Configuring the X2 PROFINET Interface on the S7-300 CPU 315-2 PN/DP

The SIMATIC S7-300 CPU 315-2 PN/DP (MLFB 6ES7 315-2EH14-0AB0) integrates a single PROFINET interface on the X2 socket that is presented externally as two RJ45 jacks (X2 P1 and X2 P2). Because the two jacks are the two faces of an internal Layer-2 switch, the CPU only exposes one logical PROFINET interface to STEP 7. NetPro therefore only allows one set of connections against that single interface, even though the topology editor will physically display both ports. This reference explains the architecture, walks through the configuration in STEP 7 V5.5, and shows the valid physical topologies for connecting one PC to two PLCs without a separate switch.

1. CPU Identification and Hardware Layout

The CPU 315-2 PN/DP is a 40 mm wide central processor designed for medium program/memory requirements and high-performance binary and floating-point arithmetic. The variant 6ES7 315-2EH14-0AB0 carries an integrated PROFINET interface with two RJ45 ports plus a combined MPI/PROFIBUS DP master-slave interface on X1. Always confirm the exact MLFB printed on the front flap before assuming the X2 architecture described here applies, because the older 6ES7 315-2EH13-0AB0 uses a different firmware base.

2. Why NetPro Only Shows One Connection

When you insert the CPU into the S7-300 station in HW Config, the PROFINET interface appears as a single PN-IO node with one IP address and one PROFINET device name. NetPro binds S7, TCP, ISO-on-TCP, and UDP connections to that single node, not to a physical socket. The two physical jacks on the front panel are represented inside the same node as port 1 and port 2; they exist for the following reasons only:

• Allowing a line topology where PLC #1 X2 P2 feeds PLC #2 X2 P1 directly without an external switch.
• Allowing the topology editor to draw a ring/line for diagnostics and port statistics.
• Providing cable redundancy at the port level (MRP) when both ends participate in an MRP domain.

Selecting the Profinet IO Topology editor in NetPro (right-click the PROFINET bus) reveals the two ports as graphical elements, but they cannot host distinct S7 connections. The dialog is informational; it does not enable port-level connection routing.

3. Prerequisites

1. STEP 7 V5.5 or later. The 6ES7 315-2EH14-0AB0 was added to the HW catalog at V5.5 SPx. With V5.4 the CPU does not appear in the catalog and HW Config will reject the station. The official entry is documented in the Siemens automation support article 36816516 (SIMATIC S7-300 CPU 315-2 PN/DP product information).
2. SIMATIC Manager or STEP 7 within the TIA Portal with S7-300 add-on. The configuration described here uses the classic STEP 7 V5.x interface (NetPro, HW Config), which is the standard toolchain for S7-300 PROFINET nodes.
3. PROFINET cabling. Cat 5e or better RJ45 patch cords. The CPU supports 100 Mbit/s full-duplex with auto-negotiation and auto-crossing on both ports.
4. PG/PC Ethernet interface on the engineering station.
5. Both CPUs powered and accessible on the same IP subnet (or with a router in between if the network design requires it).

4. STEP 7 Hardware Configuration

1. Open SIMATIC Manager and create a new S7-300 station project.
2. Open HW Config and drag the CPU 315-2 PN/DP (6ES7 315-2EH14-0AB0) from the catalog into slot 2 of the rail.
3. Double-click the PN-IO row inside the CPU to open the PROFINET interface properties.
4. On the General tab, click Properties... next to the subnet list and either assign the interface to an existing Ethernet subnet or create a new one (e.g., PN/IO_1).
5. Set the IP address (e.g., 192.168.0.10), Subnet mask (255.255.255.0), and a PROFINET device name (e.g., plc1). Use a unique name per CPU. The PROFINET name is case-sensitive and must match between the project and the device, otherwise PROFINET IO will not start.
6. Repeat steps 1–5 for the second CPU with a different IP (e.g., 192.168.0.11) and device name (e.g., plc2).
7. Save and compile the HW Config. The PROFINET interface is now ready for download.

