you are revamping in the field.

Pre-Replacement Prerequisites

Capture the following information from the existing PROFIBUS device before powering it down. Once the PROFIBUS electronics is removed, the digital parameters stored in its non-volatile memory are lost.

1. Process variable range – the calibrated 0% and 100% engineering units (e.g. 0 to 16 bar, -50 to +50 mbar for d/p flow).
2. Tag and message – 32-character HART tag (long-tag is 8 bytes), 32-character message, descriptor, and date.
3. Damping – the time constant in seconds (typical 0 to 30 s for pressure, 0 to 100 s for flow).
4. Transfer function – linear, square-root (for d/p flow), or custom curve (10-point or 30-point linearisation table).
5. PROFIBUS address – only required if you later decide to restore PROFIBUS for any reason.
6. PROFIBUS slot configuration – which AI blocks are mapped, scaling, and units, from the GSD file (Siemens 0x0B46 or 0x9700 for DSIII PA).

Export these values from SIMATIC PDM (Device Library > SITRANS P DSIII) and keep a project-archived copy. If PDM is not available, walk the device display menu and write down the parameters by hand.

Electronics Replacement Procedure

Follow the steps below. They mirror chapter 7.4.2 ("Replacing the electronic modules") and chapter 7.4.4 ("Terminal board assembly removal and replacement") of the SITRANS P320/P420 technical reference and the equivalent DSIII operating instructions.

1. Isolate the loop, wait 5 minutes for any capacitor discharge, and open the electronics-side cover.
2. Loosen the two display clamps and lift the display module off the electronics. The display is a passive LCD with a 4-pin connector; it is not polarity-sensitive.
3. Disconnect the spring-cage or screw terminals on the terminal board: "+", "-", and (if PROFIBUS) the PA shield clamp.
4. Unscrew the two captive screws on the electronics module and pull it straight out of the sensor connector. Do not twist; the connector is keyed.
5. Unclip the terminal board from the housing by releasing the two plastic latches (chapter 7.4.4 of the manual). It lifts out as a complete sub-assembly.
6. Compare the new 4-20 mA terminal board (7MF4997-1AC00) against the old PROFIBUS board. Confirm the "+" / "-" silk-screen matches the loop polarity and that the cable gland entries are in the same position.
7. Insert the new terminal board until both latches click, route the loop cable through the cable gland, and land "+" / "-" on the new screw terminals (0.5 to 2.5 mm² solid or stranded, torque 0.6 Nm).
8. Plug the new HART electronics (7MF4997-1AB or 7MF4997-1DB) into the sensor connector and tighten the two captive screws (0.6 Nm).
9. Re-seat the display, close the cover, and torque the cover screw to 10 Nm with the O-ring seated in its groove.

Terminal Board Rewiring

On the old PROFIBUS terminal board the field cables landed on a four-position connector: PA+, PA-, SHIELD, and an optional SPARE. On the new HART board the field cables land on + and - only. The shield should now be terminated in the cable gland or on the dedicated shield clamp provided inside the housing, not on a dedicated PCB pad.

Loop supply: 11.5 to 45 V DC at the transmitter terminals for standard (non-Ex) operation, 11.5 to 30 V DC for Ex ia. The DSIII draws 3.6 to 22.5 mA, and the HART signal is superimposed as a 1 mA peak-to-peak 1200/2200 Hz FSK on the loop current.

S7 Hardware Reconfiguration

After the physical swap, the SIMATIC S7 project must be changed. The S7 no longer communicates with the transmitter over PROFIBUS DP/PA, so the PA slave object is deleted and an analogue input module is added to the ET 200M / ET 200SP station. The most common replacements are 6ES7 331-1KF02-0AB0 (SM 331 AI 8x13 bit, on ET 200M) or 6ES7 134-6TD00-0CA1 (AI 4xI 2-wire, on ET 200SP), both of which accept a 4-20 mA loop and pass the raw value to the S7 CPU as a 0-27648 integer.

STEP 7 V5.x

1. Open HW Config, delete the DP/PA slave that previously represented the DSIII.
2. Drag the new AI module into the same slot (or the slot next to the existing analogue cards).
3. Set the channel measurement type to "I (current)", 4-20 mA, 2-wire transducer. The wire-break diagnostic bit becomes active when the loop current drops below 3.6 mA.
4. Compile and download the hardware configuration.
5. Replace the old FC/FB that read PIW from the PA slave with a simple load of the new PIW (e.g. L IW 256 for slot 0 of an SM 331 in slot 4 of an IM 153).

