S7-1200 Remote I/O Setup: SM 1278 vs ET 200S Profinet Stations

An S7-1200 CPU drives distributed I/O over its integrated PROFINET interface, but the field device that hangs on the cable must be matched to the signal type. The SM 1278 is a four-port IO-Link master, not a digital/analog backplane expander, and it does not accept standard SM 1200 signal modules. For cabinet-to-cabinet remote I/O carrying discrete and analog signals, the correct head modules are PROFINET devices from the ET 200 family or third-party PROFINET couplers such as the Phoenix Contact Axioline F series. This reference covers the supported architectures, the limits of the SM 1278, the TIA Portal commissioning procedure, and a BOM comparison sized for a 32-machine replication in a U.S. plant.

1. Problem Context: Replacing S5 PLCs With a Single S7-1200 and Remote I/O

Legacy S5 installations were typically paired in cabinets, one CPU per cabinet, with point-to-point wiring to local I/O. Modernizing to a single S7-1200 CPU plus a remote I/O drop in the second cabinet is a common cost-reduction strategy. The driver is straightforward: one program, one HMI project, one spare-parts pool, and a single Ethernet cable between cabinets instead of hundreds of conductors in a multi-conductor trunk.

Two conditions must be satisfied for the architecture to work:

1. The remote I/O device must speak a protocol the S7-1200 CPU supports natively (PROFINET IO is preferred, Modbus TCP is acceptable).
2. The remote I/O must accept the field signal types (24 V DC digital, 4-20 mA analog, RTD, thermocouple, etc.) the application requires.

The SM 1278 only satisfies the first condition for IO-Link sensor/actuator devices. It does not backplane a string of standard SM 1200 digital or analog modules. If the application is dominated by conventional 24 V DC discrete I/O and 4-20 mA loops, the SM 1278 is the wrong head module and an ET 200 PROFINET station is the correct selection.

2. S7-1200 Communication Interfaces Onboard

Every S7-1200 CPU in the 1211C through 1217C range ships with a PROFINET interface on the bottom of the module. The firmware version and CPU model determine the number of ports, switching capability, and supported protocols.

The two-port switch on the CPU 1215C and 1217C allows a linear daisy-chain topology without an external managed switch. For a single CPU driving one or two remote I/O drops, the CPU 1215C is usually the right pick because the second port can feed the remote I/O directly. CPU 1211C/1212C/1214C require an external switch between the CPU and remote I/O.

The S7-1200 PROFINET interface is configured as a PROFINET IO Controller only. It cannot operate as a PROFINET IO Device, which means the S7-1200 cannot be a remote I/O drop on another controller. If the architecture requires two CPUs cooperating, the only configuration is two independent S7-1200 IO Controllers exchanging data over PROFINET (or over PUT/GET, Modbus, or ISO-on-TCP).

For protocol specifications and supported connection counts, see the SIMATIC S7-1200 Programmable Controller System Manual entry on the Siemens Industry Online Support portal.

3. SM 1278 IO-Link Master: What It Does and Does Not Do

The SM 1278 (order number 6ES7278-4BD32-0XB0, firmware V2.0 and later) is a signal module (SM) that plugs into the right side of the S7-1200 CPU, in the same backplane position as any SM 1221 digital input, SM 1231 analog input, or SM 1222 digital output. It exposes four IO-Link ports compliant with the IO-Link V1.1 specification, single-channel per port, supporting IO-Link devices, standard digital actuators, and standard digital sensors.

The SM 1278 sits in the backplane like any other signal module. The signals it exchanges with the CPU are not 24 V DC inputs on an input card and not 4-20 mA on an analog card; the signals are IO-Link process data frames delivered to and from the four IO-Link ports. The CPU receives the process data as a 32-byte input slot and 32-byte output slot (slot 0 of the SM 1278 image) in the device configuration.

The module supports standard IO-Link IODD device descriptions and the S7-PCT (Port Configuration Tool) plug-in in TIA Portal. Each port can be configured for IO-Link mode, SIO (standard digital input) mode, or disabled. In SIO mode the port behaves as a simple 24 V DC discrete input and ignores the IO-Link handshake; this is useful for retrofitting an existing sensor that has no IO-Link variant.

