S7-1200 PROFIBUS Master and Slave Configuration Reference

The SIMATIC S7-1200 family is primarily a PROFINET-centric controller, but Siemens provides a dedicated PROFIBUS DP communication module (CM) portfolio that turns the CPU into a fully functional DP master or DP slave on a PROFIBUS network. This reference consolidates the hardware catalog, the STEP 7 / TIA Portal configuration path, the firmware prerequisites, and the most common field-engineering pitfalls (gateway operation, slave count limits, baud-rate selection) for a S7-1200 on PROFIBUS.

1. PROFIBUS Support Matrix for the S7-1200 Family

The S7-1200 CPU does not have PROFIBUS integrated on the front panel. PROFIBUS DP is added through a left-side communication module. Two catalog numbers are relevant for greenfield work:

Only one CM is permitted per S7-1200 CPU on the left bus; you cannot stack a master and a slave on the same CPU. To get both directions (master on one subnet, slave on another, or PN/DP gateway), you must add a second CPU in the same rack or move the master/slave role to a different controller. The official device description is published in the Siemens product finder entry for the SIMATIC S7-1200 and in the TIA Portal hardware catalog under "Communication modules > PROFIBUS."

2. Firmware and Engineering Software Requirements

PROFIBUS on the S7-1200 is not a firmware feature of the CPU itself, but the CPU must be at a firmware level that recognizes the CM and exposes it in the device configuration. The minimum dependencies are:

• CPU firmware: V2.0 or higher is the practical minimum for stable CM operation. Firmware V4.x and later is required for full TIA Portal V13+ features including symbolic I/O mapping, isochronous-mode properties for drives, and the integrated PROFIBUS diagnostics.
• STEP 7 (TIA Portal): V11 SP2 was the first release that added the S7-1200 CM 1242-5/CM 1243-5 to the hardware catalog. TIA Portal V13, V14, V15.1, V16, V17, and V18 each extended the catalog. The current path is TIA Portal V18/V19 with HSPs (Hardware Support Packages) installed.
• GSD files: For third-party slaves, install the manufacturer GSD file through Options > Manage general station description files (GSD) in TIA Portal before adding the device to the project.

Cross-check your CPU firmware with the firmware update tool SIMATIC Automation Tool or the S7-1200 system manual; the firmware version exposed in Online > Accessible devices must match the catalog entry you selected.

3. Hardware Installation Rules

1. Power down the CPU before plugging or unplugging the CM; the left-bus connector is not hot-swappable.
2. Snap the CM onto the left side of the CPU until the bus connector seats. The mechanical keying prevents inverted mounting.
3. Wire the 9-pin D-sub PROFIBUS connector (female on the cable, male on the module) following the standard PROFIBUS DP pin-out: pin 3 = RxD/TxD-P (B-line, red), pin 5 = DGND, pin 6 = VP (termination power), pin 8 = RxD/TxD-N (A-line, green). The shield is bonded to the D-sub metal housing on both ends.
4. Enable bus termination on the connectors at the physical ends of the segment only. The CM module itself has no built-in termination.
5. Respect the segment length table. At 12 Mbps, a single segment is limited to 100 m; at 1.5 Mbps it is 200 m; at 187.5 kbps and below it is 1,000 m. Repeaters (e.g., 6GK1500-0AA10) extend the network in steps.

4. Configuring PROFIBUS DP in TIA Portal

The official Siemens TIA Portal help describes the configuration sequence for adding a DP master system. The steps map directly to the Configuring PROFIBUS DP (S7-1200) chapter of the TIA Portal online documentation. A condensed procedure:

1. Open the project and double-click Devices & networks from the project tree.
2. Drag the S7-1200 CPU and the CM 1242-5 or CM 1243-5 from the hardware catalog into the network view. TIA Portal auto-inserts the CM to the immediate left of the CPU on the internal backplane bus.
3. Right-click the CM and select Add new subnet. TIA Portal creates a PROFIBUS subnet with the default name PROFIBUS_1.
4. Open the CM properties and set the PROFIBUS address. For a master, addresses 1–125 are legal; for a slave, the same range applies and the address must be unique on the segment.
5. Set the transmission speed. The master CM 1243-5 auto-detects slaves that support auto-baud, but it is best practice to lock the baud rate explicitly (e.g., 1.5 Mbps) to avoid link-down events during start-up.
6. From the catalog, drag PROFIBUS DP slaves into the subnet. The connection lines auto-render in the network view.
7. For each slave, assign I/O addresses in the slave properties — these become the I/O area of the S7-1200 CPU. Module-slot mapping on ET 200 stations is configured under the slave's device view.

5. S7-1200 as a PN/DP Gateway

A common migration scenario is to attach legacy PROFIBUS devices to a PROFINET controller such as an S7-1500. The S7-1200 with a CM 1242-5 plus the integrated PROFINET interface can serve as a low-cost PN/DP gateway. The reference application from Siemens, documented in S7-1200 as a PN/DP Gateway, describes the roles as:

• PROFIBUS side: The S7-1200 acts as a DP slave (I-slave). The DP master on the PROFIBUS segment exchanges I/O data with the CM 1242-5; the S7-1200 CPU does not host the user program that processes this data.
• PROFINET side: The S7-1200 acts as an I-device (PROFINET IO device). The PROFINET controller, for example an S7-1500, exchanges I/O with the S7-1200 CPU over PROFINET.
• Data transfer: Inside the S7-1200, the I-slave I/O area and the I-device I/O area are linked by a copy operation in the user program (typically PE/PAB on one side copied to PA/PE on the other). This makes the S7-1200 a transparent bridge between the two fieldbuses.

