Configuring SITRANS TW 7NG3242 for PT100 via HART

The SITRANS TW (order code 7NG3242) is a DIN-rail mounted, blind, 4-wire HART temperature transmitter that accepts a wide range of input types — resistance thermometers (Pt100, Pt1000, Ni100, Ni1000, Cu100), thermocouples (B, E, J, K, N, R, S, T, L, U), resistance (0–5 kΩ), millivolts (±1000 mV), and current/voltage. The factory default input is encoded in the model suffix; units shipped with suffix ...08 (as in 7NG3242-0AA08) are set to the -120 to +1000 mA DC current input at the factory. Re-tasking that hardware for a PT100 (RTD) measurement is a HART-based configuration change, not a wiring change. This article documents the hardware, software, and procedure required to commission the TW as a 2-, 3-, or 4-wire PT100 transmitter.

1. SITRANS TW Product Overview

The SITRANS TW is part of the Siemens process-instrumentation portfolio for DIN-rail mounting inside cabinets. Key physical and functional characteristics relevant to PT100 retrofit work:

Because the TW has neither display nor pushbuttons, every parameter — sensor type, range, units, damping, output curve, and write-protection state — is set digitally through HART commands, normally via Siemens PDM (Process Device Manager).

2. Decoding the 7NG3242 Model Number

The 7NG3242 model string is positional. The trailing suffix characters map to the factory configuration as follows:

The 8 digit is the operational concern. It does not mean the hardware cannot measure an RTD — the TW hardware supports it. It means the unit shipped pre-loaded with the mA input profile. Changing the profile is a HART reconfiguration task.

3. Prerequisites

Before reconfiguring a SITRANS TW for PT100, assemble the following hardware and software:

1. HART modem: A USB HART modem (e.g., Siemens 7MF4997-1AA, Mactek VIATOR Bluetooth, or Rosemount 375/475 field communicator). Communication runs at 1200 bps over the 4–20 mA loop.
2. Siemens PDM: Version 6.0 or later (PDM V9.x is current and supports SITRANS TW EDD revisions distributed with the catalog). The TW Electronic Device Description (EDD) must be installed in the PDM device catalog.
3. Power supply: 24 V DC regulated, 100 mA minimum, connected at terminals 13 and 14.
4. HART load resistor: 230 Ω minimum, 250 Ω typical (±0.1 %, ≥ 0.5 W). Required between terminals 5 and 6 so the average loop current is non-zero for Bell 202 modulation. A 650 Ω maximum applies; above 650 Ω the 4–20 mA loop no longer has headroom at 24 V.
5. Write-protect jumper: X9 must be in the open position to accept configuration writes. With X9 closed, the device rejects all HART write commands.
6. PT100 sensor: Class A or Class B per IEC 60751. Verify lead count (2-, 3-, or 4-wire) so you wire terminals 1–4 correctly on the TW.

Optional but recommended: a laptop with PDM and a verified EDD installation, a multimeter for loop-current verification, and a copy of the SITRANS TW operating manual for the page-90 HART connection instructions and page-92/93 output-jumper diagrams.

4. HART Hardware Connection

The TW exposes HART on a dedicated pair. Connect as follows:

1. Connect the 24 V DC supply positive lead to terminal 13 and the negative (return) to terminal 14.
2. Place the load resistor (230–650 Ω) between terminal 5 and terminal 6. The current output flows through this resistor. The HART modem leads clip across the resistor — never across the supply — so the modem sees the modulated current without shunting it.
3. Connect the HART modem input leads to terminal 7 and terminal 8. These are the dedicated HART communication terminals on the TW and are polarity-insensitive for the modem side.
4. Connect the PT100 sensor to terminals 1, 2, 3, and 4 as a function of lead count (see section 5).
5. Verify jumper X9 is open (write protection disabled). Closing X9 hard-locks the device against all HART writes.

The bench-test wiring is summarised below:

    +24 V DC Supply
         |
         +------- Terminal 13 (+ supply)
         |
         |       SITRANS TW 7NG3242
         |       +---- Terminal 7 (HART-A)
         |       |    +---- Terminal 8 (HART-B)
         |       |    |    +---- Terminal 5  ---+
         |       |    |    |                    |
         |       |    |    +---- Terminal 6  ---+--- 250 Ω load ---
         |       |    |                                  |
         |       |    |                            HART modem
         |       |    |                            (clips here)
         |       |    |                                  |
         |       |    +---- Terminal 4  --- PT100 lead (4W: I-; 3W: I-; 2W: link to terminal 3)
         |       |    +---- Terminal 3  --- PT100 lead (3W/4W: I+; 2W: link to terminal 4)
         |       |    +---- Terminal 2  --- PT100 lead (V- sense)
         |       |    +---- Terminal 1  --- PT100 lead (V+ sense)
         |
         +------- Terminal 14 (- supply return)

   Jumper X9: OPEN = write-enabled (required for configuration)
   Output mode jumper: 4–20 mA (default) per page 92/93 of manual

For voltage output (0/2–10 V), the load requirement between terminals 5 and 6 is > 10 000 Ω. The voltage output mode also requires the output jumper to be in the V position; confirm by reading the manual jumper diagram on page 92/93 before powering up.

