Overview
When an ET200M distributed I/O station sits 30 m from the main control cabinet, the power supply topology is no longer a parts-list detail. Cable impedance, sensor leakage, ground loops, and EMC all change behavior at that distance. This reference walks through how to design the two 24 V rails (backplane electronics and field-side sensor supply) for an ET200M station built on an IM 153-2 interface module with four SM 321 DI32 digital input modules.
The guidance below also applies to mixed configurations (DI/DO/AI on the same station); the example with four DI32 modules is the smallest useful case that exercises both 24 V rails and the PROFIBUS DP commissioning sequence.
ET200M System Architecture
ET200M is a modular distributed I/O system that mounts standard S7-300 signal modules on a passive backplane bus. The station consists of:
- An Interface Module (IM 153-x) that terminates the fieldbus and generates the internal 5 V backplane rail.
- Passive bus modules on a DIN rail that carry the backplane and the mechanical slots for SMs.
- Up to 8 or 12 Signal Modules (SM) snapped onto the bus modules.
- Optional Function Modules (FM) and Communication Processors (CP).
The IM 153-2 (6ES7153-2BA10-0XB0) is the workhorse PROFIBUS DP interface with full diagnostics and time-stamping support. The PROFINET variant is the IM 153-4 PN (6ES7153-4BA00-0XB0). For a 30 m cabinet-to-station link running 1.5 Mbps, the IM 153-2 is the standard choice.
Why ET200M Needs a Dedicated 24 V Supply
The IM 153-2 accepts 24 VDC at its top-mounted screw terminals (1L+/1M and 2L+/2M) and internally generates the 5 V backplane rail that powers all SMs. If this 24 V is shared with contactor coils, VFD brake outputs, or solenoid valves, transients couple directly into the backplane and into the digital/analog input thresholds. Symptoms seen on the HMI or in the STEP 7 diagnostic buffer are classic: sporadic I/O faults, sensor wire-break alarms that clear on their own, or PROFIBUS diagnostic interrupts with cause "external fault."
The Siemens operating instructions for the IM 153-2 call out the requirement to switch on the power supply at the ET 200M as a separate step before commissioning the fieldbus, and to observe the SF (group fault), BF (bus fault), and ON LEDs. The implication is that 24 V must be present and stable before the DP master attempts to exchange I/O data.
Reference: IM 153-2 Interface Module - ET 200M Operating Instructions (Siemens)
Two Power Rails, Not One
A correctly wired ET200M has two electrically isolated 24 V rails entering the IM:
| Rail | Function | Typical Source | Isolation |
|---|---|---|---|
| 24 V backplane rail (1L+ / 1M) | Powers IM logic, 5 V backplane, sensor reference for DI groups | PS 307 24 V / 2 A or SITOP smart 5 A | 1L+ to PE isolated, separated from 2L+ |
| 24 V load rail (2L+ / 2M) | Field-side sensor / load power, separate ground reference | SITOP smart or PSU with signal output | 1L+/1M isolated from 2L+/2M |
Both rails are nominally 24 V, but they must NOT be bonded together at the ET200M. The 1M and 2M terminals on the IM remain on separate DIN-rail terminal blocks. This is the single most important wiring rule: do not jumper 1M and 2M on the IM, and do not share the same PS output terminal block. Most field EMC problems on ET200M come from this one rule being violated.
Sizing the Backplane Supply (Rail 1L+)
The backplane rail only feeds the IM electronics and the 5 V draw of each SM. The math is straightforward.
Step 1: IM 153-2 24 V Current Draw
The IM 153-2 draws approximately 150 mA at 24 V for its own logic (PROFIBUS interface, backplane DC/DC converter, diagnostic controller). Refer to the IM 153-2 data sheet for the exact figure of the specific order number in use.
Step 2: SM 321 DI32 Backplane Current
The 32-channel digital input module 6ES7321-1BL00-0AA0 (and successor 6ES7321-1BL01-0AA0) draws up to 15 mA from the 5 V backplane. Four modules therefore draw:
- 4 × 15 mA = 60 mA at 5 V
- Translated back through the IM's DC-DC (typical efficiency ~85%): about 70-80 mA at 24 V
The IM 153-2 backplane converter delivers up to 1.5 A at 5 V. Four DI32 modules use only 4% of that budget, so headroom is not a concern.