TIA Portal

1. Open the device configuration of the ET 200SP / ET 200M station.
2. Remove the PROFIBUS PA device from the network view.
3. Insert the new AI module. In the module properties set the channel to "Current 4-20 mA 2-wire" and enable "Wire break" and "Overflow / underflow" diagnostics.
4. Map the input word to a tag (e.g. "DSIII_4434_Flow" : IW64). Scale the raw integer to engineering units in the user program with SCALE_X or a dedicated FC.

Typical scale block (SCL, TIA Portal V18)

// Inputs
i_rawWord  : INT;     // 0..27648 from AI module
i_lrvEU    : REAL := 0.0;   // engineering units at 4 mA
i_urvEU    : REAL := 100.0; // engineering units at 20 mA

// Returns the scaled engineering value
"DB_DSIII_Scale".rEUValue :=
    SCALE_X(MIN   := 0,
            VALUE := INT_TO_REAL(i_rawWord),
            MAX   := 27648.0) * (i_urvEU - i_lrvEU) + i_lrvEU;

HART / 4-20 mA Transmitter Parameterisation

Once the new electronics is powered, the transmitter is in the factory default state. The loop current will be 3.6 mA (low saturation) until the lower- and upper-range values are set. Use either the on-board keypad, a HART handheld (e.g. Siemens PDM, Emerson 475, ABB DHH805), or SIMATIC PDM over a HART modem on the loop.

Verification and Loop Check

Run the following checks before signing off the revamp. The four-point verification is intentionally redundant so that any failure pinpoints the layer at which the loop is broken: physical loop, transmitter analogue output, S7 wiring, or S7 application.

1. Loop continuity. Disconnect at the AI module, measure with a multimeter. Expect < 600 Ω loop resistance at 24 V and 20 mA; DSIII alone drops ~10 V, leaving ~14 V across the sense resistor and wiring.
2. Loop current at LRV. Apply the calibrated low pressure. Loop current should be 4.000 mA ± 0.005 mA. Read with a calibrated mA meter in series.
3. Loop current at URV. Apply the calibrated high pressure. Loop current should be 20.000 mA ± 0.005 mA.
4. S7 PIW value. Read the process input word from the AI module (e.g. IW 256 in a VAT or watch table). At 4 mA expect 0, at 20 mA expect 27648, with linear progression. With the SCALE_X block, expect the engineering unit value to track the calibration pressure.
5. Wire-break detection. Open the loop at the AI terminal. Within 5 s the AI module should set the wire-break diagnostic bit and the PIW should read 0 or 32767 (depending on the module). SIMATIC HMI or WinCC should colour the value red.

Migration Path: SITRANS P DSIII to P320 / P420

For plants that would rather replace the entire transmitter than retrofit electronics, the SITRANS P320 / P420 family is the current Siemens offering. The mechanical interface (process connection, mounting bracket, electrical housing) is dimensionally compatible with the DSIII, so a swap is often a one-to-one replacement in the same cut-out.

The P320 / P420 ships with HART 7 by default and supports the long-tag (32-character) required by EDD-based asset management tools. See the SITRANS P320/P420 technical reference for full MLFB structure, accuracy classes (up to 0.075 % of span for the P420 premium line), and Ex approvals. For new installations, specifying P320 / P420 from the start avoids the PROFIBUS-to-analogue conversion entirely; the spare-parts stock of 7MF4997-1AB / -1DB remains the correct path for the revamp of an existing DSIII fleet.

Troubleshooting Matrix

Spare-Parts and Documentation References

• SITRANS P DSIII operating instructions: chapter 7.4.2 (electronic module replacement) and 7.4.4 (terminal board assembly replacement) – the SITRANS P320/P420 technical reference PDF covers the equivalent procedure and the cross-reference table for spare electronics.
• SITRANS P320 / P420 product page – current-generation pressure transmitter family, mechanical fit compatible with DSIII cut-outs.
• Siemens HVAC pressure / flow sensor portfolio – reference for the wider transmitter / switch / sensor family used for air-side and water-side measurements.
• SIMATIC PDM Device Library for SITRANS P DSIII – required to back up the PROFIBUS parameters (PV range, damping, transfer function, tag, message) before removing the old electronics.