SM 331 6ES7331-7KF02-0AB0 Placement in S7-300 Racks with IM365: Backplane Power Budget Reference

The SIMATIC S7-300 analog input module SM 331; AI 8 x 12 bit; (6ES7331-7KF02-0AB0) can be installed in the central rack (CR) alongside the CPU 314, in an expansion rack (ER) connected through an IM 365 interface module, or in a distributed ET 200M station on PROFIBUS or PROFINET. The mechanical slot is unrestricted, but the 5 V backplane power budget of the host rack must be satisfied before the module is wired and parameterized. This reference consolidates the Siemens hardware manual data, derives the permissible load for each rack, and walks through the verification procedure used in STEP 7 / TIA Portal commissioning.

1. Module Identification and Variants

The order number 6ES7331-7KF02-0AB0 identifies the SM 331 analog input module in the 8-channel, 12-bit resolution family. The trailing index -0AB0 is the Siemens MLFB release code; later indices of the same family share the same backplane current consumption but may add diagnostic interrupts or HART support. For rack-planning purposes, treat all 7KF0x variants the same.

The 50 mA figure for the 7KF02 is sourced from the Siemens S7-300 Module Data Manual and the S7-300 Isolated Analog Input SM 331 product information (PDF). Always confirm the value against the manual revision shipped with the specific firmware/hardware combination on the customer site.

2. Physical Slot Rules for the SM 331 in an S7-300 System

The SM 331 conforms to the S7-300 single-width module form factor and accepts any slot that the host rack exposes. There is no slot-number dependency and no firmware handshake that ties the analog input to a specific position. The valid hosts for a 7KF02 are:

1. Central rack (CR / rack 0) — the rack holding the CPU 314. Slots 4 through 11 are available for signal modules. Slot 3 is reserved for the IM 365 receiver if used; slots 1 and 2 are power supply and CPU.
2. Expansion rack (ER / rack 1) — a second S7-300 baseplate connected by an IM 365 pair. The expansion rack is limited to two slots populated in practice because the IM 365 interconnect is a fixed 1 m cable and the backplane drive is deliberately limited.
3. ET 200M on PROFIBUS DP — the IM 153-x head module replicates the S7-300 I/O bus to a remote station. The SM 331 plugs into the ET 200M just as it would in a local rack.
4. ET 200M on PROFINET IO — the IM 153-4 PN head module provides the same backplane over PROFINET. Useful when the CPU 314 has no PROFIBUS master port and the cabinet layout demands a remote station.

3. IM 365 Backplane Power Budget — Authoritative Numbers

The interface module pair IM 365 (6ES7365-0BA01-0AA0) is the lowest-cost way to add a second rack to a CPU 314. The send and receive modules are ordered together; the interconnect cable is captive at 1 m. From the S7-300 Installation Manual and the Module Data reference, the following limits govern the 5 V backplane bus:

The recommended 0.8 A figure reflects the continuous load that the IM 365 send module was characterized for. The absolute maximum of 1.2 A is the protective trip of the on-board 5 V regulator — sustained operation at that level risks thermal shutdown of the send module. Field practice is to budget to 0.8 A and treat 1.2 A as a hard, never-crossed ceiling.

3.1 Power budget formula

For a single S7-300 rack (CR or ER), the 5 V backplane current sum is:

I_backplane(5V) = SUM( I_module[5V] )    [A]

Validity conditions:

• For an ER behind an IM 365: I_backplane(ER) ≤ 0.8 A (recommended) and ≤ 1.2 A (absolute).
• For the CR when an IM 365 is fitted: I_backplane(CR) + I_IM365_send(100 mA) ≤ PS_307_5V_supply, and combined with the ER not exceeding 1.2 A.
• For a CR without IM 365: the budget is the 5 A rating of the PS 307 minus all module consumption.

4. Worked Power-Budget Example for a CPU 314 + IM 365 + SM 331 System

The example below is typical of a small machine retrofit. The objective is to validate the SM 331 placement choice using only published 5 V backplane currents.

Central rack sum (excluding the SM 331 in slot 4 because the SM 331 is the module being relocated):

I_CR(5V) = 700 (CPU 314) + 100 (IM 365 send) + 80 (SM 322)
        = 880 mA
I_CR_total_with_SM331 = 880 + 50 = 930 mA

Expansion rack sum:

I_ER(5V) = 50 (SM 331 7KF02) + 25 (SM 321)
        = 75 mA

Verification against limits:

• CR headroom: 930 mA < 5 A supplied by the PS 307 5A — PASS.
• ER budget: 75 mA ≤ 0.8 A recommended — PASS (the ER has 725 mA of unused capacity).
• Combined CR + ER: 930 + 75 = 1005 mA ≤ 1.2 A — PASS (195 mA of margin to the absolute limit).

This example is a textbook case: the SM 331 in the ER is comfortable. Re-run the same arithmetic if you populate the ER with a 7NF10 (100 mA) plus an SM 332 AO 8 (120 mA) and a CP 343-1 (140 mA) — the ER sum climbs to 360 mA, still inside the budget, but combined with the CR you start to approach the 1.2 A ceiling if the CR is already loaded.

