Configuring S7-1200 as PROFIBUS DP Slave with CM 1242-5

1. Overview and Module Evolution

The SIMATIC S7-1200 PLC was originally released with a single integrated PROFINET interface on the CPU, and early firmware revisions (V1.0 through V3.0) restricted that interface to S7 communication, PUT/GET, ISO-on-TCP, TCP, UDP, and HMI panels. PROFIBUS DP master or slave functionality was not present on the CPU itself, which forced integrators using S7-200 with an EM-277 PROFIBUS DP slave module to retain the older controller for any non-Siemens PROFIBUS DP networks.

With STEP 7 / TIA Portal V11 and matching firmware updates on the S7-1200 CPU, Siemens opened the door to PROFIBUS by introducing the CM 1242-5 (DP slave) and CM 1243-5 (DP master) communication modules. The CM plugs onto the left side of the CPU or behind another CM/CP/SM, just like any other S7-1200 plug-in module, and the PROFIBUS segment is brought out to a 9-pin sub-D female connector on the front of the CM.

Today, if the requirement is to integrate the S7-1200 into an existing PROFIBUS DP network as a slave—for example behind a third-party DP master, an S7-300/400, an S7-1500, or a non-Siemens controller—the CM 1242-5 is the dedicated hardware. If the goal is to operate the S7-1200 as a master on PROFIBUS while exposing a PROFINET interface upward, the CM 1243-5 is the correct choice and is also used in the S7-1200 PN/DP Gateway (I-device / I-slave) configuration.

2. CM 1242-5 PROFIBUS DP Slave Module

The CM 1242-5 is the PROFIBUS DP slave communications module for the S7-1200 system.

Place the CM in any free slot on the left bank of the CPU. TIA Portal recognizes the module from the hardware catalog under Communications modules → PROFIBUS → CM 1242-5. After inserting the module, the purple PROFIBUS subnet port appears in the device view; drag it to the project tree's PROFIBUS subnet to bind the CM to a network.

3. CM 1243-5 PROFIBUS DP Master Module

The CM 1243-5 (article number 6GK7243-5DX30-0XE0) is the symmetric counterpart of the CM 1242-5. It implements the DP-V0/DP-V1 master, supports up to 32 DP slaves per segment (subject to repeater topology), and is required when the S7-1200 must poll PROFIBUS slaves (such as ET200S, ET200MP, SINAMICS, or third-party DP slaves).

Both CMs share the same mechanical form factor and backplane connector. The 9-pin sub-D connector carries the same pin-out as a standard PROFIBUS DP connector (pin 3 = B line, pin 8 = A line, pin 6 = VP, pin 5 = DGND, pin 4 = RTS, pin 1 & 9 = shield). Use a Siemens PROFIBUS bus connector (e.g. 6ES7972-0BA12-0XA0 with PG socket, or 6ES7972-0BB12-0XA0 without) and PROFIBUS FC cable (violet, 6XV1830-0EH10) for the trunk; terminate both ends of the segment with the switchable terminating resistor in the bus connector.

4. TIA Portal Configuration: S7-1200 as DP Slave with CM 1242-5

The configuration flow below assumes TIA Portal V16/V17/V18 and a CPU 1214C DC/DC/DC with firmware V4.4. Earlier portal versions follow the same logical steps, but the inspector window layout differs.

