Configuring Siemens DP-DP Coupler with IM308C Profibus Setup

Overview

The Siemens IM 308C is a Profibus DP master interface module for the S5-115H/F, S5-135U, S5-155U/H, and S5-155H/F PLC families. It allows an S5 station to act as a Class 1 DP master on a Profibus DP network. In real-world plants, an S5 station running an IM 308C often needs to exchange data with a second Profibus DP master that lives on a different bus segment, perhaps belonging to a different cell, a different vendor's controller, or a redundant network. The standard way to bridge two DP masters without merging their bus parameters is the DP/DP Coupler.

The DP/DP Coupler (Siemens catalog number 6ES7 158-0AD01-0XA0 for the classic version, 6ES7 158-0AD02-0XA0 for the upgraded module) is a slave on both sides: it presents a Profibus DP slave interface to the IM 308C on segment A and another Profibus DP slave interface to the second master on segment B. It copies the output data it receives from one side into the input image of the other. The bus parameters, baud rate, and master configuration of the two segments are completely independent; only the data image widths and data types must match across the coupler.

The official product description states: "The function of the DP/DP coupler is to connect two PROFIBUS DP networks with one another and to transfer data between the masters of both networks." See the Siemens support article New DP/DP Coupler Released for Delivery (ID 18491063).

When to Use a DP/DP Coupler with an IM 308C

Use a DP/DP Coupler between an IM 308C and another DP master in any of the following situations:

• The two Profibus segments run at different baud rates (e.g., 1.5 Mbit/s on the S5 cell, 12 Mbit/s on the new S7-300 cell) and cannot share a single bus.
• The S5 master must appear as a standard DP slave to the other master so the other master can use a generic slave project without needing any S5-specific GSD.
• You must isolate the bus segments electrically (the DP/DP Coupler provides galvanic isolation between the two Profibus networks).
• You are migrating an S5 cell toward S7-400 or S7-1500 and want to leave the IM 308C in service while the new master reads the same I/O image.

The DP/DP Coupler is not a router and does not extend a single bus. It is a gateway that exchanges a fixed data image between two masters.

Prerequisites

Before starting the configuration, gather the hardware, software, and documentation listed below.

Hardware Installation

1. Mount the DP/DP Coupler on a 35 mm DIN rail inside the cabinet. Allow at least 25 mm clearance above and below for ventilation and cable bend radius.
2. Connect the 24 V DC supply to the removable terminal block on top of the coupler. Observe polarity: terminal 1 is +24 V, terminal 2 is 0 V, terminal 3 is functional earth (FE). The FE terminal should be bonded to the cabinet ground bar to discharge any cable-borne transients.
3. Plug the first Profibus DP connector into the left Profibus port (segment A, the IM 308C side). Use a Profibus connector with an enabled terminating resistor on the physical end of the segment, and a connector with the terminating resistor disabled everywhere else.
4. Plug the second Profibus connector into the right Profibus port (segment B, the second master side). Repeat the termination rule for that segment.
5. Power up the coupler. The green PWR LED should illuminate steadily. The two BF (bus fault) LEDs may flash until both masters establish a connection.

The procedure mirrors the steps documented in the DP/DP Coupler installation manual: plug the power supply into the new DP/DP coupler, then plug the bus connection plugs into the PROFIBUS DP interfaces of the new DP/DP coupler.

Integrating the GSD File into Com Profibus

The IM 308C is not configured with STEP 7. It uses a dedicated Siemens tool, Com Profibus, which is supplied with the IM 308C package. The first task is to register the DP/DP Coupler's GSD file in the Com Profibus device catalog.

1. Copy the GSD file (for example SIEM8110.GSD) and the matching bitmap file (SIEM8110.BMP or SIEM8110.NMP) into the Com Profibus GSD directory, typically C:\ComProfibus\GSD or the equivalent path on your installation.
2. Start Com Profibus and open the IM 308C master project. If the project does not exist, create a new master system that matches the IM 308C you have installed.
3. Choose Options > Install GSD (the exact menu text varies slightly by Com Profibus build) and point the dialog to the GSD directory. Com Profibus will parse the GSD and add the DP/DP Coupler to the device catalog under the DP Slave > Coupler branch.
4. If the coupler does not appear in the catalog, exit Com Profibus, verify that the GSD filename extension is .GSD and not .gsd (Windows is case-insensitive, but some Com Profibus builds are not), and re-open the tool.

