S7-1500 PM vs PS Power Supply Configuration and Sizing Reference

Overview: Why Two Power Supplies on S7-1500

The SIMATIC S7-1500 splits module power into two independent domains to preserve backplane integrity when field faults occur. Field-side electronics (input filters, output drivers, sensor supply rails, actuator drivers) sit on a circuit that physically touches the wiring in the plant. Backplane-side electronics (logic ASICs, communication controllers, status LEDs, RUN/ERROR/MAINT indicators) sit on a circuit that only talks to the CPU across an isolating barrier. Optocouplers and isolated DC/DC converters enforce that barrier.

If an over-voltage, mis-wire, or short on the field side destroys an I/O module, the damage must not propagate into the backplane and the CPU. That isolation is the entire engineering reason an S7-1500 module has two distinct power input pins: one for the field-side load current and one for the backplane-side logic current.

The CPU alone cannot feed enough current through its connector to supply the backplane side of every possible expansion. Earlier SIMATIC families (S5, S7-300, S7-400) handled this by limiting module count per CPU. The S7-1500 introduces an external system power supply that augments or replaces the CPU's internal backplane feed, decoupling the available backplane budget from the CPU rating. See the S7-1500 / ET 200MP System Manual, "Power supply" section for the canonical specification.

Power Supply Type Definitions

S7-1500 uses two distinct supply concepts. The nomenclature is precise:

Load Power Supply (PM) — Module Power Supply

The Load Power Supply (PM in Siemens documentation; "Module Power Supply" in some translations) provides 24 V DC to the input and output circuits of every I/O module in the rack. It also feeds the sensor and actuator terminals wired to those modules. The PM is connected to the module's front terminal connector (X1/X2 plug) and is the source the field wiring ultimately references. The CPU and any PS module also receive their 24 V DC from a load source, normally the same PM.

If the PM fails or is switched off, the field-side electronics lose power. Inputs read 0, outputs drop, sensors and actuators de-energize. The backplane side may continue to run if a separate PS is present.

System Power Supply (PS)

The System Power Supply (PS) is a plug-in module on the S7-1500 DIN rail. It does not touch the field wiring. Its sole output is an internal system voltage bus that feeds the backplane electronics of the modules to its right (or, when installed to the left of the CPU, the CPU and modules to its right). The PS module has a green status LED and a diagnostics interface accessible from TIA Portal.

Three PS variants are catalogued for the S7-1500:

Slot Positioning Rules

The slot where a PS is installed fundamentally changes what it powers and how it interacts with the CPU. The S7-1500 backplane behaves like a daisy-chained bus; power can be injected at any of three logical positions.

Position A — No PS (CPU Standalone)

When no PS is present anywhere in the rack, the CPU alone must supply the backplane voltage to itself and to every I/O module in the rack. This works for small racks only. The CPU's backplane feed is bounded by its part number.

Position B — PS on the Left of the CPU

When the PS sits in the first slot to the left of the CPU, the PS becomes the master backplane source. The CPU draws its own backplane power from the PS (the CPU's internal feed is disabled in this topology), and every module to the right of the CPU is fed from the same PS bus. This is the standard topology for medium-sized racks.

Position C — PS on the Right of the CPU

When the PS is installed on the right side of the CPU, it starts a new, independent power segment. The CPU (or a PS on its left, if present) continues to feed everything on the CPU side; the new PS feeds every module to its right. Up to two PS modules may be placed on the right side of the CPU, creating two independent power segments.

This topology lets a single CPU feed a heterogeneous rack where some modules draw unusually high current (heavy CPs, F-modules, or technology modules) without forcing the entire rack onto a single PS that may be undersized.

