Wiring Siemens SITRANS TK-L to Twido Analog Input Modules

## Overview This technical reference documents the field-proven procedure for connecting a Siemens SITRANS TK-L 2-wire RTD temperature transmitter to a Schneider Electric Twido analog input module. The TK-L is a loop-powered (2-wire) head-mount transmitter that draws its operating current directly from the 4-20 mA current loop. Because the device is not self-powered, the analog input channel must be part of a closed series loop supplied by a nominal 24 Vdc source capable of sustaining the loop current while absorbing the transmitter's compliance voltage drop and the receiver's input burden. The Twido controller family (TWDLCAA10DRF, TWDLCAE40DRF, TWDLCDA10DRF, and modular bases) accepts a range of analog input cartridges and expansion modules. The two modules most often deployed for 4-20 mA RTD loops are the TWDAMI2HT (2 isolated current inputs, 24 Vdc sensor supply) and the TWDAMI8HT (8 mixed voltage/current inputs with 24 Vdc module supply). The TWDAMI8HT is documented on the Schneider Electric product page as an analog input module with 24 V DC supply, 8 inputs, and ranges of 0-10 V or 0-20 mA. (Schneider Electric TWDAMI8HT product page) The Twido e-catalog states that the analog input current card requires an external 20.4-28.8 Vdc supply to power sensors and preactuators. This supply is independent of the Twido controller's internal logic power and must be provided by the panel builder. The 20.4-28.8 V range corresponds to the nominal 24 Vdc industrial supply rail with the standard +/- 20 % tolerance, allowing operation from a regulated 24 Vdc PSU or a battery-backed 24 Vdc UPS bus. This document covers device identification, the underlying loop theory, the field wiring procedure, TwidoSuite configuration, signal scaling, verification, and a field-tested troubleshooting matrix. It is written for control engineers, panel builders, and commissioning technicians who have basic familiarity with 4-20 mA current loops and Twido programming but are connecting a Siemens transmitter to a Twido AI module for the first time. ## Prerequisites

Item	Specification	Notes
Twido controller	TWDLCAA10DRF, TWDLCAE40DRF, TWDLCDA10DRF, or compatible	Verify analog module slot availability
Twido AI module	TWDAMI2HT, TWDAMI8HT, or equivalent current-input cartridge	Voltage-input modules will not read 4-20 mA
Siemens SITRANS TK-L	2-wire RTD temperature transmitter	Loop-powered, head-mount form factor
RTD probe	Pt100, Pt1000, Ni100, or Cu50 as configured in the TK-L	3-wire or 4-wire recommended for lead compensation
External 24 Vdc supply	20.4-28.8 Vdc regulated, current rating per loop count	Dedicated to analog loops; do not share with relay or VFD circuits
TwidoSuite	V2.x or later	TwidoSoft supported for legacy programs
Loop cable	2-conductor shielded, twisted pair, 18-22 AWG	Belden 8760, 8719, or equivalent; shield grounded at panel end only
Calibration source	Calibrated 4-20 mA loop calibrator or mA source	0.05 % accuracy or better
Documentation	Twido hardware reference, TK-L operating instructions	Available from manufacturer product support