5. NetPro Connection Configuration

NetPro is the only place where S7, TCP, ISO-on-TCP, and UDP connections are defined. For a typical PG ↔ PLC ↔ PLC scenario, configure the following connections against the single PROFINET interface of each CPU:

1. Open NetPro from HW Config (button on the toolbar) or from SIMATIC Manager.
2. The PROFINET subnet is shown as a yellow bus; each CPU appears as a single node on it. You will not see X2 P1 and X2 P2 as separate nodes.
3. For a PG/PC connection to PLC #1: select the PG/PC station, right-click the PROFINET node, choose Insert New Connection, and pick S7 Connection targeting plc1. The connection is bound to the PROFINET IP of PLC #1, not to a specific port.
4. For a PG/PC connection to PLC #2: create a second S7 connection on the same PG/PC station, this time targeting plc2.
5. For PLC-to-PLC communication (e.g., PUT/GET or BSEND/BRCV), add an S7 connection from the plc1 node to the plc2 node. The local interface is the PROFINET interface of plc1; the partner interface is the PROFINET interface of plc2.
6. Compile and download the net to both CPUs.

If NetPro only allows a single connection per CPU, that is the correct behavior. Each connection is unidirectional in the sense of S7 connection resource consumption, but the underlying socket uses the same PROFINET node. There is no port-selection field by design.

6. PROFINET IO Topology Editor

To inspect the physical cabling you can use the topology editor:

1. In NetPro, right-click the PROFINET bus line and choose PROFINET IO Topology.
2. The editor opens a graphical view that includes both ports of the CPU as separate port objects.
3. Drag a cable from PLC #1 X2 P2 to PLC #2 X2 P1. STEP 7 will then store this as the expected topology and will report a diagnostic alarm if the cable is moved to a different port at runtime.

The topology editor is also the right place to configure Media Redundancy Protocol (MRP) clients, although the CPU 315-2 PN/DP typically acts as an MRP client, not a manager. The MRP manager is usually a SCALANCE switch.

7. Physical Network Topologies for One PC and Two PLCs

The original question describes a constraint: one PC, two PLCs, all in line, and no external switch. The following three topologies cover the realistic options. Only the first uses the internal switch of the CPU as the network hub; the second and third add a switch but keep the physical line intact.

7.1 Pure line topology using the CPU internal switch

This is the configuration the original poster wanted to build. The PC plugs into X2 P1 of PLC #1, and a patch cord runs from X2 P2 of PLC #1 to X2 P1 of PLC #2. Because the two jacks are the same switch, the PC sees both PLCs on a single subnet. PG/PC routing inside STEP 7 only needs to know the two IP addresses; the physical port is irrelevant.

7.2 Switched star with one shared switch

When the second PLC must remain reachable even if PLC #1 is powered off, drop in a managed switch (e.g., SCALANCE XC-200). The internal switch of PLC #1 is then optional, and the two CPU ports are still treated as a single PROFINET interface by STEP 7.

7.3 Cascaded internal switches (daisy-chain with two segments)

Each PLC has two free ports; the PC and PLC #2 can both share PLC #1's internal switch, and PLC #2 can then continue the line to additional devices. This is the topology most PROFINET line-installation guides recommend because it removes the need for a cabinet switch.

8. Assigning the PG/PC Interface in STEP 7

To make the engineering PC see the CPUs, set the PG/PC interface once:

1. Open Set PG/PC Interface from the Windows Control Panel (or Options > Set PG/PC Interface in SIMATIC Manager).
2. Choose the access point S7ONLINE and the physical Ethernet adapter that is wired to the PROFINET network.
3. From SIMATIC Manager use PLC > Download > Target System to push the HW Config and the NetPro connection data into each CPU. Repeat for plc1 and plc2.
4. Use PLC > Diagnostics > Online > Accessible Nodes to confirm that both CPUs appear in the list. The default broadcast discovery works because all three devices share the same IP subnet.

9. PROFINET Device Naming and Address Assignment

Two pieces of identity matter: the IP address (used by the TCP/IP stack and by S7 connections) and the PROFINET name (used for PROFINET IO discovery and AR establishment). The two are independent.

