S7-300 S7-1200 PROFIBUS DP Communication Using CM 1242-5 Module

Architecture Overview

This reference describes a homogeneous Siemens PROFIBUS DP network in which a SIMATIC S7-300 CPU with an integrated DP master interface acts as the class-1 master and an S7-1200 station equipped with the CM 1242-5 communication module acts as a DP-V0 slave. The typical hardware arrangement is the 313C-2DP (or any CPU 31x with integrated DP port, e.g. 315-2DP / 317-2DP) on the master side and a CPU 121x (firmware V4.0 or later required for CM 1242-5) on the slave side.

The CM 1242-5 is the dedicated PROFIBUS DP slave module for the S7-1200 family (article number 6GK7 242-5DX30-0XE0). It supports all standard baud rates from 9.6 kbps to 12 Mbps and a maximum payload of 244 bytes input / 244 bytes output per slave, which is well below the PROFIBUS DP-V0 theoretical maximum of 244 bytes per direction. The companion module CM 1243-5 (6GK7 243-5DX30-0XE0) is the DP master counterpart; do not confuse the two when ordering.

Because the CM 1242-5 does not expose any cyclic program blocks to the user, the slave side does not require a single line of code. The master simply reads and writes the configured I/O areas as peripheral I/O.

Hardware Prerequisites

Software Prerequisites

• STEP 7 V5.5 SP4 or V5.6 for the S7-300 master (HW Config is still the standard tool for 300-series hardware, even when commissioning the network from a TIA Portal project).
• TIA Portal V13 SP1 or later for the S7-1200 / CM 1242-5 slave configuration. The reverse-order transfer-area behaviour described in this article was verified against TIA V13, V15, and V16.
• GSD file for CM 1242-5 (file name typically SI0180B8.GSD or successor). Download from the official Siemens Product Support portal at support.industry.siemens.com (search term "CM 1242-5 GSD"). The file is mandatory because the CM 1242-5 is not in the legacy STEP 7 V5.x hardware catalogue.

Network Topology and Cable Limits

Maximum segment length depends on the configured baud rate:

A maximum of 32 stations is allowed per segment without a repeater (RS-485 physical layer). Each repeater adds a segment with another 31 nodes. Valid PROFIBUS addresses are 1 through 126; address 0 is reserved for a class-2 master (PG) and address 127 is reserved for broadcast.

S7-300 Master Configuration (STEP 7 V5.x)

Step 1 — Install the GSD

1. Open SIMATIC Manager and load the project containing the 313C-2DP station.
2. Open HW Config and select Options → Install GSD File….
3. Browse to the directory containing the extracted CM 1242-5 GSD, then choose Install. Wait for the catalogue to update.
4. The slave will appear under PROFIBUS DP → Additional Field Devices → I/O → CM 1242-5.

Step 2 — Insert the Slave on the PROFIBUS Subnet

1. Drag the CM 1242-5 from the catalogue onto the PROFIBUS(1):DP master system rail.
2. Assign PROFIBUS address 3 (master should be address 2; slaves may use any free address 1–126 except the master's).
3. Confirm the proposed baud rate (1.5 Mbps is the most common production default; 12 Mbps is permitted but reduces cable length to 100 m).

Step 3 — Configure the Transfer Areas

The CM 1242-5 GSD exposes up to 16 modular slots. Each configured slot becomes one transfer area. Two slots are sufficient for cyclic I/O exchange:

1. Double-click Slot 1 (the first universal module in the catalogue tree) and select 2 bytes I input. From the master side this is data the master reads from the slave.
2. Double-click Slot 2 and select 2 bytes O output. From the master side this is data the master writes to the slave.
3. Note the peripheral input/output addresses assigned by HW Config, e.g. PEW 256 / PAW 256. The peripheral address range is what the slave's transfer areas map into.

Step 4 — Save and Compile

1. Station → Save and Compile (Ctrl+S).
2. Download the hardware configuration to the S7-300 CPU.
3. Set the CPU to RUN. The DP master should begin polling address 3.

CM 1242-5 Slave Configuration (TIA Portal)

Step 1 — Add the Module to the Device View

1. Open the S7-1200 project in TIA Portal.
2. In Device View, drag the CM 1242-5 from the catalogue (Communication modules → PROFIBUS) into a free slot left of the CPU.

Step 2 — Configure the PROFIBUS Interface

1. Select the CM 1242-5, then open Properties → PROFIBUS interface.
2. Click Add new subnet; TIA will create a PROFIBUS_1 with the parameters you enter.
3. Set the PROFIBUS address to 3, matching the HW Config entry on the S7-300 side.
4. Set the baud rate to the same value as the master (1.5 Mbps recommended for new installations).
5. Confirm that the highest PROFIBUS address is 126 (default).

Step 3 — Define Transfer Areas

1. In the Properties → Transfer areas tab of the CM 1242-5, add a new transfer area.
2. Configure Transfer Area 1:
   • Type: Input (data the slave receives from the master)
   • Length: 2 bytes
   • Consistency: Total length (or Byte, depending on payload < 4 bytes)
   • Starting address: e.g. %IW100
3. Configure Transfer Area 2:
   • Type: Output (data the slave sends to the master)
   • Length: 2 bytes
   • Consistency: Total length
   • Starting address: e.g. %QW100

Step 4 — Compile and Download

1. Compile → Hardware (rebuild all).
2. Download the configuration to the S7-1200 CPU.
3. Cycle power to the CM 1242-5 only if the BF LED remains solid red after download.

