DP/DP Coupler vs GET/PUT for PROFIBUS Network Bridging

Overview

When two PROFIBUS DP networks must exchange process data, Siemens integrators typically choose between two fundamentally different mechanisms: S7 GET/PUT over the S7 communication layer and a DP/DP Coupler acting as a slave-to-slave gateway. Both methods move data between controllers, but they diverge sharply in architecture, throughput, electrical isolation, security boundary enforcement, and configuration overhead.

This reference consolidates field-proven criteria from the Helmholz DP/DP Coupler manual and the Siemens TIA Portal hardware catalog documentation with the engineering considerations that determine which mechanism belongs in a given machine or plant.

PROFIBUS DP Bridging Problem Space

A bridging requirement arises whenever two autonomous DP segments must exchange a bounded amount of I/O data, typically because:

• Each segment is owned by a separate DP master (Class 1) and re-architecting one master into a slave is not permissible.
• The two segments run at different baud rates or use different PROFIBUS address ranges.
• Different plant zones must remain electrically isolated for safety, grounding, or noise reasons.
• Functional or safety boundaries require that data crossing the bridge be limited to a fixed, pre-mapped set.

Each of these conditions maps directly to a strength of the DP/DP Coupler, while S7 GET/PUT solves a different problem class — fast, peer-to-peer variable exchange between cooperating S7 controllers that already trust each other.

S7 GET/PUT — Architecture and Properties

The GET and PUT function blocks (FB14 / FB15 in the standard S7 Communication package, plus the S7-1500 system block pair GET/PUT) implement the S7 Communication protocol. They allow one S7 CPU to read or write data areas of a partner CPU on the same logical S7 connection. The transport runs on top of the configured interface (MPI, PROFIBUS, or Industrial Ethernet) and is managed by the operating system, not by PROFIBUS DP slave services.

GET/PUT Configuration

1. Open NetPro / TIA Portal "Devices & Networks".
2. Right-click the source CPU → Properties → Communication → S7 Connection.
3. Define the partner CPU, the connection resource (connection ID), and the local/partner TSAPs.
4. In the user program, call FB14 GET / FB15 PUT with the configured connection ID, request and done bits, and source/destination area pointers (e.g. P#DB100.DBX0.0 BYTE 100).

Engineering Implications

• Variable granularity: GET/PUT can address arbitrary bit, byte, word, or DB areas — there is no fixed mapping table.
• Security: The local PLC programmer is trusted to only touch data for which authorization exists. There is no enforcement at the protocol boundary; any data area in the partner CPU that the connection resolves to is reachable.
• CPU support: GET/PUT is not universally available on every S7 CPU firmware/interface combination. Some older or stripped-down CPU variants do not expose S7 Communication services on the DP port — in those cases GET/PUT is not usable for PROFIBUS bridging at all, leaving the DP/DP Coupler as the only option.
• Hardware prerequisites: Both partners must have an interface capable of S7 Communication, and the link must be configured consistently.

DP/DP Coupler — Architecture and Properties

A DP/DP Coupler is a two-port PROFIBUS DP device. Internally it is implemented as two independent PROFIBUS DP slaves sharing a fixed, byte-oriented data map. Each port behaves toward its DP master exactly like any other DP slave: GSD file import, slave diagnostic, cyclic I/O exchange. The two slaves are not aware of each other — they simply mirror their input/output buffers through the device.

Functional Block Diagram

Key Technical Properties

Architectural Comparison

Throughput and Timing

The DP/DP Coupler mirrors data on every DP cycle. At 12 Mbps with a typical segment of 32 stations and the coupler occupying 16 input + 16 output slots, the bus rotation time on the slower segment dominates the propagation latency. The data crossing the coupler is therefore refreshed at the slower of the two segment cycle times, not at the combined rate.

For GET/PUT, the effective update rate is the call interval of the FB in the user OB (typically OB1 with a 100 ms cycle, but more often throttled to 50–200 ms to limit connection load). This makes GET/PUT unsuitable for tight isochronous coupling but acceptable for supervisory data exchange.

Numerical Example

Assume segment A runs at 1.5 Mbps with 24 slaves and segment B runs at 12 Mbps with 4 slaves. The DP cycle on segment A at 1.5 Mbps with that load is approximately 4–8 ms. Even though segment B is much faster, the mirror through the coupler only updates at the rate of segment A's cycle. Designers expecting near-zero latency must size both segments symmetrically.