Step 3: Total Backplane Load
| Load | Current at 24 V |
|---|---|
| IM 153-2 logic | ~150 mA |
| 4 × SM 321 DI32 (backplane portion) | ~80 mA |
| 20% margin | ~50 mA |
| Total | ~280 mA |
A Siemens PS 307 (6ES7307-1BA01-0AA0) rated 24 V / 2 A is comfortably oversized. In practice most ET200M panels use a 5 A or 10 A SITOP because the same rail also feeds the panel fan, the HMI backlight inverter, and the cabinet heater. The PS 307 S7-300 form factor is convenient because it latches onto the same DIN rail mounting footprint; it is not required.
Sizing the Field Supply (Rail 2L+)
This is the rail that powers the actual field devices connected to the DI32 channels. The SM 321 DI32 has two galvanically isolated groups of 16 inputs, each with its own 24 V / 0 V terminal pair. Each input is rated for 7 mA typical at 24 V.
Worst-Case Sensor Current
- 32 channels × 7 mA = 224 mA per module
- 4 modules × 224 mA = 896 mA continuous
Add the following to size the supply correctly:
- Sensor inrush (typical proximity switches): up to 2× for 10 ms per sensor, but non-coincident in practice; budget 1.5× for the panel as a whole.
- Cable loss at 30 m with 1.5 mm² Cu (~0.013 Ω/m loop): 0.78 Ω × 0.9 A ≈ 0.7 V drop. Acceptable for 24 V sensor supply.
- Margin for future expansion (spare channels, added SM modules): 30%.
Recommended field supply: 24 V / 2.5 A minimum, 24 V / 5 A preferred. A SITOP smart 6EP1334-3BA10 (24 V / 5 A) is a typical choice. Its relay signal output (24 V OK contact) should be wired to a digital input on the IM (or to a dedicated input module in the master PLC) to provide "field supply OK" diagnostics on the HMI.
Cable Run at 30 m: Engineering Rules
| Concern | Rule of Thumb |
|---|---|
| Voltage drop on 24 V backplane cable | Keep below 3% at full load. 1.5 mm² is fine for 0.3 A. For 2 A load, use 2.5 mm² or 4 mm². |
| Voltage drop on 24 V field cable | Keep below 5%. With 0.9 A on 30 m of 1.5 mm², drop is ~0.7 V - acceptable. |
| PROFIBUS DP cable | 6XV1830-0EH10 (Siemens purple) or Belden 3079A. Max 1.5 m stub to each station; max 1,000 m at 1.5 Mbps. |
| Cable segregation | Keep 24 V power, PROFIBUS, and signal cables in separate trays, ≥ 200 mm apart, crossing at 90°. |
| Shield grounding | Bond PROFIBUS shields at both ends if equipotential bonding is solid; otherwise single-ended at the cabinet. |
IM 153-2 Terminal Pinout
The IM 153-2 has four 4-pole screw terminals on top. The pinout, looking at the front of the module, is:
| Terminal | Signal | Function |
|---|---|---|
| 1L+ | +24 V backplane | Powers IM and 5 V backplane converter |
| 1M | 0 V backplane | Return for 1L+ |
| 2L+ | +24 V field supply | Sensor / load supply reference for SM groups |
| 2M | 0 V field supply | Return for 2L+ |
Important: the 1L+/1M and 2L+/2M terminals are bridged internally and feed downstream SM groups. If each SM group needs its own fused branch, use external terminal blocks and bring each 1L+/1M and 2L+/2M pair back to the PSU through a dedicated 2 A or 4 A miniature circuit breaker.
Commissioning Sequence
- Verify the PROFIBUS address is set correctly on the IM 153-2 (DIP switches 1-7, range 1-125; switch 8 = ON enables software addressing via STEP 7 / TIA Portal).
- Apply 24 V to the backplane rail (1L+/1M) first. The IM 153-2 ON LED should illuminate solid green.
- Apply 24 V to the field supply rail (2L+/2M). Each SM 321 should show a solid green status LED.
