Configuring Redundant PROFIBUS DP Communication with DP/DP

Overview

Connecting a Siemens SIMATIC S7-400H redundant PLC to an external redundant Distributed Control System (DCS) over PROFIBUS DP requires careful selection of the gateway device. Two officially supported topologies exist:

• Dual DP/DP couplers – one DP/DP coupler terminated on each of the two redundant PROFIBUS DP networks of the S7-400H.
• Dual Y-Links plus one DP/DP coupler – one Y-Link on each S7-400H PROFIBUS DP subnet, with a single DP/DP coupler bridging the Y-Link downstream networks to the DCS.

Both topologies are described in the Siemens S7-400H system manual and in the SIMATIC DP/DP Coupler manual. The choice between them depends on cost, the number of PROFIBUS DP nodes already on each subnet, the diagnostic capability required, and whether the DCS expects a single PROFIBUS DP segment or two independent segments.

Why a Plain PROFIBUS DP Cable Will Not Work

A PROFIBUS DP segment allows exactly one active bus master class 1 per logical network. On an S7-400H system both CPUs (CPU 0 / CPU 1) operate as a single logical DP master through the H-sync mechanism, but the segment downstream of a Y-Link still presents only one active master at any time. Two redundant S7-400H systems, or one S7-400H plus a separate DCS master, cannot share the same copper PROFIBUS DP segment without arbitration. The DP/DP coupler isolates the two masters' electrical segments and converts data between them at the application layer using configured I/O mapping.

DP/DP Coupler Hardware

The Siemens DP/DP Coupler (order number 6ES7 158-0AD01-0XA0 for the standard version) is a DIN-rail mounted gateway that links two PROFIBUS DP networks. Each side of the coupler is a fully independent DP slave with its own PROFIBUS address and its own GSD file. Data exchange is performed by configuring matching input/output byte areas on each side; the coupler copies the output area of side A into the input area of side B and vice versa on every PROFIBUS cycle.

The coupler has no IP address and no parameter assignment in STEP 7 beyond the GSD file and a slave address. Configuration is identical to that of any DP slave: drop the GSD into the hardware catalog, place the device on the DP master system, and assign the input/output byte ranges.

Topology Option 1 – Dual DP/DP Couplers

In this configuration the S7-400H's two PROFIBUS DP master systems (H-CPU 0 subnet and H-CPU 1 subnet) each carry a DP/DP coupler. The DCS is also split into two PROFIBUS DP networks, and each one terminates on the DCS side of one coupler.

Addressing Principle

The two couplers appear to the S7-400H as two independent DP slaves with different PROFIBUS addresses (commonly chosen as 33 on subnet A and 34 on subnet B). The DCS side also sees two independent slaves. The PLC programmer reads/writes the data for coupler #1 from its own process image (e.g. PIB 100 … PIB 131) and the data for coupler #2 from a different range (e.g. PIB 200 … PIB 231). The DCS-side application must then arbitrate which of the two incoming data sets to use as the live value.

Failure Behaviour

• S7-400H failover (CPU 0 → CPU 1): the active master moves to CPU 1, the DP/DP coupler on subnet A loses its master, the DCS loses its data from subnet A but continues receiving data from subnet B through DP/DP coupler #2. The DCS controller application must detect the loss of input area A and switch over to input area B.
• DP/DP coupler failure: only the segment connected to that coupler is affected. The opposite coupler and segment continue running.
• One PROFIBUS segment break: identical to a CPU failover scenario for that segment; the redundant segment continues.

Topology Option 2 – Dual Y-Link plus One DP/DP Coupler

The Y-Link (order number 6ES7 197-1LA04-0XA0 for the PROFIBUS DP variant) is the Siemens-branded PROFIBUS PA / DP coupler with master redundancy. It connects a redundant PROFIBUS DP master system to a single-channel downstream PROFIBUS DP/PA network. Two Y-Links on the S7-400H side give the DCS a logical single PROFIBUS DP network terminated by one DP/DP coupler.

Advantages Over the Dual Coupler Layout

• The DCS sees a single PROFIBUS DP network with a single DP/DP coupler slave; no DCS-side redundancy arbitration code is required for I/O.
• The Y-Link handles the master redundancy switchover transparently to the DCS.
• Lower PROFIBUS address count and shorter DCS-side configuration.

Disadvantages

• Two Y-Links cost significantly more than two plain DP/DP couplers.
• The downstream segment becomes a single point of failure for the DP/DP coupler itself; a redundant DP/DP coupler cannot be installed without bringing the Y-Link redundancy concept into question.
• Y-Link adds ~6 ms of cycle-time overhead per DP cycle.

STEP 7 / TIA Portal Configuration

The DP/DP coupler is treated as a standard DP slave in both STEP 7 V5.x and TIA Portal. The official Siemens documentation for placing the device in the hardware catalog is in the TIA Portal help and in the STEP 7 hardware configuration manual.

