Siemens SM 331 6ES7331-7KF02-0AB0: Mana and Comp+ Bridge Wiring

Module Identification and Scope

The 6ES7331-7KF02-0AB0 designation identifies a specific Siemens SIMATIC S7-300 analog input (AI) module. The Siemens article number (MLFB) decodes as follows:

For the official module specification, refer to the SIMATIC S7-300 Automation System Module Data manual on the Siemens Industry Online Support portal. The manual contains the complete pin assignment, measurement range tables, and diagnostic interrupt descriptions for the -7KF02- module.

Module characteristics relevant to the Mana/Comp+ bridge question:

• 8 differential analog inputs arranged in 4 groups of 2 channels
• Resolution: 13 bits plus sign (effective 14 bits including polarity on bipolar ranges)
• Configurable measurement: voltage (±80 mV, ±250 mV, ±500 mV, ±1 V, ±2.5 V, ±5 V, ±10 V, 1-5 V), current (±3.2 mA, ±10 mA, ±20 mA, 0-20 mA, 4-20 mA), RTD (Pt100, Pt200, Pt500, Pt1000, Ni100, Ni120, Ni200, Ni500, Ni1000, Cu10), resistance (0-150 Ω, 0-300 Ω, 0-600 Ω, 0-3 kΩ, 0-6 kΩ), and thermocouples (B, E, J, K, L, N, R, S, T, U)
• Galvanic isolation: 500 V AC between channels and the S7-300 backplane bus
• Update time: approximately 10 ms per channel group (channel-pair dependent)
• Front connector: 20-pin (6ES7392-1AJ00-0AA0 screw-type or 6ES7392-1BJ00-0AA0 spring-type)

Front Connector Terminal Layout

The 6ES7331-7KF02-0AB0 connects to field wiring through a 20-pin front connector. Terminal 10 (COMP+) and terminal 11 (MANA) sit adjacent to each other at the right side of the upper channel group, immediately before the start of the lower channel group. The complete terminal assignment for voltage and current modes is summarized below per the Siemens S7-300 module data manual:

Transducer Type Fundamentals

Before deciding whether the Mana (terminal 11) to Comp+ (terminal 10) bridge is necessary, the transducer interface must be classified. The SM 331 supports three families of field devices on its 4-20 mA current inputs:

2-Wire (Loop-Powered) Transmitters

A 2-wire transmitter derives its operating power from the 4-20 mA current loop itself. The transmitter has only two connection terminals; the loop current is both the power supply and the measurement signal. Typical examples: head-mounted temperature transmitters (Rosemount 644, WIKA T32), pressure transmitters, pH probes with integrated head transmitters. The PLC must source the loop current, typically 24 V DC at the transmitter terminals, dropping to 12-15 V at the transmitter after loop resistance and sense resistor losses.

3-Wire Transmitters

3-wire devices have separate power supply connections (often labeled V+ and GND, or Vs+ and GND) and a separate signal output. The signal output shares the same ground reference as the power supply but draws no significant current from the PLC input. The SM 331 does not provide 24 V for 3-wire devices on the COMP+ terminal; an external 24 V supply is required for the transmitter power input.

4-Wire (Self-Powered) Transmitters

4-wire transmitters have an independent mains-powered or separately-powered supply and provide a current output that is fully isolated from the supply. The PLC input only senses the 4-20 mA signal; it does not source loop power. Typical examples: laboratory-grade analyzers, mains-powered mass flow controllers (Bronkhorst, Endress+Hauser Promass).

The Mana/Comp+ Bridge Function Explained

The bridge between terminal 10 (COMP+) and terminal 11 (MANA) is a common source of confusion because its name (compensation) and its electrical function are often conflated with the loop supply function for 2-wire transmitters. In the 6ES7331-7KF02-0AB0, the COMP+ terminal is an internal 24 V supply that exists specifically to power an external cold-junction compensation (CJC) sensor when the analog input channels are configured for thermocouple measurement with external reference temperature.

For thermocouple inputs, the measured thermoelectric voltage is the difference between the hot junction (measuring point) and the cold junction (terminal block). To obtain an absolute temperature reading, the cold-junction temperature must be known and the equivalent thermoelectric voltage added. Two reference methods exist:

• Internal CJC: A temperature sensor mounted on the module's terminal connector reports the cold-junction temperature to the module firmware. Used when the thermocouple wires terminate directly at the SM 331 front connector and ambient temperature at the connector is well-controlled.
• External CJC: An external Pt100 (or other RTD) is mounted in an isothermal block at the field termination point, away from the module. The Pt100 is wired to the COMP+ and MANA terminals (and one channel input) so the module can read the cold-junction temperature and apply the compensation.