5. PT100 Sensor Wiring (2-, 3-, and 4-Wire)

PT100 lead resistance is the dominant error source at 0.1 °C accuracy. The TW supports all three lead configurations. Use the configuration that matches the physical probe and the required accuracy:

3-wire is the most common industrial configuration and is the recommended default unless the probe is short. The TW compensates the third-wire error internally; the lead-resistance correction is active only when the device is configured for 3-wire RTD in PDM. Verify the selected wire count in the PDM menu after upload-from-device.

6. Configuration via Siemens PDM

Siemens PDM is the only widely-supported configuration tool with an EDD for the TW. The device library on the Siemens support portal hosts the EDD and the user manual. Launch PDM and proceed as follows:

1. Install the EDD: From the SITRANS TW support page, run the EDD installer. After installation, the TW appears in the PDM device catalog under Sensors > Temperature > SITRANS TW.
2. Open PDM and select the TW: In PDM, choose Device > Communication > Open or double-click the TW entry in the plant view.
3. Establish HART communication: Confirm the HART modem is across the 250 Ω load and that 24 V is on terminals 13/14. PDM performs a poll and identifies the TW by its HART long tag.
4. Upload from Device: This is the most important step. Click Upload from Device (or press F5) to pull the current live configuration into PDM. The on-screen matrix only shows live values after this step. Working from the suffix digit alone is a common error.
5. Set the sensor type: Navigate to the input configuration matrix. Set Sensor Type to Pt100 (IEC 60751). Set Connection to 3-wire (or 2-/4-wire to match the probe).
6. Set the range: Define the lower-range value (LRV) and upper-range value (URV) in the engineering units required, typically °C. For example, 0 °C = 4 mA and 200 °C = 20 mA.
7. Set damping: Default is 0 s. Increase to filter thermocouple-grade or noisy RTD installations; 1–5 s is typical.
8. Set the output: 4–20 mA linear is the default. Failure mode (low or high on sensor break) is also selectable.
9. Download to Device: With jumper X9 open, click Download to Device. PDM writes the new configuration. The TW confirms with a status byte change in the HART response.
10. Verify and upload again: Click Upload from Device to confirm the values were persisted. Compare against what you downloaded.

PDM does not display every HART variable as a live HART menu; the on-screen matrix is generated from the EDD and represents the TW's parameter set. Where the EDD does not show a parameter, use a generic HART command (Command 3 to read PV, Command 15 to read device variables, Command 35/45 for range) to interrogate the device directly.

7. Output Mode and Jumper Settings

The TW supports two output modes, hardware-selected by an internal jumper:

0–20 mA operation is non-trivial: HART modulation requires a non-zero DC average, and 0–20 mA has zero average when the loop is at 0 %. The TW manual (page 90) describes the additional steps required. For a standard PT100 retrofit, use 4–20 mA and avoid the 0–20 mA mode unless the controller mandates it.

8. Verification Procedure

After downloading the configuration, validate the loop end-to-end:

1. Loop current at LRV: Apply a calibrator resistance equal to the PT100 value at LRV (e.g., 100.00 Ω for 0.0 °C). Measure 4.000 mA ± 0.005 mA across the 250 Ω load with a precision multimeter in mA mode.
2. Loop current at URV: Apply the PT100 value at URV. Measure 20.000 mA ± 0.005 mA.
3. Linearity: Apply 25 %, 50 %, 75 % of span values. The current should track to within the TW's stated linearity (typically ± 0.05 % of span).
4. HART PV read-back: Read HART Command 3 (dynamic variable PV in engineering units). The PV in PDM/°C should match the applied resistance within ± 0.1 °C for a Class A Pt100.
5. Sensor-break detection: Disconnect one RTD lead. The output should drive to the configured failure-mode value (high or low) within 1 s.
6. Write protection: Close jumper X9. Attempt a HART write. The TW should respond with a write-protected error (HART response code 16). Reopen X9 if further changes are required.

9. Troubleshooting Matrix

10. Alternative: SITRANS TH100 for Dedicated PT100 Applications

If the application is a fixed PT100 measurement and the model-8 input type has caused repeated reconfiguration effort, consider the SITRANS TH100. The TH100 is designed exclusively for Pt100 resistance thermometers per IEC 60751 in 2-, 3-, or 4-wire connection. Key differentiators versus the TW:

The TH100 reduces commissioning steps for a single-sensor-type cabinet and is the preferred choice when a fleet of identical PT100 measurements is in scope. The TW remains the right pick when a mixed RTD/Tc/mA panel must share spares and a single configuration tool.

11. Spare-Parts and Sizing Notes

• Loop voltage budget: V_supply - (I_max × R_load) - V_TW. With 24 V, 250 Ω, and the TW's typical 11 V drop, the budget is 24 - (0.020 × 250) - 11 = 8 V of margin, well within spec.
• 250 Ω load is preferred over 230 Ω for HART because the extra headroom (0.02 A × 20 Ω = 0.4 V) tolerates supply sag during DC inrush.
• For HART multidrop polling (up to 15 devices on one pair), each TW must be assigned a unique short address via HART Command 6; the loop then operates at 4 mA constant per device.
• Spare units: stock a TW (not the TH100) as a universal spare when cabinets have a mix of PT100, thermocouple, and mA inputs. Stock the TH100 as a dedicated PT100 spare where input variety is fixed.

FAQ