S7-400H Profibus Master Setup: Y-Link and DP Slave Integration

System Overview: S7-400H Redundant Profibus Master

The SIMATIC S7-400H (CPU 412-3H, 414-4H, 416-4H, 417-4H) is a fault-tolerant controller platform designed for processes that cannot tolerate a single point of failure. The H suffix designates the high-availability / hot-standby architecture, in which two identical CPUs run the same user program in lock-step and synchronize over fiber-optic redundancy links. For Profibus DP networking, the H-station typically uses one or two CP 443-5 Extended communication processors, each wired to a separate physical Profibus segment (Profibus(1) and Profibus(2)). The redundant Profibus segments terminate at a pair of IM 153-2 interface modules mounted in a redundant ET 200M station with an active backplane.

When a third-party DCS or a single-sided DP slave (one with only one Profibus port) must be integrated into the redundant system, an additional component is required: the Y-Link. The Y-Link allows a non-redundant Profibus segment to be attached to a redundant S7-400H system without breaking the fault-tolerant architecture. The Y-Link consists of two IM 153-2 modules and a Y-coupler, and from the H-CPU side the entire downstream network appears as a single DP slave. From the third-party DCS side, the Y-Link acts as the DP master on the lower segment.

Required Hardware and Software

Y-Link Architecture and Limitations

The Y-Link is the only Siemens-approved method to attach a non-redundant Profibus DP segment to a redundant S7-400H. The Y-Link occupies one Profibus address on each of the two H-CPU segments. Inside the Y-Link, the active bus module is selected by the active H-CPU; the inactive bus module continues to monitor the bus but does not drive it. Switchover between Y-Link bus modules is bumpless from the application perspective - the same I/O area is always available, but the data is sourced from whichever module is currently active.

For a single-sided Profibus slave, the Y-Link acts as a Profibus DP master on the lower segment. The third-party DCS, in turn, becomes a Profibus DP slave to the Y-Link, configured using its vendor GSD file. From the S7-400H point of view, the Y-Link is just one more DP slave; the third-party slaves are visible only on the lower segment of NetPro.

The hardware limit is the single-slave envelope: 244 bytes IN / 244 bytes OUT. This is independent of the number of slaves behind the Y-Link. For a single-sided Profibus slave using a Y-Link, the S7-400H is constrained to 244 bytes of cyclic input and 244 bytes of cyclic output per slot configuration. Plan the I/O count before commissioning.

Step-by-Step Configuration

Step 1: Install the Y-Link and Slave GSD Files

1. Download the Y-Link GSD file from the Siemens Product Support portal. The standard GSD is shipped with STEP 7, but for new firmware versions you may need the latest revision (for example SIEM816D.GSD for the Y-Link).
2. Copy the GSD file and the matching bitmap (e.g. SI0816D.BMP) into the STEP 7 GSD directory: C:\Program Files\Siemens\Automation\S7\Sys\PG\GSD or ...\STEP7\S7data\GSD.
3. Open SIMATIC Manager and select Options > Install GSD File. After installation, the Y-Link appears in the HW catalog under PROFIBUS DP > DP-Slaves > Y-Link.
4. Repeat for every third-party DP slave. The vendor GSD file is usually available on the vendor website or shipped on a CD with the slave. Place the file in the same directory and install.

Step 2: Configure the H-CPU Pair and the CP 443-5

1. Open SIMATIC Manager and create a new S7 project. Insert a SIMATIC 400H station (e.g. 412-3H or 414-4H depending on your application).
2. Open HW Config and place both CPUs in slot 3 of rack 0 and rack 1. The H-system requires both CPUs in the same slot position in two separate UR2-H racks.
3. Insert the CP 443-5 Extended in the same rack as the CPU. Configure a Profibus subnet for Profibus(1) with the desired master Profibus address (typically 1 or 2). The CP 443-5 acts as a passive slave to the H-CPU for diagnostic data and as a DP master for the lower-level devices.
4. Duplicate the configuration for the second CPU in rack 1, and configure the second CP 443-5 on Profibus(2). Profibus(1) and Profibus(2) are two physically separate buses, each terminated at a different Y-Link bus module.
5. Configure the synchronization module in slot 1 of each CPU (for CPU 41x-H with synchronization module interface).

Step 3: Place the Y-Link and the DP Slaves

1. In HW Config, drag the Y-Link from the catalog onto Profibus(1). Assign it a unique Profibus address (e.g. 3).
2. Open the Y-Link's properties and place it on Profibus(2) as well. The Y-Link must appear on both Profibus segments with the same logical address (3).
3. Open the lower Profibus subnet (the single-sided side of the Y-Link) and place the third-party DP slaves using their vendor GSD files. Assign a unique address to each slave (4, 5, 6, ...).
4. For each slave, open the configuration table and define the I/O modules (digital inputs, digital outputs, analog inputs, analog outputs). The configuration on the Y-Link's lower side must match the slave's actual physical I/O.