The bridge between terminal 10 and terminal 11 supplies the excitation current for that external Pt100. Without the bridge, the external CJC circuit has no reference and the module cannot measure the cold-junction temperature. This is the external compensation referenced in Siemens technical guidance for the -7KF02- module.

For all non-thermocouple applications (voltage, current, RTD), the bridge has no functional role in signal conditioning. Whether or not to install the bridge depends on whether the user intends to use the channel for thermocouple measurement with external CJC. The default wiring diagrams in the S7-300 module data manual show the bridge installed for thermocouple external CJC and absent for all other measurement modes.

Wiring Procedure: 2-Wire 4-20 mA Transmitter

A 2-wire loop-powered transmitter requires no Mana/Comp+ bridge. The wiring is direct: the transmitter is inserted in series with the channel input, and the SM 331 sources the 24 V loop supply internally.

1. Confirm the channel group measurement type in STEP 7 V5.x or TIA Portal: "2-wire current (4-20 mA)" or "2-wire current (0-20 mA)".
2. Connect the transmitter positive (+) lead to the channel M+ terminal (terminals 1, 3, 5, 7 for channels 0-3; terminals 12, 14, 16, 18 for channels 4-7).
3. Connect the transmitter negative (-) lead to the corresponding channel M- terminal (terminals 2, 4, 6, 8 or 13, 15, 17, 19).
4. Verify that COMP+ (terminal 10) and MANA (terminal 11) are not bridged unless external CJC is also being powered from the same front connector.
5. Tighten the front connector screws to 0.6-0.8 N·m (screw-type) or use the spring-release tool (spring-type).
6. Apply 24 V to the CPU / IM power supply and verify the input current reading matches the loop measurement.

Wiring Procedure: 4-Wire 4-20 mA Transmitter

A 4-wire transmitter is fully isolated from its own power supply. The SM 331 input only senses the current; it does not source any power for the loop. The Mana/Comp+ bridge is also not required.

1. Wire the transmitter's 4-20 mA output (+) to the channel M+ terminal.
2. Wire the transmitter's 4-20 mA output (-) to the channel M- terminal.
3. Provide 110/230 V AC or 24 V DC to the transmitter's separate power input terminals per the transmitter manual. Do not connect the transmitter power ground to MANA or to any PLC terminal.
4. Confirm in STEP 7 that the channel is configured for "4-wire current" or "current" measurement with the appropriate range (4-20 mA, 0-20 mA, ±20 mA).
5. Skip the Mana/Comp+ bridge.

For grounded 4-wire transmitters with non-isolated supply, the Siemens S7-300 module data manual explicitly notes that the interconnection between MANA and M- is not required. This means the channel M- terminal does not need to be tied to MANA inside the panel; the transmitter's signal return path is independent. If the transmitter output is grounded at the transmitter end, leave MANA floating (do not bond to PE) to avoid ground-loop currents that would inject common-mode noise into the analog reading.

Wiring Procedure: Thermocouple with External CJC

External CJC is the only scenario that requires the Mana/Comp+ bridge on a 6ES7331-7KF02-0AB0. The external CJC uses a Pt100 (or Pt1000, depending on module firmware and configuration) in an isothermal block at the field termination. The bridge supplies the excitation current for this Pt100.

1. Mount the Pt100 sensor in the isothermal terminal block at the thermocouple termination point.
2. Wire the Pt100 in 2-wire mode: one lead to COMP+ (terminal 10), the other lead to MANA (terminal 11).
3. Install the bridge (jumper wire) between terminal 10 and terminal 11 if the module's CJC firmware expects the external Pt100 excitation to be sourced from this path. Confirm by reviewing the module's HW configuration in STEP 7: if "Reference junction" is set to "External" and the channel measurement type is "Thermocouple", the bridge may be required per the firmware version.
4. Wire the thermocouple positive (+) lead to the channel M+ terminal and the negative (-) lead to the channel M- terminal, observing polarity per the thermocouple type (Type K: + is chromel, - is alumel; Type J: + is iron, - is constantan).
5. Configure the channel in STEP 7 / TIA Portal for the correct thermocouple type and "External" reference junction. The module will read the Pt100 resistance through the COMP+/MANA path and apply the cold-junction correction automatically.