5. SM 331 7KF02 Channel-Level Electrical Specification

The 7KF02 is an isolated 8-channel AI module with 12-bit resolution (plus sign). Channel configuration is per-channel in STEP 7 hardware configuration. Typical specs from the Siemens product information PDF:

6. Configuration in STEP 7 / TIA Portal

The 7KF02 is recognized automatically by STEP 7 V5.5 and by TIA Portal V13 SP1 and later, provided the Hardware Support Package (HSP) for the S7-300 is installed. Configuration procedure:

1. Open the S7 project in STEP 7 V5.5 or TIA Portal and load the hardware catalog SIMATIC S7-300 > SM 300 > AI-300 > AI 8x12 Bit (6ES7 331-7KF02-0AB0).
2. Drag the module to the destination slot. For an ER, the rack itself is added via Insert > Expansion Rack > IM 365; the IM 365 is automatically paired on the CR slot 3.
3. Open the module properties — Inputs tab — and select the measurement type per channel group (voltage, current, RTD). The default after a fresh insert is ±10 V, which is the lowest-risk selection for bench commissioning.
4. Enable the diagnostic interrupt if wire-break detection on a 4…20 mA loop is required. Wire break is supported in current modes and for PTC.
5. Assign the input addresses in the process image. The 7KF02 occupies 16 bytes (8 channels × 16-bit word). Default starting address is PIW 256 for the first SM 331 inserted in slot 4.
6. Compile the hardware configuration and download to the CPU 314. After restart (STOP → RUN), the analog values appear at the configured PIW addresses.

7. Verification Procedure After Mechanical Installation

Field verification ensures the backplane power budget is not exceeded in practice. Run the checks in the order listed so that a power-related fault is caught before the analog wiring is touched.

1. Power-supply output voltage check — With the CPU in STOP and the rack powered, measure 5 V between the backplane connector's +5 V and M terminals at the slot that will receive the SM 331. Acceptable range: 4.75 V to 5.25 V.
2. Module insertion and SF LED — Insert the SM 331 with power off; restore power and observe the SF (red) LED. The LED should extinguish within 2 s after the CPU performs the module identification. A persistent SF LED with the CPU in RUN indicates a parameter assignment error or an open thermocouple/RTD loop on a configured channel.
3. Backplane current measurement — Insert a calibrated clamp meter (DC, mA range) around the 5 V feed wire of the rack (not the PS 307 output — clamp around a single module feed if accessible) or read the value in STEP 7 via PLC > Module Information > Diagnostic Buffer. Confirm the live reading matches the static budget calculation within ±10 %.
4. Analog value plausibility — Apply a known reference (e.g. 10.000 V from a calibrator) to channel 0 and verify PIW 256 returns 27648 ± 4 LSB. If the value is 32767 (overflow) or 0, recheck the channel group measurement-type setting.
5. Diagnostic interrupt test — Disconnect the 4…20 mA loop on a configured channel and confirm an OB82 entry in the diagnostic buffer. The OB82 event timestamp and slot number should match the physical channel.

8. When to Choose ET 200M Instead of an IM 365 Expansion Rack

ET 200M is the correct choice when one or more of the following applies:

• The distance between the CPU 314 and the I/O exceeds 1 m (the captive length of the IM 365 cable).
• Multiple I/O clusters are required (the IM 365 only supports one expansion rack).
• The machine layout forces distributed I/O across several cabinets. Each ET 200M station receives a 5 V budget of its own through the IM 153 head module and is not subject to the 0.8 A / 1.2 A limit of the IM 365.
• Redundancy (S7-300 with CPU 315-2 DP / PN or higher) requires a switched DP/PN line to the I/O.

The CPU 314 has a single MPI/DP combined port; if PROFIBUS is already used for a drive or an HMI, the ET 200M must share that line. With a CPU 314C-2 PN/DP or a CPU 314C-2 DP the second port is available and a clean PROFINET line can be added.

9. Troubleshooting Matrix for the SM 331 + IM 365 Combination

10. Field-Proven Caveats

• Hot-swap of the SM 331 is not supported on the S7-300 local rack. Remove and insert modules only with the rack powered down. Hot-swap is supported only on ET 200M with the IM 153-2 / IM 153-4 in DPV0/DPV1 mode and only when the head module is configured for it.
• Channel-to-channel isolation is present, but channel group isolation is the practical limit: 2 channels share a common-mode return. Wire differential inputs within a group; do not assume channel-to-channel isolation between groups.
• If the 7KF02 is used in a 4-wire RTD configuration, the excitation current is internal; do not stack 4-wire RTDs from different modules onto a single cable run — common-mode noise will degrade the 12-bit result.
• The PS 307 5A is a 24 V-input supply. If the cabinet uses 120/230 V AC mains, the PS 307 5A (6ES7307-1EA01-0AA0) accepts both; the 2A variant (6ES7307-1BA01-0AA0) is 24 V DC input only.
• STEP 7 V5.4 SP3 and earlier do not include the 7KF02 in the default hardware catalog. Install the HSP0009 (or equivalent for the STEP 7 version) before configuring the module.

11. Related Documentation References

• S7-300 Isolated Analog Input SM 331; AI 8 x 16 Bit — product information (Siemens Support, PDF)
• SIMATIC S7-300 S7-300 Module Data Manual entry for 6ES7331-7KF02-0AB0 (Siemens Support, MDM)
• SIMATIC S7-300 Automation System — Hardware and Installation manual (printed 6ES7398-8FA10-8AA0, available on the Siemens Support portal under entry ID 15343415)
• STEP 7 V5.5 — Configuring Hardware and Communication Connections (Siemens Support entry ID 18652678)