Step 4: NetPro Network Configuration

1. Open NetPro from SIMATIC Manager and select the S7-400H station. The two Profibus subnets (Profibus(1) and Profibus(2)) are visible as two bus segments.
2. Verify that the Y-Link appears on both subnets and that the third-party slaves appear only on the lower (single-sided) subnet connected to the Y-Link.
3. Compile and download the NetPro configuration (NetPro > PLC > Download to Target Station). With the H-system, the project is downloaded to both CPUs.
4. Use the Download to Target Station wizard and ensure that both H-CPUs are reachable (typically both on the same Ethernet subnet for online access).

DP Slave I/O Mapping and Addressing

Profibus DP maps the modules of a DP slave to the digital and analog standard addresses of the DP master. The mapping is configured in HW Config. When the slave is inserted into the bus, STEP 7 automatically proposes input/output addresses in the I/Q and PIW/PQW area of the H-CPU. You can edit these addresses as long as they do not conflict with the existing I/O or with the redundant ET 200M station.

The slave's perspective is the mirror image: inputs to the master are outputs from the slave, and outputs from the master are inputs to the slave. When configuring the third-party DCS as a slave, ensure that the vendor's I/O definition matches this mirror. The slave receives the data in its own input area and the master receives the data in its own input area; the addressing is a pure mailbox between two controllers.

In the table-style reference given in the source, a CPU 315-2DP is used as a DP slave with the same mapping: 1 byte of digital inputs at I 0.0 - I 0.7, 2 bytes of digital outputs at Q 0.0 - Q 1.7, 2 words of analog inputs at PIW 512 / PIW 514, and 4 words of analog outputs at PQW 512 - PQW 518. The slave is configured with a mirror set of modules: 1 byte of digital output (which the master reads as I 0.0 - I 0.7), 2 bytes of digital input (which the master writes as Q 0.0 - Q 1.7), 2 words of analog output (master reads as PIW 512 / PIW 514), and 4 words of analog input (master writes as PQW 512 - PQW 518). The size and order of the modules must match on both sides.

Required Organization Blocks

For a redundant S7-400H to operate, the following OBs must be loaded in the S7 program (in addition to OB 1):

For DP-master-specific behavior, the most relevant OBs are OB 82 (diagnostic interrupt from a DP slave), OB 86 (DP master system failure), and OB 70 / OB 72 for H-system redundancy. Loading dummy (empty) OBs for OB 70, OB 72, OB 80-87, OB 100, OB 121, and OB 122 is the standard commissioning practice to prevent the CPU from going into STOP if any of these events occur.

Sample Program Code

After STEP 7 compiles the HW Config and writes the address mapping, the cyclic OB 1 program simply accesses the configured I/Q and PIW/PQW addresses. No special function call is required to read a Profibus slave; the data is read by the DP master automatically every Profibus cycle and deposited in the input area. The addressing is straightforward: a slave occupying "Master I Slave Q 1 B unit" at Profibus address 1 is accessed in the master program by its absolute input / output addresses - for example I 1.0 for the first bit of the digital input byte, or Q 1.0 for the first bit of the digital output byte.

Example - read two analog inputs and one digital input from the third-party DCS at slave address 4:

// OB 1 - cyclic section
// Assume: digital input at I 4.0, analog at PIW 540 / PIW 542

// Latch the digital input into a flag
      A     I     4.0
      =     M     100.0      // Process flag

// Read the first 16-bit analog input and scale it
      L     PIW   540
      ITD                     // Integer to double integer
      DTR                     // Double integer to real
      L     2.764800e+01      // Scaling factor 27648 / 100
      *R
      T     MD   200          // Store scaled value (REAL) in MD 200

// Read the second analog input
      L     PIW   542
      ITD
      DTR
      L     1.000000e+00
      *R
      T     MD   204

Example - write a setpoint to the third-party DCS via the configured analog output at PQW 540:

// Scale an internal REAL setpoint (MD 300) into a 16-bit integer
// and write to PQW 540
      L     MD   300          // REAL setpoint, 0.0 ... 100.0
      L     2.764800e+01      // 27648 / 100, Siemens S7 normalization
      *R
      RND                     // Round to 32-bit integer
      T     PQW  540

Example - consistent read of a Profibus slave data area using SFC 14 (only required for buffers larger than 4 bytes):

// SFC 14 "DPRD_DAT" - consistent read from a DP slave
// LADDR := W#16#0200 (start address of the slave input area, here IB 512)
// RET_VAL := MW 50
// RECORD := P#M 100.0 BYTE 32 (target area in the flag area)

      CALL  "DPRD_DAT"
        LADDR  := W#16#0200
        RET_VAL:= MW50
        RECORD := P#M 100.0 BYTE 32
      NOP   0

Profibus Diagnostics Using FB 125 / FC 125

Siemens supplies the standard library "SIMATIC_NET" with diagnostic blocks for Profibus:

• FB 125 "DP_DIAG" - reads the diagnostic data of a DP master system. Output includes the slave address, the diagnostic status, the slot number, and the manufacturer-specific diagnostic bytes.
• FC 125 "DP_DIAG" - older variant for STEP 7 V5.