Hardware Catalog Entry

1. Install the GSD file supplied with the DP/DP coupler. In STEP 7 V5.x use Options → Install GSD File. In TIA Portal use Options → Manage general station description files (GSD).
2. Open the hardware catalog and navigate to PROFIBUS DP → Gateway → DP/DP Coupler → 6ES7 158-0AD01.
3. Drag the device onto each PROFIBUS DP master system. Assign a unique PROFIBUS address on that master system (recommended: addresses 33 and 34 on the two S7-400H subnets).
4. Open the device properties and configure the input and output byte areas. Use the same byte count on both sides of the coupler; the maximum is 244 bytes per direction.

Sample Configuration – STEP 7 V5.x (HW Config)

Sample Configuration – TIA Portal V20 (Devices & Networks)

1. Select the DP master system of CPU 0 in the network view.
2. From the catalog pane, locate PROFIBUS DP → Gateway → DP/DP Coupler.
3. Drag the device onto the master system. Assign PROFIBUS address 33.
4. In Device view, drag input and output modules into the slots. The address assignment is automatic; rename the I/O tags in the PLC tag table.
5. Compile and download to the S7-400H station.

DCS-Side Configuration

The DCS must import the GSD of the DP/DP coupler and place the device on its PROFIBUS DP master system. The byte layout on the DCS side must mirror the layout on the S7-400H side, but with input and output reversed: whatever the S7-400H writes to QB 100..163 on subnet A appears as IB 100..163 on the DCS side of coupler #1.

For ABB 800xA / Compact Control Builder environments, Siemens provides an application example. The ABB-side configuration tool treats the DP/DP coupler as a PROFIBUS DP slave on the DCS network; the I/O area on the ABB side is mapped into ABB internal variables via the fieldbus configuration editor.

Diagnostic and Status Indicators

The DP/DP coupler exposes the following LED states for fast field diagnostics:

Within STEP 7 the DP/DP coupler generates the following standard DP diagnostics:

• 0x01 — Slave not in master project (configuration mismatch)
• 0x02 — Slave present but configuration differs from master
• 0x0C — Slave watchdog expired
• 0x0F — Invalid slave response / parameterization error
• 0x40 — Extension available (read with DPNRM_DG SFB 52 / RDREC)

Cycle-Time and Throughput Calculations

When planning redundant PROFIBUS DP communication, size the cycle time and the byte budget for both directions. The PROFIBUS DP cycle time on a segment with n slaves and a configured baud rate B can be approximated as:

T_cycle_ms ≈ (Sum of all slave I/O bytes × 11 bits / byte + 200 bits overhead per slave + 50 bits inter-frame gap) / (B in bit/s) × 1000

For the dual-coupler topology the S7-400H and the DCS each add their own PROFIBUS DP cycle time on their side of the coupler. The end-to-end latency from PLC tag update to DCS tag update is:

T_latency ≈ T_cycle_PLC + T_cycle_DCS + T_coupler_processing (~ 1 ms typical)

For 64 bytes each direction and 1.5 Mbit/s baud rate, T_cycle on a sparsely loaded segment is typically 3 – 5 ms per side. End-to-end latency in a healthy dual-coupler redundant system is therefore on the order of 7 – 12 ms. The Y-Link topology adds the Y-Link processing time and is typically 12 – 20 ms end-to-end.

Commissioning Procedure

1. Verify PROFIBUS DP cable length, terminating resistors, and baud rate on both subnets. Switch off terminating resistors on intermediate nodes.
2. Power up the DP/DP coupler(s). Confirm the ON LED is green and BF1/BF2 are off.
3. Download the STEP 7 / TIA Portal hardware configuration to the S7-400H. Observe that BF1 on each coupler goes from red-flashing to off.
4. Download the DCS hardware configuration. Confirm that the DCS sees the coupler as a live DP slave with all configured I/O bytes.
5. Perform a cross-traffic test — write a known pattern to QB 100 on the S7-400H side, read it back as IB 100 on the DCS side, and vice versa.
6. Trigger an S7-400H failover (CPU stop on the active CPU). Confirm that the DCS switches to the data set from the surviving coupler with no lost communications.
7. Pull a PROFIBUS connector on one subnet. Confirm that the opposite subnet continues to deliver data to the DCS.
8. Power-cycle one DP/DP coupler. Confirm that the opposite coupler continues to serve the DCS and the S7-400H surfaces a clear diagnostic.

Failure Mode Matrix

Selecting Between the Two Topologies

For most greenfield integrations between an S7-400H and a modern redundant DCS, the dual Y-Link topology is preferred when DCS-side simplicity outweighs cost. For retrofit or cost-sensitive applications the dual DP/DP coupler topology provides full redundancy at the expense of DCS-side arbitration logic.

Related Documentation

• SIMATIC DP/DP Coupler Manual (Siemens Support, Entry ID 1179382)
• TIA Portal V20 — DP/DP Coupler in the Hardware Catalog
• Connecting PROFIBUS DP slaves to a redundant CPU (109807303)
• PROFIBUS — DP/DP Coupler product finder
• ABB PROFIBUS Links & Coupler datasheet (DS/NDL-EN)

FAQ