Configuring CPU 315-2 DP as PROFIBUS Slave: Master-Slave Setup

Configuring CPU 315-2 DP as a PROFIBUS DP Slave: Master-Slave Configuration with MPI-Routed HMI Access

This technical reference describes how to connect two SIMATIC S7-300 CPU 315-2 DP controllers via PROFIBUS DP so that one CPU acts as the DP master and the second CPU acts as a DP slave. It also documents how to wire an HMI panel to the master's MPI port and collect process data from both CPUs using SADD routing. The procedures apply to STEP 7 V5.x projects as well as the equivalent block construction in TIA Portal when migrating older SIMATIC S7-300 stations.

The CPU 315-2 DP integrates two physical interfaces in a single module: an MPI/PROFIBUS-DP combined port (labelled X1, RS-485) and a second dedicated PROFIBUS-DP port (labelled X2, RS-485). Both interfaces can be configured for DP-Master, DP-Slave, or passive DP-Slave operation, which is what makes the master-slave arrangement between two 315-2 DP controllers feasible without external coupler hardware. The same configuration is also used inside compact SIMATIC C7 systems; for example, the SIMATIC C7-633 DP control system integrates a CPU 315-2 DP that explicitly supports master or slave operation on PROFIBUS-DP.

1. Overview of the Target Topology

The network consists of three logical nodes:

• DP Master CPU — a CPU 315-2 DP (order number 6ES7315-2AFxx or 6ES7315-2AHxx) configured as a DP master on its X2 PROFIBUS port.
• DP Slave CPU — a second CPU 315-2 DP (same family) configured as a passive DP slave on its X2 PROFIBUS port.
• HMI panel — an MP/TP/OP panel connected to the master's X1 MPI port; data from the slave is reached via SADD routing through the master.

PROFIBUS DP is used purely for deterministic cyclic I/O exchange between the two CPUs (typically a few words of inputs and outputs mapped into the master's process image). The HMI panel is intentionally kept on MPI so that any firmware update or re-download of the slave project can still be performed from a single programming device that "rides" the master's MPI port and tunnels through to the slave via DP.

2. Prerequisites

2.1 Hardware

2.2 Software

3. CPU 315-2 DP Interface Details

The CPU 315-2 DP exposes two nine-pin sub-D female connectors labelled X1 and X2. Understanding their default role is essential because STEP 7's interface selection directly determines which mode is active.

At 1.5 Mbit/s (the default DP bus speed) the maximum segment length is 200 m; at 187.5 kbit/s it is 1,000 m. Choose the lower speed when mixing older DP slaves or when extending the segment across plant floors.

4. PROFIBUS DP Master-Slave Concept

In a PROFIBUS DP master-slave relationship, the master polls the slave cyclically. The slave never initiates communication. From the master's perspective the slave is simply a "virtual" DP device with a fixed number of input and output bytes. From the slave's perspective the master is a DP master to which a slave must declare its I/O footprint using a GSD file or via STEP 7's DP-Slave component definition.

Three configurations are possible on the CPU 315-2 DP:

1. DP-Master (DPM): the CPU owns the bus token and exchanges data with DP slaves.
2. DP-Slave (DPS): the CPU only responds when addressed; it cannot initiate any DP transfer.
3. Passive DP-Slave: identical to DP-Slave but the CPU cannot be parameterised by STEP 7 through the DP segment unless a routing-capable master is on the same bus.

For CPU-to-CPU cyclic exchange, mode (2) or (3) on the slave CPU is appropriate. The master CPU uses mode (1).

5. STEP 7 NetPro Configuration — Master Side

1. Open the SIMATIC Manager and create a new project (File > New > Project). Insert a SIMATIC 300 Station for the master CPU.
2. Open the HW Config of the master station and insert a RACK-300, then a PS-307, then a CPU 315-2 DP. Double-click the X2 (DP) row of the CPU and select PROFIBUS(1) as the interface type with DP master role.
3. Assign the master PROFIBUS address 2 and a bus profile of 1.5 Mbit/s.
4. In NetPro (button "Network" or menu Options > Configure Network) right-click the CPU and select "Insert New Connection". Choose "CPU-to-CPU (S7 Connection)" only if you want open S7 communication (PUT/GET, BSEND/BRCV). For pure cyclic I/O mirroring, no S7 connection is needed; the slave is inserted as a DP slave instead.
5. To insert the second CPU as a DP slave, switch back to HW Config of the master station, open the PROFIBUS(1) subnet and drag a CPU 31x entry from the hardware catalog under "PROFIBUS-DP > SIMATIC 300 > CPU 31x" into the bus line. A DP-slave pairing dialog appears.
6. In the pairing dialog select the slot that defines the slave's I/O footprint. A typical default provides 32 bytes of inputs and 32 bytes of outputs; reduce this to the real requirement (e.g., 8 in / 8 out) to shorten bus cycle time.