Security and Network Isolation

The most underappreciated differentiator is the security boundary. With GET/PUT, the local PLC programmer can read or write any data the partner CPU exposes through the configured S7 connection. There is no protocol-level mechanism preventing a programmer from extending the area pointer and touching areas beyond the original design intent.

The DP/DP Coupler behaves as a fixed-mapping firewall:

• Each half is a slave with a strictly declared slot configuration.
• Only the bytes mapped into those slots traverse the device.
• A DP master on segment A has no protocol path to any device on segment B other than the bytes mirrored into the coupler's slots.
• The two segments can run at different baud rates and use entirely different PROFIBUS address schemes without coupling their address spaces.

This is the recommended pattern when segment A and segment B belong to different organizational units, different vendors, or different safety zones, and the contract is to exchange a bounded, well-defined data set only.

Hardware Selection

Siemens offers the 6ES7 158-0AD02-0XA0 DP/DP Coupler as the standard catalog reference, documented in the TIA Portal hardware catalog under Other field devices > PROFIBUS DP > Gateway. Third-party equivalents (Helmholz 700-158-0AD02, INSEVIS, etc.) are GSD-compatible and are typically used when extended diagnostic blocks, integrated PROFIBUS diagnostics, or vendor-specific configuration tools are preferred. Always import the GSD file matching the article number and firmware revision of the physical device.

Configuration Walk-Through (TIA Portal V20)

1. In the TIA Portal device catalog, locate the DP/DP Coupler under the appropriate path (per the Siemens documentation) and drag two instances onto the project — one per segment.
2. Connect each instance to the corresponding DP master system of segment A and segment B respectively.
3. Assign the PROFIBUS addresses (DIP-switched or software-set; the Helmholz manual describes both methods).
4. Open the device view of the first instance and configure the desired number of input/output slots (up to 16 each, total ≤ 244 bytes per direction).
5. Mirror the same slot configuration on the partner instance so that input slot 0 of segment A maps to output slot 0 of segment B and vice versa.
6. For data areas larger than 4 bytes, use SFC14 DPRD_DAT / SFC15 DPWR_DAT (or the equivalent system blocks on S7-1500) to guarantee consistent read/write across the consistency boundary.
7. Compile and download to both CPUs; verify that both masters report the coupler as "OK" without configuration fault.

Diagnostics and Fault Handling

The DP/DP Coupler reports standard DP-V0 diagnostics plus extended module diagnostics. The first six bytes conform to the PROFIBUS DP standard:

Bytes 6 and above carry extended module diagnostics. Per the Helmholz manual, these blocks convey port-level status (link down on one side, bus fault, power supply loss). On the master side, evaluate SFC13 DPNRM_DG (or DPNRM_DG system block on S7-1500) to read the full diagnostic frame and surface the cause on the HMI.

Failure Behavior

If segment A goes dark, the coupler continues to drive segment B with the last received output values from segment A's side (or, depending on configuration, substitutes configured substitute values). This fail-safe behavior is one of the primary arguments for the coupler over GET/PUT in process applications where controlled fall-back is part of the safety concept.

Selection Decision Matrix

Field-Proven Caveats

• Consistency: 128-byte consistent window applies to data crossing the coupler. If your structure exceeds that, split into multiple slots with offsets in HW Config.
• Dual supply: Wire both 24 V inputs from independent sources for the documented redundancy behavior. A single supply defeats the fail-safe path.
• Bus termination: Each PROFIBUS segment must be terminated per the standard. The coupler ports do not include built-in termination; use the standard PROFIBUS connector with termination switched ON at the segment ends.
• Address conflicts: Because each coupler half is a slave with its own PROFIBUS address, ensure those addresses do not collide with other slaves on either segment.
• Cyclic OB placement: On S7-1500, place DPRD_DAT / DPWR_DAT calls in a cyclic OB matched to the DP cycle, not in OB1 if OB1 runs faster than the bus rotation.
• Vendor mixing: When using a Helmholz or third-party coupler with Siemens masters, the GSD must be installed manually and the device inserted from the catalog after restart of the engineering tool.