CPU Standalone Power Budget

When no PS is installed, the CPU feeds the backplane from its internal converter. The available budget is small and depends on CPU part number:

PS on Left of CPU — Combined Power Budget

Installing a PS to the left of the CPU and setting the CPU property "Connection to supply voltage L+" to "No connection to supply voltage L+" disables the CPU's internal backplane feed. The PS becomes the master source for the entire rack. The available wattage scales with the PS rating:

The 1516-3 yields slightly less expansion wattage than the 1511/1513 because the 1516-3 draws more from the PS for its own higher-performance CPU core. The expansion column is what remains after the CPU's own consumption is subtracted from the PS rating.

Adding Power — PS (Left) + CPU Internal Feed

When the CPU property "Connection to supply voltage L+" is set to "Connection to supply voltage L+" while a PS is on the left, the CPU continues to feed the backplane from its own internal source in addition to the PS. The available backplane budget is the sum of the CPU's standalone budget (10 W for 1511/1513, 12 W for 1516-3) and the PS expansion contribution from the table above. TIA Portal exposes this combined figure in the device view when both sources are configured.

PS on Right of CPU — Independent Power Segment

Installing a PS to the right of the CPU creates a fresh power segment. The CPU (or a left-side PS) feeds everything to its left; the right-side PS feeds everything to its right. The two segments are isolated from each other on the backplane power rail.

A rack may use up to two PS modules on the right of the CPU. Each one starts its own segment. Common applications:

• Separating F-modules onto a dedicated PS so a fault in the standard segment cannot de-power the safety segment.
• Isolating high-current CPs (for example, CP 1543-1 with extensive routing load) onto their own PS.
• Splitting a long rack geographically so a single PS does not have to drive an excessive backplane length.

Sizing Calculation: Module Wattage Budget

To size the PS, sum the backplane consumption of every module in the segment, add 10% margin, and select the next PS rating above the sum.

Sample backplane consumption values (verify against each module's data sheet; values below are typical for catalogued S7-1500 modules):

For a representative rack of 8 DI + 4 DO + 2 AI + 2 AO + 1 CP, the backplane consumption is roughly 8(1.0) + 4(1.0) + 2(1.7) + 2(1.7) + 1(3.5) = 20.4 W. A PS 25 W (6ES7 505-0KA00-0AB0) on the left of the CPU is sufficient; a CPU 1516-3 standalone (12 W) is not.

Diagnostic Behavior on Insufficient System Power

If the projected (configured) rack demands more backplane power than the installed PS plus CPU feed can provide, the CPU enters a controlled fault sequence:

1. CPU detects power budget shortfall during startup or runtime.
2. CPU shuts down the backplane bus to all modules in the affected segment.
3. Diagnostic buffer records the event with timestamp and segment identifier.
4. CPU attempts to restart; if the shortfall persists, the cycle repeats.
5. STOP LED flashes (typically 2 Hz) to indicate "system power supply fault" or "configuration does not match".

Connect to the CPU with TIA Portal and read the diagnostic buffer for the specific event. Common entries include "Insufficient system voltage", "Module cannot be supplied", or "System power supply missing or faulty". Cross-reference the event against the S7-1500 System Manual diagnostics chapter. The official Siemens FAQ (Industry Online Support Entry ID 73191930) describes how STEP 7 (TIA Portal) verifies configuration consistency for S7-1500 system power supplies.

Other Conditions That Trigger the Same Fault

• PS is installed in TIA Portal hardware catalog but not physically mounted (or vice versa).
• PS is mounted but its 24 V DC input is missing or under-voltage.
• PS is mounted but set to a different rating in the device configuration than the physical model.
• Module order in the real rack does not match the device view in TIA Portal.

Configuration in TIA Portal

Wire the PS into the TIA Portal device view before assigning modules to its segment. The procedure is the same whether the PS sits left or right of the CPU.