1. Add the CM 1242-5. Open the device configuration of the S7-1200 station. In the hardware catalog, navigate to Communications modules → CM 1242-5. Drag the module to an empty slot to the left of the CPU. TIA Portal assigns the module to slot 101 (or the next free slot).
2. Create or open a PROFIBUS subnet. Click the purple PROFIBUS port on the CM 1242-5. In the inspector window Properties → General → PROFIBUS interface, click Add new subnet to create a PROFIBUS (1.5 Mbit/s default) subnet, or assign an existing subnet from the project tree.
3. Assign the PROFIBUS address. In Properties → PROFIBUS address, enter the desired slave address (1–125). Valid range is 1 to 125; 0 is reserved for the master class 2 (PG) and 126 is the default for newly shipped units. The address set in TIA Portal must match the address selected by hardware switches (if present) on the CM or set in the software (no DIP switch on CM 1242-5; address is purely a configuration value).
4. Configure slot mapping. In Properties → Operating mode, leave the CM in DP slave mode. Under Module parameters → I/O configuration, define up to 32 slots of input/output data. Each slot accepts 1 to 32 bytes; the total across all slots must not exceed 240 bytes input + 240 bytes output. Typical mapping uses one or two slots matching the data width exchanged with the partner master (e.g., slot 0 = 4 bytes input / 4 bytes output, slot 1 = 8 bytes input /amp; 8 bytes output).
5. Address the I/O in the S7-1200 program. In the PLC tags, the CM 1242-5 I/O areas map to process image or direct I/O addresses beginning at %I and %Q addresses derived from slot index. TIA Portal displays the assigned input address (e.g., I100..I107) and output address (e.g., Q100..Q107) in the device view. Reference these directly in the user program; no PUT/GET or BSEND/BRCV calls are required for DP-V0 cyclic exchange.
6. Compile and download. Right-click the device → Compile → Hardware (rebuild all), then download the hardware configuration to the CPU. The CM 1242-5 receives its configuration automatically at CPU startup.
7. Configure the partner master. On the DP master station (S7-1500 with CM 1542-5 or any third-party DP master), install the GSD file of the CM 1242-5 (filename SIEM812B.gsd for DP-V0, or SIEM812D.gsd for DP-V1; both ship with TIA Portal and are also available from the Siemens Industry Online Support under entry ID 49855653). Place the CM 1242-5 on the master's PROFIBUS subnet at the address assigned in step 3 and configure the master's I/O slots to match the slot layout from step 4.

For step 6 the official Siemens TIA Portal Online Help describes the procedure in the chapter PROFIBUS → Configuring a DP master and slave device → Assigning PROFIBUS addresses to the CM 1243-5 module and DP slave (the same inspector workflow applies symmetrically to CM 1242-5). Reference: SIMATIC S7-1200 Manual Collection – PROFIBUS address assignment.

5. I-Slave Mode and PN/DP Gateway

The S7-1200 supports a PROFINET IO device / PROFIBUS DP slave gateway—referred to by Siemens as the S7-1200 PN/DP Gateway—in which the controller acts simultaneously as:

• A PROFINET IO device (I-device) on the upper-level PROFINET controller, and
• A PROFIBUS DP slave (I-slave) on the lower-level PROFIBUS master.

This is the configuration described in the official Siemens application document S7-1200 as a PN/DP Gateway, available from the Siemens Industry Online Support: S7-1200 as PN/DP Gateway (PDF). The use case covered by that document places an S7-1500 as the PROFINET controller, an S7-300 as the PROFIBUS DP master, and an S7-1200 in between. The I-slave functionality on the PROFIBUS side uses the CM 1242-5; the I-device side uses the integrated PROFINET port of the S7-1200 CPU.

To configure I-slave mode:

1. Add the CM 1242-5 to the S7-1200 device configuration as described in section 4. Set the operating mode to DP slave.
2. In the device view, switch to Properties → I-slave / I-device configuration. Define the transfer areas: each area is a contiguous I/O block (input, output, or input/output) that is exposed both to the upper PROFINET controller (as I-device slots) and to the lower PROFIBUS DP master (as DP slave slots). Configure a name (e.g., TA_1_IN), direction, length in bytes, and the start address in the S7-1200 process image.
3. On the PROFINET side, export the I-device GSD file (right-click the S7-1200 → Export → PROFINET GSD) and import it into the PROFINET controller's project (S7-1500). Place the I-device on the PROFINET subnet and configure the controller's I/O slots to mirror the transfer area lengths.
4. On the PROFIBUS side, install the CM 1242-5 GSD (SIEM812D.gsd) on the DP master (S7-300 in the reference application) and configure the slave slots to match the transfer area lengths.
5. Compile, download, and verify the data path. A successful configuration lights the BF LED on the CM 1242-5 as off and the SF LED as off; the PROFINET port shows link and activity on its integrated switch.