6. STEP 7 NetPro Configuration — Slave Side

1. In the same project, insert a second SIMATIC 300 Station (File > Insert > Station > SIMATIC 300). This becomes the DP slave CPU.
2. Open its HW Config and insert a RACK-300, PS-307, and a CPU 315-2 DP. Double-click the X2 (DP) row and choose PROFIBUS(1) with role DP slave. Set slave PROFIBUS address 3.
3. Assign the same bus profile (1.5 Mbit/s) used on the master.
4. Insert your application program blocks (OB1, FB/FC, DB). The process data that must be exchanged is moved into the I/O area you selected in step 5 of section 5. For example, if the slave exposes inputs starting at PIW 256, copy your data into a DB and use SFC 14 (DPRD_DAT) only if you used a DP-V1 slot; for the classic CPU-to-CPU DP-Slave mode used here, direct process-image access is sufficient.
5. Save and compile (Station > Save and Compile). The download to the slave CPU can be performed either through the master's MPI port (SADD routing) or via a separate programming cable on the slave's MPI.

7. HMI Routing (SADD Concept)

SADD routing allows a panel configured with the master's MPI node to read or write tags that physically live in the slave CPU. STEP 7 implements this through a "S7 connection" with the Active Connection Establishment flag disabled on the slave and enabled on the master, while the panel uses a single "Routing" connection type.

7.1 Configuring the Routing Connection in NetPro

1. In NetPro select the master CPU, right-click and choose "Insert New Connection".
2. Pick the slave CPU as the partner. The connection type is S7 connection.
3. In the connection properties, tab "General", confirm:
   • Local ID = 1 (auto-assigned)
   • Partner address = PROFIBUS address 3
   • Connection path traverses the PROFIBUS subnet
4. Generate and download the routing table (Options > Configure Network > Download to all Stations).

7.2 Configuring the HMI Project

1. Open WinCC flexible (or ProTool). Create a new project.
2. In the connection editor add a connection with type SIMATIC S7-300/400, protocol S7 Routing, point to the master CPU's MPI address (2), and specify the partner's PROFIBUS address (3) inside the routing extension. The wizard field "Partner CPU / Routing via" defines which intermediate node forwards the request.
3. Repeat for the master CPU without routing enabled for direct master tags.
4. Compile and transfer the HMI project. The panel now lists two connections: one direct, one routed.

8. Cyclic I/O Handshake Sample

The following snippets show how to exchange 8 bytes in each direction between the two CPUs. The master writes to the slave's outputs at PQB 256..263, and reads the slave's inputs at PIB 256..263. The slave does the same with PIW/PQW at the same offset.

8.1 Master CPU — OB1 Excerpt (STL)

      L     DB10.DBW 0        // Master setpoint word 1
      T     PQW 256           // -> Slave PIB 256
      L     PIB 256           // Slave status word 1
      T     DB10.DBW 2        // Store in master DB

8.2 Slave CPU — OB1 Excerpt (STL)

      L     PIB 256           // Master setpoint word
      T     DB20.DBW 0
      L     DB20.DBW 2        // Slave status word
      T     PQW 256           // -> Master PIB 256

The bus cycle time t_cycle for the configuration above with 8 input + 8 output bytes at 1.5 Mbit/s is approximately 1.5 ms plus the slave's DP-Slave response time (typically < 1 ms). For full PROFIBUS cycle calculations refer to PROFIBUS DP Cycle Time in the Siemens STEP 7 online help.