The gateway pattern is widely used to preserve installed PROFIBUS drives and remote I/O when a plant transitions its controller layer to PROFINET. The CPU firmware level (V4.4 or higher is recommended) and the TIA Portal version must both support the I-device role on the PROFINET interface; check the CPU's "Operating mode" property in the device configuration.

6. Programming the PROFIBUS Interface

PROFIBUS I/O on the S7-1200 is treated like centralized I/O from the user's perspective once the slave is configured. Standard access mechanisms apply:

• Direct I/O access: Use %IW (input word) and %QW (output word) addresses that were assigned during configuration. The simplest ladder logic reads inputs and writes outputs in OB1.
• PUT/GET (for slave-to-slave via master): Not applicable on the S7-1200 — use S7 communication blocks PUT / GET over PROFINET or the PROFIBUS S7 services if a CM 1243-5 is configured as a class-2 master.
• Diagnostics: Call RDREC and WRREC to read/write acyclic diagnostics records. The standard slot 0 record index 0 returns the DP-V1 diagnostic frame; slot 0 index 1 returns identifier-related diagnostics.
• Process image update: The process image partition PIP 0 is updated automatically; if the slave is assigned to PIP 1–3, attach it to an OB that runs in the assigned time slice (e.g., OB 61 for PIP 1).

7. Diagnostics and Status Indicators

The CM modules expose LED status indicators on the front bezel. The behavior is the same as on the S7-300 CP 342-5 family:

From the TIA Portal online view, Online & diagnostics > PROFIBUS > Diagnostic buffer on the CM gives access to the structured diagnostic entries. Use PROFIBUS > Station list to verify which slaves are visible on the wire versus which are configured, which is a fast way to detect duplicate addresses or missing terminations.

8. Comparing the S7-1200 CM with the S7-300 CP 342-5

For new applications, the S7-1200 CM is preferred because of the TIA Portal integration and the smaller footprint. For brownfield S7-300 migrations, leaving the existing CP 342-5 in place and connecting it to a new S7-1500 over PROFINET (via PN/DP gateway pattern or a separate gateway) is often simpler than rewiring.

9. Common Field-Engineering Pitfalls

1. Adding two CMs to the same CPU. Only one PROFIBUS CM can be installed on a single S7-1200 CPU. If both master and slave roles are needed, deploy two CPUs or use a third-party gateway.
2. CPU firmware too old. Firmware V1.x CPUs will not enumerate the CM; the hardware catalog entry exists but the slot remains empty in the device view after download. Upgrade the CPU to V4.x.
3. Address conflict. The S7-1200 master defaults to PROFIBUS address 2. If a slave is also configured for 2, the master will refuse to start the data exchange; the BF LED flashes 0.5 Hz.
4. Missing GSD. Third-party PROFIBUS slaves (frequency inverters, valves, weighing modules) require their GSD/GSML files installed before the catalog shows them.
5. Termination power not supplied. Some PROFIBUS connectors require 24 V on pin 6 (VP) to power the internal bus terminator. If pin 6 is left floating and the segment relies on bus-powered terminators, the master sees reflection-induced errors.
6. Auto-baud mismatch with older slaves. A 1.5 Mbps master trying to slave a 45.45 kbps device will not start. Lock the baud rate in the CM properties to match the slowest device on the segment.
7. PN/DP gateway data flow direction. In a gateway, copy the PROFINET inputs into the PROFIBUS I-slave outputs in OB1; mismatched direction is the leading cause of "data exchange running but values do not move" complaints.

10. Sizing a PROFIBUS Network with an S7-1200 Master

A CM 1243-5 master supports up to 32 DP slaves per segment, with up to 244 bytes of input and 244 bytes of output data per slave. A useful quick check on network size:

• Total cyclic I/O data (bytes) = sum of inputs + outputs of every slave.
• The cycle time is determined by the slowest slave's response plus bus arbitration. A 1.5 Mbps network with 16 slaves at the per-slave maximum typically updates in < 10 ms; at 12 Mbps the same network updates in < 2 ms.
• For deterministic drives, set the slave's "Watchdog" time to 3× the master poll cycle to tolerate one missed telegram without a station drop.

If a deployment approaches 32 slaves or 2×244 bytes per slave on a high-throughput segment, segment it with a repeater and balance the bus load between two masters, or move to PROFINET where the S7-1200 PROFINET interface supports more than 32 devices.

11. Migration and Replacement Notes

When a S7-1200 on PROFIBUS is being replaced by an S7-1500 or an ET 200SP controller, the PROFIBUS network can be preserved in two ways:

1. Use an S7-1200 as a PN/DP gateway. Keep the S7-1200 with the CM 1242-5 installed, configure it as an I-slave on PROFIBUS and as an I-device on PROFINET, and let the new PROFINET controller treat the S7-1200 as a sub-device (see Section 5).
2. Use a DP/PN coupler. Siemens offers the DP/PN coupler (6GK1411-1AD00) for transparent one-way data exchange between a PROFIBUS master and a PROFINET controller. The S7-1200 is removed entirely.

For completely new installations, PROFINET is the preferred path because the S7-1200's PROFINET interface is faster, supports topology-based diagnostics, and is included on the CPU. The PROFIBUS CM remains the right choice for any application that must talk to a brownfield PROFIBUS device — drives, field instruments, remote I/O from older installations.

12. Quick Reference Specifications