## Identifying the Twido Analog Input Module Twido analog input modules fall into two categories. Voltage-input modules such as TWDAMU2HT accept 0-10 V signals. Current-input modules such as TWDAMI2HT and the multi-range TWDAMI8HT accept 0-20 mA or 4-20 mA. For any TK-L connection, the analog input module must be a current-input type. Attempting to read a 4-20 mA signal on a 0-10 V input will not produce a valid reading because the current is converted to a voltage across the channel's burden resistor; the channel will saturate or read near zero. The TWDAMI8HT product page lists 8 inputs spanning 0-10 V and 0-20 mA, with a 24 V DC module supply. (TWDAMI8HT) For 2-input configurations, TWDAMI2HT provides two isolated 4-20 mA / 0-20 mA inputs. Some Twido analog cartridges are integrated into the controller base (for example, the TWDLCAE40DRF base includes a built-in analog input cartridge); these are still subject to the same wiring rules and the same external 24 Vdc supply requirement. The Twido e-catalog states that the analog input current card requires an external 20.4-28.8 Vdc supply to power sensors and preactuators. The 24 V sensor supply terminal on the analog module is intended to source the loop, but the panel builder must provide the 24 Vdc rail. Do not backfeed the Twido's internal logic power into the analog loop, and do not share the analog 24 Vdc rail with inductive loads such as relay coils, contactors, or VFD braking resistors. ## SITRANS TK-L Transmitter Basics The SITRANS TK-L is a 2-wire loop-powered RTD temperature transmitter in a compact head-mount form factor (typically DIN B connection heads). According to the Siemens product announcement archive, the TK-L is described as a 2-wire RTD temperature transmitter. (Siemens press release) The transmitter accepts a 2-wire, 3-wire, or 4-wire RTD input (Pt100, Pt1000, Ni100, or Cu50, depending on the variant and configuration) and produces a 4-20 mA output proportional to the configured temperature range. Because the transmitter is loop-powered, the maximum loop load resistance is determined by the supply voltage, the transmitter's minimum terminal voltage, and the maximum loop current. The general loop budget equation is: R_L_max = (V_supply - V_TK-L_min) / I_max For a 24 V supply, an 11 V typical TK-L minimum compliance at 20 mA, and a 20 mA loop current, the budget is approximately (24 - 11) / 0.020 = 650 Ω. The Twido AI channel's input burden (typically 250 Ω for current-input channels) and the wire resistance must be subtracted from this budget. Confirm the exact transmitter compliance and Twido channel burden from each manufacturer's published datasheet before final design. The TK-L has only two connection terminals. There is no separate power input and signal output. This is the defining characteristic of a 2-wire device: the same two wires carry both the supply voltage and the modulated output current in series. The two terminals are typically labeled + and - on the SITRANS TK-L housing. A label inside the connection head shows the wiring diagram specific to the TK-L variant. ## Loop-Powered 2-Wire Transmitter Theory A 2-wire, 4-20 mA loop is a series circuit. The transmitter, the load, and the power supply are all in one current path. The transmitter regulates its internal current draw to a value between 4 and 20 mA proportional to the measured temperature. The receiver (PLC analog input) senses this current and converts it to a digital value. Three wiring rules apply: 1. The 24 Vdc source must have enough voltage to overcome the transmitter's minimum compliance AND the receiver's input impedance AND any series wiring resistance. Total voltage drop = V_TK-L + V_burden + V_wire. 2. Current flows in a single loop; there is no parallel return path. The same wire that supplies power to the transmitter also carries the signal. 3. Polarity matters. Reverse-polarity protection on the TK-L is documented in the Siemens datasheet, but on the Twido side the analog input polarity is fixed by the terminal block labeling. The 4 mA live-zero is a fault-detection feature. A reading of 4 mA represents the low end of the measured range (e.g., 0 °C if the TK-L is configured for 0-200 °C). A reading of 0 mA indicates a broken wire, a failed transmitter, or a wiring fault. A reading of 20 mA represents the high end of the measured range. A reading above 20 mA (typically up to 21 mA or 22 mA) may indicate an over-range condition or a sensor break in NAMUR NE43 high burnout mode. ## Wiring Procedure The field-proven wiring sequence for connecting a Siemens SITRANS TK-L to a Twido current analog input is:

1. Mount the TK-L inside the connection head of the RTD assembly. Wire the Pt100 (or other RTD) leads to the TK-L input terminals per the TK-L data sheet. Use 3-wire or 4-wire RTD connection to compensate for lead resistance.
2. Mount the Twido AI module on the Twido controller's expansion bus (or insert the analog cartridge, depending on the base model). Verify the module is firmly seated and the bus connector is engaged.
3. Provide an external 24 Vdc supply compliant with the Twido e-catalog's 20.4-28.8 V range. This supply may be sourced from the panel's 24 Vdc bus, but it must be dedicated to the analog loop and not shared with noisy loads such as relay coils, contactors, or VFD braking resistors.
4. Run a shielded 2-conductor cable from the panel to the field. Connect the cable shield to ground at the panel end only. The field end of the shield should be cut back and insulated to prevent ground loops. Maintain at least 200 mm separation from VFD power cables and 100 mm from 120/230 Vac control wiring.
5. At the transmitter, connect the +24 Vdc conductor to the TK-L (+) terminal.
6. Connect the TK-L (-) terminal to the Twido AI channel (+) terminal.
7. Connect the Twido AI channel (-) terminal to the 24 Vdc return (0 V).
8. Tighten terminal screws to the manufacturer's specified torque (typically 0.5-0.6 N·m for Twido analog terminal blocks). Use wire ferrules on stranded conductors to prevent stray strands from shorting adjacent terminals.
9. Apply power and verify the loop current with a clamp meter or by inserting a series mA meter. The reading should settle near 4 mA at ambient temperature when the TK-L is configured for a temperature range that includes ambient.