FB 125 is the recommended way to display the diagnostic state of every slave on the HMI or in the user program. The call is:

// FB 125 - DP_DIAG, single instance DB 125
      CALL "DP_DIAG", DB125
        DP_MASTER_SYS := 1          // Profibus(1)
        EXTERNAL_DP_INTERFACE := FALSE
        MANUAL_MODE := FALSE
        SINGLE_STEP_SLAVE := 0
        SINGLE_STEP_OP := 0
        ACK_BUTTON := M 200.0
        DIAG_OUT := DB125.DBD0
        RET_VAL := MW 210
        BUSY := M 201.0
      NOP   0

Commissioning and Verification

1. Power on the redundant ET 200M and verify that both IM 153-2 modules are in RUN with the SF and BF LEDs off.
2. Power on the Y-Link and the third-party slaves.
3. Connect the STEP 7 PG to either H-CPU. Open Accessible Nodes (PLC > Accessible Nodes) and confirm both H-CPUs are visible.
4. Download the hardware configuration and the user program. Both H-CPUs accept the download and synchronize.
5. Open Monitor/Modify in HW Config and verify that the configured input addresses (I 0.0, PIW 512, etc.) are updated as the slave changes its outputs.
6. Force an output in the output area (Q 0.0, PQW 512) and confirm that the corresponding value arrives at the slave. Many DCS vendors provide a Profibus slave test utility that displays the process image.
7. Open the diagnostic buffer of the H-CPU (PLC > Diagnostic Buffer) and confirm no OB 70, OB 72, or OB 86 events are pending. If events are present, the OB event class tells you the affected station.
8. Trigger a master-reserve switchover (H-system panel button or WinCC control) and verify that DP communication continues without interruption. The H-system re-routes the lower Profibus segment through the surviving Y-Link bus module within one Profibus cycle.
9. Pull the fiber-optic sync link on the master CPU and verify that the reserve CPU takes over and the DP slaves see no interruption. The diagnostic buffer of the reserve CPU should show OB 72 with the master-reserve switchover event.

Troubleshooting Matrix

Limitations and Field-Proven Caveats

• Y-Link as a single slave: The Y-Link appears as one DP slave to the H-CPUs. The sum of all slaves behind the Y-Link must still fit in 244 bytes IN / 244 bytes OUT. Plan your I/O count before commissioning.
• GSD maintenance: When the third-party DCS firmware changes its GSD revision, the existing Y-Link configuration must be re-validated. Always re-install the GSD and re-compile HW Config before commissioning the new firmware.
• Symbolic vs. absolute addressing: For clarity, especially when integrating with a third-party DCS, prefer absolute addressing (I, Q, PIW, PQW) for the Profibus data. The symbol table can still hold meaningful names; just resolve to the absolute address. The configuration table entry "Master I Slave Q 1 B unit" with address 1 is bit-level accessible - use I 1.0 / Q 1.0 or assign a symbol that points to those addresses.
• Hot-standby behavior: The user does not need to manage which H-CPU is currently the master. Both CPUs read the same input area; both write the same output area. The active H-CPU drives the Profibus segments. The reserve CPU executes the same program but its CP 443-5 is in passive mode.
• Master port on the CPU vs. CP 443-5: Some S7-400 CPUs have an integrated Profibus DP port. For the S7-400H, this built-in port is generally used for H-system synchronization modules, not for third-party slaves. Use the CP 443-5 Extended for the Y-Link and third-party slaves. The integrated DP port can be used as a non-redundant DP master if the application does not require CP 443-5 redundancy, but the standard H-system uses the CPs.
• One Y-Link per lower segment: Do not cascade Y-Links. A Y-Link must be the first segment downstream of the H-CPUs. Cascade by adding a second Y-Link to a different pair of CPs, or move the slaves to a non-redundant DP master outside the H-system.
• Watchdog time: Set the DP watchdog appropriately. With redundant segments and Y-Link switchover, the lower segment may go through a brief reconfiguration that exceeds the default 10 ms watchdog. Typical values are 100 ms - 1 s for non-critical slaves, with longer values for slaves that are slow to come back online.

Key Siemens Documentation References

• SIMATIC S7 Y-Link structure FAQ (entry ID 43688632)
• S7 CPU as single-sided slave (using Y-Link) to an S7-400H redundant system (entry ID 19951154)
• SIMATIC S7-400H Fault-Tolerant Systems, System Manual (entry ID 109476411)
• CP 443-5 Extended, Manual (entry ID 21489980)

Frequently Asked Questions