Configuring the IM 308C Side of the Coupler

1. In the Com Profibus network view, drag the DP/DP Coupler from the device catalog onto the Profibus line that represents the IM 308C segment. The tool assigns the next free slave address; if you need a specific Profibus address, edit the slave's properties dialog and enter it manually (valid range 1 to 125).
2. Open the slot configuration of the new slave. The DP/DP Coupler exposes a configurable number of input and output slots. Add slots until the data width matches the data width you will configure on the other side of the coupler. Common configurations are 16 inputs / 16 outputs (one word each way), 32 inputs / 32 outputs, or 64 inputs / 64 outputs.
3. For each slot, set the data type: Input or Output from the perspective of the IM 308C. The slot is one byte, one word, or one double-word; pick the granularity that matches the data you want to move.
4. Record the start addresses. The IM 308C presents the data to the S5 CPU in the peripheral I area (P area) and the extended P area. The P addresses that Com Profibus displays are the addresses you must use in the S5 program (e.g., PY 128 for a single byte at P address 128).

Configuring the Second Master Side

The second master on segment B is configured independently. The only requirement is symmetry across the coupler: for every input slot on the IM 308C side there must be a same-size output slot on the other side, and vice versa.

• Symmetry rule: An input area on the IM 308C side must appear as an output area of the same size and data type on the other side. An output area on the IM 308C side must appear as an input area of the same size and data type on the other side.
• Example: IM 308C configures 16 bytes of input at slot 0; the second master must configure 16 bytes of output at slot 0 of its DP/DP Coupler instance. The coupler simply copies the output bytes of side A into the input bytes of side B and the output bytes of side B into the input bytes of side A.
• Mismatched sizes: The DP/DP Coupler will still power up and exchange the configured data, but anything outside the symmetric region is undefined. Avoid asymmetric configurations: they make field diagnostics very difficult because the symptom (random values) looks like a wiring problem rather than a configuration error.

Data Image and S5 Address Mapping

The IM 308C places the DP slave's I/O into the S5 peripheral image. The starting P address depends on the IM 308C slot in the S5 rack and the offset configured in Com Profibus. A typical default for the first IM 308C is P address 0. If you stack a second IM 308C, Com Profibus automatically assigns the next available range.

Note that the actual P addresses depend on the project. Always read them from the Com Profibus configuration printout, not from a generic example.

Version 2 Coupler Specifics

The DP/DP Coupler Version 2 (6ES7 158-0AD02-0XA0 and related variants) extends the original V1 design. Differences relevant to an IM 308C integration include:

• Larger maximum data image: V2 supports up to 244 bytes input and 244 bytes output per side, partitioned into slots. V1 was limited to 32 bytes input / 32 bytes output per side in many firmware revisions.
• Extended diagnostics: V2 exposes additional diagnostic slots that report the state of the opposite Profibus segment (e.g., "partner bus failure"). You can map these into the IM 308C's P area and read them from the S5 program to detect when the second master drops out.
• GSD revision: The V2 GSD is not interchangeable with the V1 GSD. You cannot load a V1 GSD into Com Profibus and expect to access V2 features, and you cannot load a V2 GSD into Com Profibus older than V5.1.
• Configuration tool support: Com Profibus V5.1 or higher is required to import the V2 GSD. If you are running a legacy Com Profibus, plan an upgrade of the engineering PC before swapping the coupler hardware.

Verification

1. Power cycle the DP/DP Coupler and observe the front-panel LEDs. PWR should be solid green, BF1 and BF2 (bus fault per segment) should be off once both masters have completed the startup sequence.
2. In Com Profibus, use the online view (Target System > Online or equivalent) to read the diagnostic buffer of the IM 308C. A clean session shows no DP slave diagnostics for the coupler slave address.
3. Force a known pattern on the output side of one master (for example, write 16#AAAA to the first output word in the S5 program or the STEP 7 project) and read the corresponding input on the other master. Repeat the test in the opposite direction. Both directions must pass before commissioning is complete.
4. Pull the Profibus connector on the second-master side while monitoring the IM 308C diagnostic buffer. The DP/DP Coupler should report a partner bus failure (V2 only) and the IM 308C should log a slave diagnostic event. Reconnecting the connector should clear the fault within one DP cycle.
5. Pull the 24 V supply on the DP/DP Coupler. The IM 308C should report the slave as failed within the configured watchdog time. Restore the supply and verify that the data exchange resumes without restarting the S5 CPU.

Troubleshooting Matrix

Field-Proven Caveats

• The DP/DP Coupler does not filter or interpret the data. It is a transparent byte mover. Any endian mismatch, scaling error, or handshaking protocol must be implemented in the application programs of the two masters.
• If the S5 cell is later replaced by an S7-1500 with a Profinet/Profibus gateway, the DP/DP Coupler remains in the rack and continues to act as a Profibus slave to the new master. The data image is preserved, which simplifies migration.
• Always print and archive the Com Profibus slot table for the coupler slave. This is the single most useful document when troubleshooting a field incident three years later.
• The DP/DP Coupler does not support DP-V2 services such as isochronous mode or slave-to-slave communication. If the second master requires these, you need an IE/PB Link or a Profinet/Profibus gateway instead.

FAQ