Extending S7-1200 I/O Modules PROFINET and PROFIBUS Remote

1. Problem Definition: Separating S7-1200 CPU from I/O Modules

When a control cabinet is physically too small to host both an S7-1200 CPU and its expansion modules (SM 1221 DI, SM 1222 DQ, SM 1231 AI, SM 1232 AQ, SM 1234 AI/AQ), the engineer must relocate the I/O away from the CPU while preserving deterministic, real-time communication. The original query is unambiguous: keep the CPU inside the operator console and place the SM signal modules near the field devices to shorten analog wiring runs and reduce noise pickup.

The S7-1200 system was designed for a compact, single-row architecture. There is no official Siemens bus extension cable that allows the standard local I/O backplane to span more than approximately 2 meters. Any attempt to extend the proprietary backplane ribbon beyond that distance is unsupported and may fail EMC and timing tests. The practical answer is to abandon the local I/O bus and migrate to a fieldbus-distributed topology.

2. S7-1200 Local I/O Backplane Limits

The S7-1200 communication bus between the CPU and SM/CM/CP modules uses a proprietary side-connector ribbon. According to the S7-1200 Programmable Controller System Manual (06/2021 edition):

• Maximum 1 signal module (SM) per CPU row in compact configurations.
• CPU 1211C: 0 SMs (no local expansion).
• CPU 1212C: up to 2 SMs.
• CPU 1214C: up to 8 SMs.
• CPU 1215C / 1217C: up to 8 SMs plus 3 CM/CP modules.
• Maximum physical row length: limited by the 1 m dual-pin ribbon that ships with each SM. Total mechanical stretch must remain below 2 m of cumulative ribbon length to keep the integrated logic within timing budget.

3. Solution Matrix: Three Architectures for Remote I/O

The following table summarizes the three proven architectures available on the S7-1200 platform. Selection depends on existing hardware, required update time, and cable distance.

4. Solution 1: I-Device (Second S7-1200 as Remote I/O)

The I-Device (intelligent IO-Device) configuration turns a slave S7-1200 into a distributed I/O station while exposing its process data to the master S7-1200 through a defined PROFINET interface. TIA Portal V13 SP1 or higher is required.

Hardware required

• Master CPU: any S7-1200 firmware V4.0+ with on-board PROFINET port.
• I-Device CPU: any S7-1200 (1212C/1214C/1215C/1217C), firmware V4.1+ recommended.
• PROFINET cable: Siemens IE FC TP Standard Cable (6XV1840-2AH10) or IE FC TP Trailing Cable (6XV1870-2B) for moving installations.
• RJ45 connectors: IE FC RJ45 Plug 180 (6GK1901-1BB10-2AA0), 4-core.

Configuration procedure

1. Insert the I-Device CPU in the TIA Portal project, add the required SM signal modules.
2. In the I-Device CPU device configuration, open Properties → PROFINET interface → Operating mode and select IO Device.
3. Configure the transfer areas: define input and output slots for the PROFINET interface. Each slot can hold up to 244 bytes of input data and 244 bytes of output data.
4. Compile the I-Device project and export the GSD file of the I-Device.
5. In the master CPU project, install the GSD under Options → Manage device description files (GSD).
6. Drag the I-Device from the catalog onto the master PROFINET subnet, assign device name and IP address.
7. Download both projects and verify the device name assignment through PRONETA or the master CPU online diagnostics.

5. Solution 2: ET200S over PROFINET

The ET200S family uses an interface module (IM) that hosts standard ST or HS plug-in I/O modules. With firmware V11 SP2 and a CPU 1215C, the IM151-3 PN can be added directly through the catalog. The IM151-3 PN (6ES7151-3BA23-0AB0) supports up to 63 I/O modules and exposes the standard PROFINET diagnostics.

Maximum topology

• Copper (100BASE-TX): 100 m segment length between two devices.
• Fiber (100BASE-FX) using IE FC Media Converter: up to 3 km.
• Maximum of 255 devices per subnet, 8 stations in a line topology.

Configuration follows the standard PROFINET device-installation workflow. The IM151-3 PN GSD file is part of TIA Portal V11 SP2 HSP0080 or higher.

6. Solution 3: ET200SP over PROFINET (Recommended for New Designs)

The ET200SP is the current generation of distributed I/O. The interface module IM155-6 PN (6ES7155-6AU01-0BN0 or newer -6BA01) combined with BaseUnit type A0/B0 provides a compact, hot-swappable I/O rack that can be expanded up to 32 or 64 modules depending on variant.

The HF variants support Media Redundancy Protocol (MRP) for ring topologies, which is critical in motion or high-availability applications.

7. Solution 4: PROFIBUS DP with CM 1243-5

If the installation already has a PROFIBUS infrastructure or the distance exceeds 100 m and copper PROFINET is not feasible, the CM 1243-5 (6GK7243-5DX30-0XE0) can be installed on the S7-1200 to act as a PROFIBUS DP master. The CM 1243-5 sits in the leftmost CM/CP slot of the CPU row.