Complete specifications, wiring diagrams, and IODD handling are documented in the S7-1200 SM 1278 IO-Link Master Manual.

3.1 The Critical Question: Can Standard SM 1200 Modules Be Attached to the SM 1278?

No. The SM 1278 is the same module class as SM 1221, SM 1222, SM 1231, and SM 1232. Each occupies one slot in the right-side signal-module string and terminates the backplane at that slot. The S7-1200 backplane does not branch or extend through a signal module. Additional SM 1200 modules are placed to the right of the SM 1278 (or of the CPU, if the SM 1278 is omitted at that slot) and not behind it.

If the second cabinet requires 32 digital inputs and 16 digital outputs of conventional 24 V DC signaling, the SM 1278 cannot carry that load. The SM 1278 only carries IO-Link-compatible field devices. A separate PROFINET head module (ET 200 or equivalent) in the second cabinet is the correct device.

4. ET 200 Family Remote I/O Options

Siemens offers three PROFINET-capable ET 200 platforms compatible with the S7-1200 as IO Controller: ET 200S, ET 200SP, and ET 200MP (the latter is sized for S7-1500 I/O and is usually overkill for a 32-point cabinet drop). For the typical S5-to-S7-1200 retrofit with budget pressure, the choice is between ET 200S and ET 200SP.

4.1 ET 200S (IM 151-x PN)

The ET 200S is a legacy modular distributed I/O system still in production and stocked. The PROFINET interface module is the IM 151-3 PN (order number 6ES7151-3BA60-0AB0) or IM 151-3 PN HF (6ES7151-3BB60-0AB0). The HF variant supports PROFINET IRT (isochronous real-time) for motion applications; the standard IM 151-3 PN supports PROFINET RT only, which is sufficient for non-motion digital and analog I/O.

The ET 200S uses PM-E (power module) and PM-D (power module with diagnostics) supplies to segment the 24 V DC load bus for digital outputs. Each output module typically needs its own PM-D to provide switched 24 V DC per channel, which raises the slot count and the BOM cost compared to ET 200SP. For a 32-point retrofit cabinet, expect 3-4 power modules in addition to the interface module.

The ET 200S is the right pick if the S5 retrofit must reuse an existing ET 200S installation, if spare parts are already on the shelf, or if the system has an unusual module the ET 200SP does not offer (e.g., certain fail-safe modules in legacy approvals).

4.2 ET 200SP (IM 155-6 PN)

The ET 200SP is the current-generation distributed I/O platform. The PROFINET interface module is the IM 155-6 PN BA (basic), IM 155-6 PN ST (standard), IM 155-6 PN HF (high feature), or IM 155-6 PN HS (high-speed). The ST variant (6ES7155-6AU00-0BN0) is the right pick for non-motion S7-1200 retrofits: it has two PROFINET ports (for daisy-chain or ring) and supports the full range of I/O modules.

ET 200SP uses a BaseUnit system: each I/O module snaps onto a BU (base unit) that carries the backplane and the 24 V DC supply. The first slot is always a BU with a CPU-side power feed; the last slot is a BU with an aux-feed or a dark BU to close the backplane. This design eliminates the discrete PM-E / PM-D power modules of the ET 200S and cuts the slot count roughly in half for the same I/O count.

Typical digital input module: DI 8x24VDC HF (6ES7131-6BF00-0BA0). Typical digital output module: DQ 8x24VDC/0.5A HF (6ES7132-6BF00-0BA0). Typical analog input: AI 4xU/I/RTD/TC ST (6ES7134-6HD00-0BA1).

Full module catalog, configuration rules, and PROFINET device description (GSD) installation are in the ET 200SP System Manual.

5. Phoenix Contact and Third-Party PROFINET I/O

Siemens PROFINET IO Controllers interoperate with any PROFINET IO Device that has a certified GSD file. Phoenix Contact is the most common third-party source in U.S. plants for several reasons: stocking is excellent, technical support is on par with Siemens for PROFINET commissioning, and the Axioline F platform offers a denser I/O count per millimeter of DIN rail than ET 200SP at slightly higher per-channel cost.