STEP 7 / TIA Portal Hardware Configuration

For the -7KF02- module in STEP 7 V5.x or TIA Portal V13 and later, the channel measurement type is configured in the hardware catalog → SM 331 → AI8x13Bit → properties → Inputs → channel x → Measurement type. The relevant options for this bridge question are summarized in the table below:

For the hardware configuration in TIA Portal, navigate to:

1. Device view → select the SM 331 (6ES7331-7KF02-0AB0)
2. Properties → Inputs → Channel 0 (or relevant channel)
3. Measurement type → "Thermocouple" or "RTD" or "Voltage" or "Current"
4. If Thermocouple: set "Reference junction" to "Internal" (uses module sensor) or "External" (requires COMP+/MANA bridge + external Pt100)
5. Set the integration time / noise rejection (50 Hz, 60 Hz, 400 Hz) per the application; 50 Hz is the default for European mains environments
6. Compile the hardware configuration and download to the CPU

Diagnostic Verification and Field Checks

After wiring is complete and the module is configured, perform the following verification sequence before commissioning the loop:

1. Loop voltage check (2-wire mode): With the transmitter connected and powered, measure the DC voltage between the channel M+ and M- terminals. The voltage should be between 12 V and 22 V for a healthy 2-wire loop. If the voltage is below 10 V, the loop resistance is too high (check wire gauge and length) or the transmitter is current-limiting.
2. Loop current check (2-wire and 4-wire): Insert a precision ammeter in series with the loop (or use the transmitter's test terminals) and verify the current at known process conditions. For 4 mA = lower range value (LRV), 20 mA = upper range value (URV).
3. Channel value readout: In STEP 7 monitor the input word (e.g., IW x) for the channel. For 4-20 mA, 0 corresponds to 4 mA and 27648 corresponds to 20 mA, with overrange up to 32511 at 23.52 mA for Siemens S7 scaling.
4. CJC temperature verification (thermocouple mode): If external CJC is configured, monitor the channel's reference temperature reading in STEP 7 (under diagnostic data or via the diagnostic interrupt OB) and confirm it tracks ambient at the terminal block within ±1 °C.
5. Bridge voltage check: With external CJC in use, measure the DC voltage between terminal 10 (COMP+) and terminal 11 (MANA). Expect approximately 24 V DC; if the reading is 0 V, the bridge is open or the internal PTC fuse is open.

The Siemens S7-300 Module Data manual documents the diagnostic interrupt behavior for the -7KF02- module. Specific diagnostic events that fire on wiring faults include:

Common Wiring Errors and Troubleshooting

Error: Bridge installed on a 2-wire 4-20 mA loop

Symptom: Channel reads 7FFFH (overflow / error); diagnostic interrupt "configuration error" or "wire break" fires. Cause: Bridging COMP+ (24 V) directly to MANA (analog ground) creates a short-circuit on the internal 24 V compensation supply, triggering the module's overload protection or blowing the internal PTC fuse. Resolution: Remove the bridge. Wire the 2-wire transmitter between the channel M+ and M- terminals only, with no bridge.

Error: Bridge omitted on external CJC thermocouple

Symptom: Thermocouple channel reads erratically with temperature reading offset by the ambient temperature at the terminal block. Cause: The external Pt100 has no excitation current and the module cannot determine the cold-junction temperature. Resolution: Install the bridge between terminals 10 and 11, confirm the external Pt100 is wired correctly, and verify in STEP 7 that "Reference junction = External".

Error: Reading drift / ground loop on 4-wire transmitter

Symptom: Channel reading has 50/60 Hz common-mode noise or slow drift. Cause: The 4-wire transmitter output ground is bonded to PE at the transmitter while MANA is also bonded to PE at the PLC cabinet. The ground loop picks up noise. Resolution: Remove the PE bond at one end (typically at the PLC, leaving the transmitter end as the single ground reference). Use shielded twisted-pair cable with the shield grounded at the PLC end only.

Error: 2-wire loop reads 0 mA / 4 mA on power-up

Symptom: When the CPU starts, the input reads LRV (0% / 4 mA) regardless of process. Cause: The channel is configured for "Voltage" or "4-wire current" instead of "2-wire current". The internal 24 V loop supply is disabled. Resolution: Change the channel measurement type in STEP 7 to "2-wire current (4-20 mA)" or "2-wire current (0-20 mA)" and re-download the hardware configuration.

Error: Reading saturates at 20 mA on every channel

Symptom: All channels on the module read URV (20 mA / 100%) immediately after wiring. Cause: The 24 V COMP+ supply has been shorted, most commonly by an incorrectly placed bridge on a non-CJC application. The module's internal PTC fuse is now open. Resolution: Remove the short, power-cycle the module. If the fuse is permanently open (module does not recover), the SM 331 must be replaced (the PTC is not user-serviceable; Siemens part number for the replacement is the same 6ES7331-7KF02-0AB0).

Glossary of Terminal Functions

Frequently Asked Questions