The resulting series loop is: 24 Vdc (+) → TK-L (+) → TK-L (-) → Twido AI (+) → Twido AI (-) → 24 Vdc (-). Current flows continuously through this path. The TK-L modulates the current between 4 mA (low scale) and 20 mA (high scale) to represent the measured temperature. For multi-channel modules such as TWDAMI8HT, each channel is a separate current loop. Each loop requires its own 2-wire run from the transmitter. The 24 Vdc return bus is shared, but the + side of each loop is dedicated to that channel. Never parallel two 4-20 mA transmitters on a single input channel; the currents will sum incorrectly and the input will read a meaningless value. ### Wiring Diagram ## Cable Selection and Routing Industrial 4-20 mA loops are low-energy signal circuits but they are susceptible to coupled noise from VFD power cables, switchgear, and welders. The cable used should be a twisted pair with an overall shield and a drain wire. Belden 8760 (1 pair, 18 AWG), Belden 8719 (1 pair, 24 AWG), or equivalent are typical choices. For runs exceeding 200 m, use a heavier gauge (18 AWG or 16 AWG) to keep the loop resistance well below the budget. Route analog cables in dedicated cable trays or conduit separated from power cables by at least 200 mm. Cross power cables at right angles if intersection is unavoidable. Do not run analog cables in the same conduit as 480 Vac motor feeders or VFD output cables. Ground the cable shield at the panel end only; the field end should be cut back, insulated with heat-shrink, and left floating to avoid ground loops. ## Configuring the Analog Input in TwidoSuite

1. Open the project in TwidoSuite and select the controller.
2. Navigate to the analog input module configuration table (right-click the module in the hardware tree and select Configure).
3. Set the input type to Current. If the module supports both 0-20 mA and 4-20 mA, select 4-20 mA for live-zero diagnostic.
4. Enable the channel and assign it a symbol name such as TIC_1001 (temperature indicating controller, loop 1001).
5. If the module offers input filtering, set the integration time to 50 ms or 100 ms to reject 50/60 Hz mains pickup.
6. Save the configuration and download it to the controller. Cycle power if required by the module firmware.
7. In the program, use the %IW (analog input word) address associated with the channel. For example, the first channel of the analog module attached to a TWDLCAA10DRF is typically %IW0.0, but the exact address depends on the controller base and the slot order. Confirm the address in the TwidoSuite I/O table.
8. Verify the raw count is incrementing as expected by monitoring the address in the TwidoSuite Animation Table editor.

If the module supports per-channel burnout detection, enable it. The Twido AI module will flag an underrange condition (reading below 4 mA on a 4-20 mA channel) as an open-loop fault. Some module firmware versions drive the input word to a defined error value (e.g., -32768) on burnout; consult the module user manual for the exact behavior. ## Signal Scaling and Engineering Units The raw input word from a Twido AI module is a signed integer in the range supported by the module firmware. Older Twido modules return a 12-bit value in 0-4095 counts; newer modules return a 16-bit value in 0-65535 or bipolar -32768 to +32767. The exact format depends on the module. Consult the Twido analog module user manual for the format of the specific module in use. To convert a 4-20 mA reading to engineering units such as degrees Celsius: Temperature = ((raw - raw_at_4mA) / span) × (T_high - T_low) + T_low Where:

• raw = current input word
• raw_at_4mA = input word at 4 mA (typically 0 or 4 mA equivalent)
• span = input word at 20 mA minus input word at 4 mA
• T_high = high end of the TK-L configured temperature range
• T_low = low end of the TK-L configured temperature range