For a 32-replication retrofit, mix the platforms by slot only if the application demands a specific Phoenix Contact module (e.g., a certain intrinsically safe barrier not offered by Siemens). The TIA Portal installs Phoenix Contact GSD files through Options > Manage General Station Description (GSD) Files, after which the device appears in the hardware catalog under Other Field Devices > PROFINET IO > Phoenix Contact.

WAGO (750-375 fieldbus coupler for PROFINET) and Turck (TBEN-S PROFINET block I/O) are the next most common third-party options for the U.S. market. Both are GSD-certified and appear in TIA Portal identically.

6. Multi-CPU PROFINET Link vs Single CPU + Remote I/O

For a cost-driven retrofit, the architecture decision is binary:

For a budget-driven 32-replication project the single-CPU-plus-ET-200SP architecture is cheaper per site. The savings come from eliminating one S7-1200 CPU per pair (CPU 1215C with HMI capability is roughly $700-900 list, the IM 155-6 PN ST is roughly $300-400 list) and from halving the I/O count needing configuration in TIA Portal because the right cabinet simply becomes a PROFINET device under the left cabinet's IO Controller.

The cost premium of the two-CPU architecture is justified only when:

• Cabinet 2 must operate independently when cabinet 1 is powered down.
• Local commissioning (in a controlled environment with no CPU) is required for testing field wiring.
• Safety certification requires a redundant local stop circuit that cannot depend on the PROFINET link.

For a recovery-mode budget retrofit of 32 stations, these conditions are rarely all met. The single-CPU-with-remote-I/O architecture is the default.

7. Cost Comparison for a 32-Plant Replication

The example assumes one of the 32 machines has 32 digital inputs, 16 digital outputs, 8 analog inputs (4-20 mA), and 4 IO-Link sensors in the secondary cabinet. The primary cabinet has the HMI, the S7-1200 CPU, and the local I/O for the local cabinet. The two cabinets are linked by a single Cat 6 PROFINET cable.

The SM 1278 architecture is the cheapest at the BOM level, but only if the second cabinet's field devices are all IO-Link. If the second cabinet is conventional 24 V DC and 4-20 mA, the SM 1278 cannot host those devices and a PROFINET head module is required. In that case, the ET 200SP is the BOM winner.

These BOMs are list-price estimates for planning; negotiate with authorized Siemens distributors (in Cincinnati and the surrounding region, several stocking distributors compete) for the actual 32-unit run. Multi-unit and project pricing typically brings 20-35% off list for volume orders.

8. TIA Portal Configuration Procedure for S7-1200 + PROFINET Remote I/O

The configuration below adds an ET 200SP station as a PROFINET IO Device on the integrated PROFINET port of a CPU 1215C, in TIA Portal V18 or later. Phoenix Contact and other third-party devices follow the same flow with their GSD files installed first.

8.1 Prerequisites

• TIA Portal V18 (or V17 with the S7-1200 V4.6 HSP installed).
• S7-1200 CPU 1215C, firmware V4.6 (recommended minimum).
• IM 155-6 PN ST (or IM 151-3 PN, or third-party PROFINET coupler).
• 24 V DC power supply sized for the I/O count (1.5-3 A typical for a 32-point drop).
• Cat 6 PROFINET cable, RJ45, industrial grade (e.g., 6XV1870-3QN20), length under 100 m per segment.

8.2 Step-by-Step Hardware Configuration

1. Open the TIA Portal project containing the S7-1200 CPU. Confirm the CPU has been added with the correct order number and firmware version.
2. In the Project Tree, expand the CPU and double-click Device Configuration. The right pane shows the CPU and its signal-module slots.
3. Click the PROFINET interface on the CPU. In the Properties pane, set the IP address (e.g., 192.168.0.10), subnet mask (255.255.255.0), and the IO Controller role. The CPU is always the IO Controller; no role change is needed.
4. Right-click the PROFINET subnet line in the Device Configuration view and select Add PROFINET IO Device. Alternatively, drag the IM 155-6 PN ST from the Hardware Catalog (under Other Field Devices > PROFINET IO > Siemens AG > ET 200SP > Interface Modules > PROFINET).
5. On the IM 155-6 PN ST, assign IP address 192.168.0.11, subnet 255.255.255.0, and assign it to the CPU as its IO Controller.
6. Drag the I/O modules from the catalog into the slots of the IM 155-6 PN ST. The slot numbering is fixed: slot 1 = first I/O module, slot 2 = second, etc. Set addresses (e.g., IB 100 for inputs, QB 100 for outputs) in the Properties of each module.
7. Download the project to the CPU with the PROFINET network cable connected. The CPU assigns the configured name and IP to the IM 155-6 during the download; this is the PROFINET naming step, not just an IP push.
8. After download, open Online & Diagnostics on the IM 155-6 to verify the modules are recognized and the PROFINET status is "OK".