- Connect the PROFIBUS cable. The BF LED on the IM will flash red until the DP master (e.g., S7-315-2 DP, ET200S master, or CPU 1515-2 PN with PROFIBUS CM) comes online.
- Power up the DP master. The BF LED should extinguish; the SF LED should extinguish; all module status LEDs should show solid green.
- In STEP 7 / TIA Portal, open the online diagnostics view of the IM 153-2 and verify the diagnostic buffer shows "no faults," no "station failure," and no "module removed / inserted" events.
Reference for steps 2-4: IM 153-2 Operating Instructions, "Switching on ET 200M" procedure
Verification Checklist
| Check | Expected Result |
|---|---|
| Voltage at IM terminals 1L+/1M | 24 V ±5% (20.4 V - 26.4 V) under load |
| Voltage at IM terminals 2L+/2M | 24 V ±5% under load |
| Voltage at far-end sensor terminals | ≥ 20.4 V with all inputs active |
| IM 153-2 ON LED | Solid green |
| IM 153-2 BF LED | Off after master connection |
| IM 153-2 SF LED | Off |
| Each SM 321 ON / STATUS LED | Solid green |
| STEP 7 online diagnostics | "No faults" for station and each module |
| Field supply OK signal contact | Closed = supply OK |
Troubleshooting Matrix
| Symptom | Likely Cause | Action |
|---|---|---|
| IM 153-2 ON LED off | No 24 V on 1L+/1M, or polarity reversed | Measure 1L+/1M with multimeter. Verify terminal assignment against the wiring diagram. |
| IM 153-2 BF LED flashing red | No PROFIBUS communication with master | Check DP address, bus terminator (ON at both ends only), cable continuity, and that the master is in RUN. |
| IM 153-2 SF LED on, BF off | Configuration mismatch or faulty SM | Read the IM diagnostic buffer; check that the configured module list matches the physically installed SMs. |
| Inputs read "0" sporadically on one DI32 | Field supply 2L+ missing or low | Measure voltage at the SM 321 sensor terminals with all inputs active. Should be ≥ 20.4 V. |
| Inputs read wire-break on an input that has a sensor | Sensor supply polarity reversed at the field device, or cable break | Check the sensor wiring; verify the input group jumper (terminals 1L+ and 2L+) is fitted on the DI32 if you are using the internal sensor supply. |
| All modules drop off intermittently | Shared 1M / 2M jumper, ground loop | Remove any jumper between 1M and 2M. Confirm separate PSUs for backplane and field supply. |
Frequently Asked Questions
Can I power the ET200M and the field sensors from the same 24 V PSU?
Technically yes if the PSU is sized for both and the wiring segregates 1L+/1M from 2L+/2M inside the station, but Siemens best practice and most field-proven panels keep the backplane rail and the field rail on separate PSUs. This isolates sensor transients and ground loops from the backplane 5 V converter and prevents stray bus faults.
What size PSU do I need for an IM 153-2 with four SM 321 DI32 modules?
Backplane rail: about 0.3 A continuous. A Siemens PS 307 6ES7307-1BA01-0AA0 (24 V / 2 A) is sufficient. Field rail: 0.9 A continuous for 128 active sensor inputs, plus 30% margin and inrush headroom. A 24 V / 5 A supply such as SITOP 6EP1334-3BA10 is recommended.
How many SM 321 DI32 modules can an IM 153-2 support?
Up to 12 SMs in a single ET200M station, limited by the IM 153-2 backplane current budget and the physical bus module length (830 mm). Four DI32 modules use only 60 mA of the available 1.5 A backplane budget and are well within capacity.
Do I need a separate PSU for outputs as well as inputs?
If SM 322 DO32 modules are added, the load rail for actuators (valves, contactors, indicators) should be a third 24 V rail with its own breaker, because actuator inrush can be 10-20x the holding current. The IM 153-2 2L+/2M terminals are typically reserved for sensor / auxiliary supplies, not heavy actuator loads.
What gauge 24 V cable should I run for a 30 m ET200M link?
For 0.3 A backplane load, 1.5 mm² is fine. For 2 A field supply, use 2.5 mm² to keep voltage drop below 3%. If the field load exceeds 3 A, step up to 4 mm² or move the PSU into the ET200M sub-panel.