Worked example: TK-L configured for 0-200 °C, AI module returning 0-4095 counts with 4 mA = 0 counts, 20 mA = 4095 counts. At 100 °C: Temperature = ((2048 - 0) / 4095) × (200 - 0) + 0 = 0.5 × 200 = 100 °C At 50 °C: Temperature = ((1024 - 0) / 4095) × 200 = 50.0 °C A simple scaling routine in TwidoSuite structured text or ladder:

// Inputs: %IW0.0 = raw count (0-4095), %KW0 = T_low, %KW1 = T_high, %KW2 = raw_at_4mA, %KW3 = span
// Output: %MW100 = temperature in tenths of a degree (0-2000 for 0-200 °C)
LD %IW0.0
SUB %KW2
MUL %KW1
DIV %KW3
ADD %KW0
MUL 10
ST %MW100

For best results, use the built-in SCALE function block in TwidoSuite, which handles integer scaling with rounding. Configure the SCALE block with the raw input range (0-4095) and the engineering output range (0-2000 for 0-200 °C in tenths of a degree). For a temperature range that includes negative values (e.g., -50 to +150 °C), use the bipolar input range of the AI module if supported, and configure the SCALE block accordingly. Confirm bipolar support in the module datasheet; some Twido AI modules are unipolar only. ## Verification and Commissioning After wiring, perform the following verification before placing the loop in service:

1. Power up the Twido controller and the external 24 Vdc supply. Verify the AI module's status LED is solid green (no fault).
2. Disconnect the TK-L loop at the transmitter end. Insert a calibrated mA source in series with the loop. Inject 4.000 mA and verify the input reads the low-scale raw count. Inject 20.000 mA and verify the input reads the high-scale raw count. This validates the wiring, the Twido input module, and the scaling end-to-end.
3. Inject 12.000 mA (mid-scale) and verify the input reads the mid-scale raw count and the scaled engineering value matches the expected midpoint.
4. Reconnect the TK-L. Apply a known temperature to the RTD (e.g., ice bath at 0.0 °C, boiling water at 100 °C adjusted for altitude, or a dry-block calibrator). Verify the input tracks the actual RTD temperature within the tolerance of the TK-L and the RTD probe (typically +/- 0.3 °C for a Class A Pt100 plus the TK-L error of approximately +/- 0.1 °C).
5. Check for ground loops by measuring voltage between the cable shield and the 24 Vdc return at the field end with a high-impedance multimeter. The reading should be near zero (less than 1 V). A non-zero voltage indicates a ground loop that can introduce noise.
6. Monitor the loop over 10-15 minutes. Verify stability; typical 4-20 mA loops hold within +/- 0.01 mA of nominal if the supply is clean and the wiring is sound.
7. Document the verification readings (raw count, mA, engineering value, ambient temperature) on the loop sheet for future reference.

## Troubleshooting Matrix The following matrix lists common faults, their indicators, and the corrective action.

Symptom	Likely cause	Diagnostic	Corrective action
Loop reads 0 mA continuously (input word at minimum)	Open circuit in the loop	Measure voltage across the loop with a multimeter; expect 24 V supply minus TK-L drop minus burden	Check terminal tightness at TK-L and Twido; inspect cable for breaks; verify the 24 Vdc fuse
Loop reads 0 mA; supply voltage present at the transmitter	TK-L failed internally	Substitute a known-good TK-L or mA simulator	Replace TK-L; verify input range and RTD type match the configuration
Loop reads saturated high (>20 mA equivalent or input word at maximum)	TK-L output stage shorted or supply voltage too high	Measure actual loop current with a clamp meter; check supply voltage under load	Replace TK-L; verify supply is within 20.4-28.8 Vdc; check for parallel signal paths
Loop reads noisy or unstable (jitter more than +/- 0.05 mA)	Ground loop in the cable shield	Disconnect field-end shield and re-measure; check for multiple ground points	Ground shield at panel end only; isolate the field end with heat-shrink
Loop reads noisy only when VFD or large motor starts	Inductive coupling from power cables	Check cable segregation; measure induced voltage on the loop with oscilloscope	Increase physical separation from power cables; use double-shielded cable; install RC snubbers on the offending load
Loop reads negatively (input word at minimum, negative, or AI error flag)	Wiring reversed (AI+ and AI- swapped) or module set to voltage input	Check polarity at Twido terminal block; verify module configuration	Swap AI+ and AI- wires; reconfigure the channel for current input
Loop reads 4 mA regardless of actual temperature	RTD probe open or disconnected	Measure RTD resistance at the TK-L terminals with the transmitter de-energized	Check RTD wiring; replace RTD probe if open; verify TK-L burnout mode configuration
Loop reads 21 mA regardless of actual temperature	RTD probe shorted or TK-L configured for high burnout (NAMUR NE43 high)	Measure RTD resistance; check TK-L configuration per datasheet	Replace shorted RTD; reconfigure TK-L burnout mode if undesired
Input word reads -32768 or AI module fault LED on	Module not configured in TwidoSuite or module address conflict	Check TwidoSuite hardware tree; verify no duplicate addresses	Add the module to the configuration; assign correct address; download to controller
Loop reads correctly at the bench but drifts in the field	Temperature coefficient of the cable; poor RTD lead compensation	Measure loop current at ambient and at elevated temperature; compare to expected	Switch to 3-wire or 4-wire RTD connection; use higher-grade cable; recalibrate TK-L in situ