The Reverse Transfer Area Rule

The most common failure mode on first attempt is "no diagnostic error, but the data on the slave is what I wrote from the master to itself". This is caused by a mirroring mistake between the slot configuration on the master side and the transfer area configuration on the slave side.

The rule is:

What the master considers Output is Input for the slave, and vice-versa. Each master slot N corresponds to slave transfer area N.

Worked example with two 16-bit areas:

If the order is reversed (master Slot 1 = Output, master Slot 2 = Input), the slave must mirror it: Transfer Area 1 = Input, Transfer Area 2 = Output. The GSD slot numbers always correlate one-to-one with the slave's transfer area numbers regardless of direction.

Programming the Data Exchange

Because the CM 1242-5 is a slave, the S7-1200 user program only manipulates the process image addresses that the transfer areas point to. A 16-bit hand-shake example in structured text:

// S7-1200 side (slave), cyclic OB1
#SlaveSetpoint   := %IW100;        // value written by S7-300 master
#SlaveFeedback   := %QW100;        // value read back by S7-300 master
%QW100 := INT_TO_WORD(#ActualSpeed); // local feedback to master

On the S7-300 master the same data is read and written as peripheral I/O:

// S7-300 side (master), OB1 (ST)
L PEW 256;          // read 16-bit feedback from CM 1242-5
T MW 200;           // buffer for evaluation
L MW 100;           // local setpoint
T PAW 256;          // write to CM 1242-5

No special function blocks (FCs/FBs) are required on the slave. The CP integrates itself into the peripheral address space of the master.

LED Status and Diagnostic Indicators

The CM 1242-5 has three status LEDs that must be interpreted together with the S7-300 master's DP diagnostics.

Reading the S7-300 Diagnostic Buffer

If the master CPU is in STOP with SF (system fault) on, the diagnostic buffer will contain the exact cause. In STEP 7:

1. PLC → Module Information.
2. Click the Diagnostic Buffer tab.
3. Read the most recent events; the codes most commonly seen are:
   • Event ID 39 84 — DP slave fault (slot diagnostics indicate which slave failed).
   • Event ID 38 04 — Slave parameter assignment error (configuration length or consistency mismatch).
   • Event ID 38 05 — Slave configuration error (slot does not match GSD).

Click Open Block on a station-fault event to drill into the slot-level diagnostics and see exactly which slot number is failing. This is the fastest path to diagnosing the "SF only when payload > 1 byte" problem reported by multiple integrators.

Troubleshooting Matrix

Advanced — Communicating Through a Y-Link

When the DP master is a redundant H-system (e.g. S7-400H), the CM 1242-5 cannot be connected directly to either CPU. A Y-Link (e.g. 6GK1 416-2CK00-0AA0 for PROFIBUS, or 6ES7 197-1LA04-0XA0 with appropriate IM) must be inserted. The Y-Link is configured in the H-station's PCS 7 / STEP 7 HW Config as a proxy for the CM 1242-5. Inside the Y-Link, the lower-level PROFIBUS master exchanges data with the CM 1242-5 transparently. The transfer-area configuration in the TIA Portal for the CM 1242-5 remains exactly as described above; only the upstream master changes.

Payload Limits and Consistency

• Maximum 244 bytes input and 244 bytes output per slave — confirmed in the GSD catalogue.
• For payloads ≤ 4 bytes, "Unit" or "Byte" consistency is sufficient. The data is transferred atomically with the peripheral I/O update of the CPU.
• For payloads > 4 bytes, always set consistency to "Total Length". On the S7-300 master this requires DPRD_DAT / DPWR_DAT system function blocks rather than direct PEW/PAW access, otherwise the slave may read partially updated data within the same cycle.

Verification and Commissioning Checklist

1. After download, power-cycle the CM 1242-5 once to ensure the parameter assignment from the master is fresh.
2. Confirm BF off and SF off on the CM 1242-5 within 5 seconds of master RUN.
3. On the S7-300 side, monitor the peripheral addresses in Monitor/Modify. Write a known pattern (e.g. 16#AAAA) to PAW 256 from the master and read PEW 100 on the slave — values must match.
4. Reverse the check: write a known pattern from the S7-1200 to %QW100 and verify the same value appears at PEW 256 on the master.
5. Toggle a free status bit in both directions at 100 ms to confirm the update rate matches the configured baud rate (≈ 1 ms cycle at 1.5 Mbps with two 16-bit areas).
6. Unplug the bus connector — both stations must flag a station failure within the watchdog time (default 10 s × 2 base time on S7-300).

Reference Documents

The behaviour of the CM 1242-5 within TIA Portal, including the transfer-area editor described above, is documented in the SIMATIC S7-1200 Communication Function Manual:
Configuring a PROFIBUS S7 connection — TIA Portal manual collection.

Additional supporting material is available on the Siemens Industry Online Support portal:
support.industry.siemens.com — entry point for S7-1200 system manual, S7-300 system manual, CM 1242-5 operating instructions, and the CM 1242-5 GSD file.

FAQ