• IP address — assigned in HW Config and pushed via PLC > Ethernet > Assign Ethernet Address.
• PROFINET name — assigned with the Profinet IO Diagnostics tool or with the Assign PROFINET Device Name wizard. The name is stored in non-volatile memory on the CPU and survives power cycles.

A common commissioning error is to download the project to the wrong target. STEP 7 will warn if the MAC address of the online CPU does not match the MAC address recorded in the project, but the warning can be suppressed. Always verify with Accessible Nodes before trusting the warning.

10. Connection Types and Their Limits

The S7-300 CPU 315-2 PN/DP supports the standard SIMATIC connection resource pool. The exact number of configurable S7 connections depends on the firmware version; do not invent a number for the -0AB0 revision without verifying against the device manual. The list below is the set of connection types you can choose from in NetPro, all bound to the single PROFINET node:

• S7 Connection — for S7 functions (PUT/GET, BSEND/BRCV, USEND/URCV) between SIMATIC PLCs and to S7-compatible HMI panels.
• S7 Connection (unspecified partner) — used for routing through the CPU as a gateway to other subnets.
• TCP / ISO-on-TCP / UDP — for open-communication blocks (T_CON, T_SEND, T_RECV, T_USEND).
• HTTP — the CPU has a small diagnostic web server that exposes the same X2 IP.

11. Troubleshooting Matrix

12. Verification and Commissioning Checklist

Run the following checks after every HW Config / NetPro download to confirm that the X2 interface and its connections are healthy:

1. Link LEDs on X2 P1 and X2 P2 — both should be solid green on every powered CPU. A flashing or off LED indicates cabling or auto-negotiation trouble.
2. PN-IO status in HW Config online view — should report "No fault" and the PROFINET name should match the project.
3. Accessible Nodes — both PLCs should appear with their configured IPs and PROFINET names.
4. Connection status in NetPro online — every S7 connection should be "established". A "not established" status points to a partner IP, subnet, or resource issue.
5. CPU diagnostic buffer — read with PLC > Diagnostics > Diagnostic Buffer. PROFINET events are time-stamped and give the exact port and partner involved.
6. Topology editor — compare online topology with the configured topology. A mismatch raises a maintenance alarm but does not stop communication.
7. PG/PC ping — ping 192.168.0.10 and ping 192.168.0.11 from the engineering station should respond in < 1 ms on a 100 Mbit/s segment.

13. Field-Proven Caveats

• Don't trust port LEDs alone. A 100 Mbit/s link is established on any Cat 5e cable up to 100 m even if the cable is wired wrong internally. Use the CPU's port-statistics counters (read via the web server or via the PROFINET diagnostics) to catch late or dropped frames before they show up as application errors.
• PROFINET name is case-sensitive. PLC1 and plc1 are different names to the device. STEP 7 will not warn if you re-assign the wrong name during a maintenance download.
• Browser-based web server requires JavaScript in older firmware. If you cannot reach the diagnostic pages, check the IP and HTTP port (default 80) before assuming the interface is down.
• Avoid cascading more than 50 PROFINET devices in a line when each device is a 2-port switch. The 100 Mbit/s bandwidth is shared and PROFINET IRT frames are sensitive to jitter. Use a star with a managed switch for high-jitter applications.
• Factory reset the PROFINET name if a CPU is moved from one project to another. The name is non-volatile; without resetting, the CPU will refuse to join the new AR.

14. Related Standards and Further Documentation

The PROFINET specification (IEC 61784-2 / IEC 61158) defines the line and ring topologies that this CPU supports. The PROFINET installation guideline (ZVEI / PI) gives the practical limits on cable length, number of devices, and grounding. The CPU-specific behavior for the X2 internal switch is documented in the SIMATIC S7-300 CPU 315-2 PN/DP device manual on the Siemens support pages; the entry point for the 6ES7 315-2EH14-0AB0 is the support article referenced in step 1 of the prerequisites above.