## Compatibility Notes for the TK-L and Twido AI Channel The Twido AI channel's input burden is part of the loop resistance. Typical Twido current-input channels present approximately 250 Ω to the loop. For a 24 V supply and an 11 V TK-L minimum compliance, the remaining budget for wire resistance is approximately: R_wire_max = (V_supply - V_TK-L_min - V_burden) / I_max R_wire_max = (24 - 11 - 5) / 0.020 = 400 Ω Where V_burden = I_max × R_burden = 0.020 × 250 = 5 V. This 400 Ω budget supports approximately 1500-2000 m of 18 AWG cable (about 21 Ω per 305 m / 1000 ft) or 600-800 m of 22 AWG cable. For longer runs, use a 30 Vdc supply, a low-burden Twido channel, or a HART isolator with a built-in 250 Ω loop resistor to reduce the burden on the channel. Always cross-check the exact compliance of the specific TK-L variant against the Twido channel burden before final design. The TK-L datasheet provides the supply voltage vs. output current curve; the Twido module datasheet provides the channel burden. The two values must sum to less than the available supply voltage at the maximum loop current. ## Safety and EMC Considerations - De-energize the panel and lock out the 24 Vdc supply before working on the analog loop. The 24 Vdc rail is normally safe (below the 50 Vdc extra-low-voltage threshold), but the 120/230 Vac supply upstream of the PSU is not. - Do not route analog loop cables in the same conduit as 480 Vac motor feeders or VFD output cables. The capacitive coupling can inject several volts of common-mode noise into the loop, which the Twido AI channel may interpret as a fluctuating reading. - Use shielded cable with the shield grounded at the panel end only. Multiple ground points create ground loops that inject 50/60 Hz hum into the loop. - For installations near VFDs or large motor starters, install a 4-20 mA signal isolator (such as a Phoenix Contact MINI MCR or equivalent) between the field and the Twido AI channel. The isolator breaks ground loops and provides a clean differential input to the Twido channel. - Verify that the panel's 24 Vdc PSU is rated for the total analog loop current. As a rule of thumb, size the PSU at 125 % of the connected analog load plus the Twido controller's logic current draw. ## Configuration Backup and Documentation Once the loop is verified, save the TwidoSuite project archive to a versioned location (e.g., a project server or the panel's document pocket). Print the loop sheet with the following minimum information:

• Loop tag (e.g., TT-1001, TIC-1001)
• Service description (e.g., Reactor outlet temperature)
• Twido controller tag and AI module slot
• AI module type (TWDAMI2HT, TWDAMI8HT, etc.)
• Channel number and PLC address (e.g., %IW0.0)
• Transmitter type (Siemens SITRANS TK-L) and serial number
• RTD type (Pt100, Pt1000, etc.) and configuration
• TK-L configured range (e.g., 0-200 °C)
• Calibration date and technician initials
• As-found and as-left mA readings at 25 °C (or other reference temperature)

This documentation is required for ISO 9001, IEC 61511 (functional safety), and most end-user quality systems. It is also invaluable for future troubleshooting. ## FAQ