8.3 TIA Portal Configuration for the SM 1278 (IO-Link)

1. Add the SM 1278 to the right side of the CPU in the device configuration (slot 1 of the signal modules).
2. Open the S7-PCT (Port Configuration Tool) from the project tree, right-click the SM 1278, and launch the S7-PCT for this device.
3. For each of the four IO-Link ports, assign the IODD of the connected device. S7-PCT reads the IODD and exposes the process data and diagnostic data of the sensor.
4. Compile and download. The IO-Link process data appears as a contiguous data block in the CPU's I/O image; the IODD parameters are accessible via the Port Configuration Tool for runtime adjustment.

TIA Portal configuration and the PROFINET device replacement procedure are documented in the SIMATIC PROFINET with TIA Portal Commissioning Manual.

9. Hardware Sizing and I/O Count Planning

The S7-1200 imposes a hard limit on the number of I/O modules in the local backplane: eight signal modules per CPU, with the SM 1278 counting as one of those eight. The right-side I/O count plus the SM 1278 plus any communications modules (CM 1241, CP 1243) cannot exceed the eight-slot limit. Verify this on each replication site, because the original S5 design may have used more signal modules than fit under the new rule.

The S7-1200 PROFINET IO Controller supports up to 16 PROFINET IO Devices. The combined input process image limit is 1024 bytes; the combined output process image limit is 1024 bytes. For a single remote drop with 32 DI / 16 DO / 8 AI, the byte count is trivial: 4 bytes in + 2 bytes out + 16 bytes AI = well under 1024.

For longer distances between cabinets, use PROFINET fiber-optic media converters (e.g., Siemens SCALANCE XF204-2BA) or specify the ET 200SP with an FO interface module. The S7-1200 PROFINET port is copper-only; for fiber from the CPU, add a SCALANCE XC206-2SFP between the CPU and the remote drop.

10. Verification and Commissioning Checklist

After hardware install and TIA Portal download, run the following checks before the machine is released to production. Each item has a pass/fail criterion and a method.

11. Choosing the Right Architecture for the 32-Site Roll-Out

The decision matrix is the following:

• If the second cabinet is dominated by IO-Link sensors and actuators: SM 1278 on the CPU backplane plus four IO-Link cables to the remote cabinet. Lowest BOM, simplest configuration, but requires IO-Link sensors at every I/O point.
• If the second cabinet is dominated by conventional 24 V DC and 4-20 mA signals: ET 200SP IM 155-6 PN ST as the head module. Best mix of BOM cost, modern diagnostics, and stocking availability. PROFINET cable replaces the multi-conductor trunk.
• If the application requires fast local commissioning of the second cabinet independent of the CPU: Two S7-1200 CPUs linked over PROFINET (or a master-master connection). Higher BOM, two programs to maintain, but each cabinet can be commissioned solo.
• If the application is mostly analog and signal density is high: Phoenix Contact Axioline F. Higher per-channel cost but denser modules and the option of integrated IO-Link masters on the same backplane.

For the typical S5-to-S7-1200 retrofit in a U.S. light-industrial plant, the ET 200SP path is the best mix of cost, availability, and commissioning speed. The Phoenix Contact Axioline F path is the right call when the existing control panel already uses Phoenix Contact, or when spare-parts consolidation with Phoenix Contact inventory is a goal. The SM 1278 path is the right call only when the second cabinet is, or can be made, an IO-Link field device cluster.

For a deeper overview of remote I/O selection criteria applicable beyond the Siemens line, the Advantech white paper on remote I/O selection (How to Select the Right Remote I/O for Your Industrial Applications) covers the same four-axis decision (protocol, I/O type, environment, integration) for the Advantech ADAM and WISE lines.

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