1. Open the project and switch to Device View of the S7-1500 station.
2. Drag the correct PS from the hardware catalog (Hardware catalog → SIMATIC S7-1500 → Power supply) into the desired slot.
3. For a PS on the left of the CPU: open the CPU properties → "System power supply" and set "Connection to supply voltage L+" to "No connection to supply voltage L+".
4. For a PS on the right of the CPU: leave the CPU's "Connection to supply voltage L+" at its default (Connection to supply voltage L+).
5. Drag I/O modules into the rack. TIA Portal displays the wattage consumed and remaining in the bottom status bar of the device view.
6. Compile the hardware configuration and download to the CPU.

TIA Portal enforces consistency: if the configured modules draw more power than the segment can supply, the compile generates a warning and the online diagnostic block reports a configuration error before the CPU ever attempts to start the rack. The "SIMATIC S7-1500 Automation System" manual (Industry Online Support Entry ID 59191792) documents the full power-supply chapter.

Grounding and Reference Conductor Practice

The PM (load supply) and the PS (system supply) may be sourced from separate 24 V DC power supplies in the cabinet. When they are sourced separately, interconnect the M (0 V) terminal of both supplies at a single point and bond that point to cabinet ground. This protects against ground loops that can otherwise bias the isolation barrier and degrade analog accuracy or inject noise into communication modules.

Use one PE terminal per DIN rail segment, bonded to the cabinet backplane. Do not daisy-chain PE from one module to the next; star-ground each module's PE terminal to the common PE bus.

Cross-Reference: SIMATIC S7-1500 vs ET 200MP

The ET 200MP distributed I/O station uses the same PS and PM modules as the S7-1500 because it shares the same backplane interface. An IM 155-5 PN HF or IM 155-5 DP HF station can mount a PS 25 W or PS 60 W on the left of the interface module; the interface module's behavior is analogous to the CPU's. PS selection rules, segmenting rules, and diagnostic behavior are identical. See the S7-1500 / ET 200MP System Manual, "Power supply" section for the shared specification.

Comparison with Building Automation PS-24V

The PS-24V used in Schneider Electric's SmartX server family is a category-distinct device: it powers a SmartX AS-P or AS-B server and its connected I/O modules over an isolated RS-485 bus. It is not a backplane injector like the S7-1500 PS. Documented in the EcoStruxure Building Operation WebHelp, "PS-24V" topic, the PS-24V outputs 24 V DC at a fixed rating without the slot-positioning, segmenting, and CPU-budget logic that defines the S7-1500 PS family. Engineers migrating between platforms must not interchange these devices or assume their specifications map.

Replacement and Migration Notes

• PS 25 W (6ES7 505-0KA00-0AB0) is the most common PS in standard cabinets; specify it for racks drawing under 19.5 W expansion budget after CPU consumption.
• PS 60 W DC (6ES7 505-0RA00-0AB0) accepts 24, 48, or 60 V DC input; useful in substations or DC microgrid cabinets where 48 V or 60 V DC is the distribution voltage.
• PS 60 W AC (6ES7 507-0RA00-0AB0) accepts 120/230 V AC or DC; the standard choice for cabinets fed from a single-phase AC source.
• When migrating from S7-300 to S7-1500, retain the existing 24 V DC load supply and add a PS only if the new rack exceeds the CPU's standalone budget.
• When replacing a PS in a running rack, de-energize the entire rack first; hot-swap is not supported on the S7-1500 PS family.

Commissioning Verification Checklist

After wiring and configuring the PS, walk through this checklist before commissioning the I/O:

1. Verify the 24 V DC at the L+ terminal of the CPU and the PS with a calibrated multimeter; tolerance ±1% at the terminals.
2. Verify the polarity at every module's front connector before applying power.
3. Verify the PE-to-cabinet bond at every module.
4. Power up the PS first, then the CPU. Observe the green DC OK LED on the PS, then the RUN LED on the CPU.
5. In TIA Portal → Online & Diagnostics → Diagnostic buffer, confirm no "System power supply" entries.
6. In TIA Portal → Device View, hover each module and confirm the displayed wattage is within the segment budget.
7. Force a test input and confirm the corresponding process image updates in the watch table.

FAQ