Configuration parameters

• Baud rate: 9.6 kbps to 12 Mbps.
• Maximum PROFIBUS segment length depends on baud rate: 1200 m at 9.6 kbps / 187.5 kbps; 400 m at 500 kbps; 200 m at 1.5 Mbps; 100 m at 3 Mbps / 6 Mbps / 12 Mbps.
• Maximum stations per segment: 32 (including repeaters), 126 total per PROFIBUS network.
• Connector: PROFIBUS connector with 35° cable outlet (6ES7972-0BA52-0XA0) or 90° outlet.
• Termination: switch ON at both physical ends of the segment, OFF at intermediate stations.

On the ET200S side use IM151-1 (6ES7151-1BA02-0AB0) for standard DP or IM151-1 HF (6ES7151-1CA00-0AB0) for high-feature. On the ET200SP side use IM155-6 DP HF.

8. Cable, Connector, and Distance Reference

9. TIA Portal Configuration: Step-by-Step

The example below uses an S7-1215C master CPU (192.168.0.1) and an ET200SP HF station (192.168.0.2) on PROFINET.

1. Open TIA Portal V16 or higher and create a new project.
2. Insert a CPU 1215C DC/DC/DC (6ES7215-1AG40-0XB0). Add the rack-mounted CM 1243-5 only if PROFIBUS is required.
3. Open Devices & Networks, drag an IM155-6 PN HF from the catalog to the PROFINET subnet.
4. Connect the master CPU port to the IM155-6 PN HF port with a drag line.
5. Assign IP addresses: master 192.168.0.1 / 255.255.255.0, ET200SP 192.168.0.2 / 255.255.255.0.
6. Assign a PROFINET device name: et200sp-station1. Note: PROFINET device name is mandatory; IP alone is insufficient.
7. Insert ET200SP modules into the IM155-6 HF station: DQ 16x24VDC/0.5A (6ES7132-6BH01-0BA0), DI 16x24VDC (6ES7131-6BH01-0BA0), AI 4xU/I/RTD (6ES7134-6HD01-0BA1), AQ 2xU/I (6ES7135-6HB00-0CA1).
8. Configure IO cycle: in PROFINET interface → Real-time settings → IO cycle time, set 1.0 ms for HF stations, 4.0 ms for ST.
9. Assign symbolic IO tags in the master program: "ET200SP".DI16_1.IW0, etc.
10. Compile, download to CPU, and use Online & Diagnostics → PROFINET device name assignment for the ET200SP.

10. Verification and Commissioning

After download, perform the following verification sequence:

1. In Online & Diagnostics → Diagnostics, confirm the ET200SP shows Station OK and no diagnostic interrupts.
2. Use PRONETA or Topology Editor to confirm the cable plant. Verify that the PROFINET device name matches the configuration.
3. Force a sample input: monitor %IW0 on the master CPU and toggle a field input. Confirm the value updates within one PROFINET update cycle.
4. Write a sample output: set %QW0 and confirm the field device energizes within one PROFINET update cycle.
5. Check bus load with Wireshark or PRONETA: PROFINET bus load should remain below 50% under normal operation. If it exceeds 70%, increase the update time.
6. Verify EMC: confirm shield bonding at both ends of the PROFINET cable through the IE FC RJ45 Plug's shield clamp.

11. Troubleshooting Matrix

12. Choosing the Right Architecture

Pick I-Device (2nd S7-1200) when the user already owns two S7-1200 CPUs and wants a low-cost, fast-commissioning solution with PROFINET at 100 m. It is the closest equivalent to keeping the original "single PLC" feel because both controllers run TIA Portal projects.

Pick ET200SP over PROFINET for any new design. It is the current Siemens generation, has the highest density per mm of DIN rail, supports hot-swap, and offers HF variants with sub-millisecond update times. HF modules also support MRP rings, which is mandatory in many modern automotive and packaging lines.

Pick ET200S over PROFINET only if the cabinet already has ET200S ST or HS modules that must be retained. ET200S is in phase-out.

Pick PROFIBUS with CM 1243-5 when the plant already has PROFIBUS infrastructure, the distance is between 100 m and 1.2 km, or when the installation environment is so electrically noisy that copper PROFINET is impractical without fiber converters. The CM 1243-5 supports up to 16 PROFIBUS slaves in the S7-1200 master configuration.

13. Field-Proven Caveats

• PROFINET requires a device name in addition to an IP address. Forgetting the device name is the single most common reason an ET200 station shows "Station Failure" even when the IP is correct.
• The IE FC RJ45 Plug must be crimped with the Siemens IE FC Stripping Tool (6GK1901-1GA00). Standard RJ45 crimps will damage the cable geometry and degrade return loss.
• The shield of PROFINET copper cable must be bonded at both ends through the connector shield clamp, not at a single point. Failure to bond both ends is the most common cause of EMC-related diagnostic interrupts.
• The CM 1243-5 occupies a CM/CP slot. On a CPU 1215C you have three slots; on a CPU 1214C you have only one. Plan accordingly.
• Update time must be ≥ 4× the send clock. TIA Portal will reject update times below the bus cycle limit.