6. PROFIBUS Address Assignment on the CM

The CM 1242-5 has no hardware address switches; the PROFIBUS address is part of the device configuration and is downloaded with the project. To assign or change it after commissioning, follow the official Siemens procedure:

1. Open the project in TIA Portal and connect to the target CPU via PROFINET (online → accessible devices).
2. Click the purple PROFIBUS box on the CM 1242-5 in the device view. Switch the inspector window to Properties → General → PROFIBUS interface → PROFIBUS address.
3. Edit the Address field (1–125). Avoid addresses already used by the master class 2 (PG), other slaves on the same segment, or master addresses (the master is typically address 1 or 2).
4. Right-click the CM and select Download to device → Hardware configuration. The CM does not require a STOP of the CPU to accept the new address; it simply re-enters the data exchange cycle after a brief reconfiguration.
5. Verify the new address by reading the diagnostics buffer of the CM (online → diagnostics → diagnostic buffer) and by browsing the master station: the slave should appear under the new address with status Data exchange within a few seconds.

7. Migrating from S7-200 EM-277 to S7-1200 CM 1242-5

The S7-200 EM-277 (article number 6ES7277-0AA22-0YA0 for the 22.5 kbit/s version, or 6ES7277-0AA21-0YA0 for the 12 Mbit/s variant) provides a PROFIBUS DP slave interface for the S7-200 family. When replacing the S7-200 with an S7-1200, the CM 1242-5 takes over the DP slave role with the following differences:

The master does not need to be re-cabled, but its slave definition must be re-imported with the new GSD (SIEM812B.gsd or SIEM812D.gsd) and the slot sizes must be re-entered. Plan a brief downtime for the master reconfiguration. After downloading, validate by reading the master's diagnostics: the new slave must transition from Wait Prm (parameterization) to Wait Cfg (configuration) to Data Exchange.

8. Diagnostics, LEDs, and Buffer Entries

The CM 1242-5 has four LEDs on the front:

From TIA Portal, additional detail is available in Online → Diagnostics → Diagnostic buffer for the CM and in the web server of the CPU (if enabled). Typical diagnostic buffer entries include:

• "Configuration error: length of input/output inconsistent" – The slot layout in the master does not match the CM 1242-5. Recheck the master project.
• "Address conflict on PROFIBUS" – Another node on the segment uses the same address. Scan with the master to identify the duplicate.
• "Watchdog timeout" – The master stopped polling. Check the master's CPU state and its bus parameters.
• "Parameter assignment fault: PNO identifier 0xXY" – DP-V1 service mismatch; the master is sending parameter data that the CM does not accept (e.g., DPV1 enabled when not configured).

9. Specifications and Limits Summary

10. Troubleshooting Matrix

11. Field-Proven Commissioning Checklist

1. Verify mechanical: CM seated and locked into the S7-1200 left bank. CPU recognizes it in Online → Accessible devices.
2. Verify electrical: 24 VDC at CPU power terminals; backplane LEDs steady green on CPU and CM.
3. Verify network: 5 V between pin 5 and pin 6 of the CM sub-D connector with master powered and termination enabled at both ends.
4. Verify configuration: PROFIBUS address in TIA Portal matches the address set on the master side; I/O lengths in master slot table match CM I/O configuration; GSD file version matches between master and TIA Portal catalog.
5. Verify data exchange: All four LEDs on CM show ON green, SF off, BF off, TXD/RXD flashing; master's slave list shows status Data Exchange; read/write a known test word from the user program and observe it at the master.
6. Verify diagnostics: Diagnostic buffer is empty on both CPU and CM after 15 minutes of continuous operation. Configure SF and BF LEDs to trigger a CPU diagnostic interrupt (optional) so user program can react.

12. Notes on Coexistence with PROFINET

Adding the CM 1242-5 does not consume the S7-1200 PROFINET port. The CPU's integrated PROFINET interface continues to support S7 communication, HMI panels, web server, and—on firmware V4.0 or higher—PROFINET IO controller/IO device with up to 16 IO devices. When the CM 1242-5 is installed and configured as a DP slave, the PROFINET port is fully independent. Use the same PN/DP Gateway pattern (PROFINET IO controller on top, PROFIBUS DP master below) when the S7-1200 must bridge between two non-Siemens networks; this is the architecture validated in the Siemens application document referenced in section 5.