9. Verification & Diagnostics

9.1 Online Diagnostics in STEP 7

1. Open HW Config of the master, right-click the slave CPU in the PROFIBUS line and select "Monitor/Modify".
2. Status "OK" and green icon confirm successful DP-Slave parameter assignment. A yellow triangle indicates a parameter mismatch (typically different bus profile or GSD revision).
3. Use the diagnostic buffer of the slave CPU (CPU > Module Information > Diagnostic Buffer) to inspect DP-V0 events. Event ID 3942 means "DP slave not found", 3943 means "DP slave parameter error".

9.2 LED Indicators on the CPU 315-2 DP

9.3 HMI Tag Test

1. On the panel, enter the service menu and select “Tag diagnosis” (WinCC flexible) or “Tag simulation” (ProTool).
2. Force a bit in the master DB and verify that the slave receives it within one bus cycle.
3. Force a bit in the slave DB and verify that the panel updates its routed tag within the configured update time (default 1 s).

10. Troubleshooting Matrix

11. Compatibility Notes and Migration

11.1 ProTool vs WinCC flexible vs WinCC (classic)

Routing the HMI across an MPI/DP boundary is supported by:

• ProTool V6.0 SP3 and later — the connection must be defined explicitly as "Routing" in ProTool and the master must be a CPU that supports S7 routing (CPU 314, CPU 315-2 DP, CPU 317, CPU 319).
• WinCC flexible 2005 SP1 and later — same routing concept; multiple routed connections are possible.
• WinCC (classic, V7.x) — routing across DP-to-MPI is not supported. Use WinCC flexible or migrate to TIA Portal.

11.2 TIA Portal Equivalent

When porting the project to TIA Portal (V13 SP1 or higher), the topology is identical: two S7-300 stations connected by a PROFIBUS subnet, the slave CPU configured as a DP-Slave in Devices & Networks, and a Comfort/Advanced panel with two HMI connections (one direct, one routed). The SADD routing is configured automatically when the connection path traverses the PROFIBUS subnet and the master CPU is online.

11.3 Use Cases

The same configuration pattern is field-proven for the following applications:

• Two-axis motion slaves each running FM 353/354 positioning modules, controlled by a master CPU that hosts the HMI and additional remote I/O on the same DP segment.
• Master CPU 314C-2 DP gathering fast local I/O on MPI while routing data to/from an IM 151-1 DP slave station on the backplane bus.
• C7-633 DP acting as a compact master controller with two additional C7-633 DP units acting as slaves, sharing recipe data and status.

12. Field-Proven Cautions

1. Terminating resistors: Only the first and last physical device on the PROFIBUS segment may have the terminating resistor ON. With two CPUs and a panel on MPI this is rarely a problem, but when adding a repeater, double-check its DIP switches.
2. Address uniqueness: Set the master PROFIBUS address to 2, the slave to 3, and never assign an MPI address that collides with a DP address on the same physical node. CPUs interpret the number as DP if the port is in DP mode.
3. Bus profile: Mixing 1.5 Mbit/s and 12 Mbit/s on the same segment is impossible; profile must be uniform.
4. Cable shielding: Connect the cable shield at every connector clamp; leave the shield ungrounded at the panel side if the panel is far from the cabinet to avoid ground loops.
5. STEP 7 version drift: Older SP versions of STEP 7 V5.4 do not always download the routing table correctly. Use V5.4 SP5 or V5.5 to be safe.
6. Power-on sequence: Power the master first so that the slave sees a valid master during its own boot, otherwise the slave may enter STOP with diagnostic buffer entry "DP master not found".

13. Quick Commissioning Checklist

• [ ] Master CPU 315-2 DP powered first, X2 in DP-Master mode, address 2
• [ ] Slave CPU 315-2 DP powered second, X2 in DP-Slave mode, address 3
• [ ] Bus profile 1.5 Mbit/s identical on both stations
• [ ] Terminating resistors ON only at the two extreme nodes
• [ ] HW Config of master includes the slave as a DP-Slave with correct slot length
• [ ] NetPro S7 connection between master and slave downloaded to both CPUs
• [ ] Panel connection defined as "Routing" with partner PROFIBUS address 3
• [ ] HMI transfer tested for both direct and routed connections
• [ ] Cyclic I/O loopback verified with simple counter increment
• [ ] Diagnostic buffer of slave contains no entries after one hour of operation