S7 Routing Through Siemens TP Panels: Why It Fails

Problem Statement: Ethernet-to-PROFIBUS Pass-Through Through a TP Panel

Engineers frequently attempt the following topology when commissioning or maintaining S7-300 systems:

The intent is to load, monitor, or modify the STEP 7 program on a SIMATIC S7-300 CPU (CPU 314, CPU 314C-2 DP, CPU 315-2 DP, etc.) without having to physically connect the programming device (PG) to the PROFIBUS (MPI/DP) interface of the PLC. The PG connects to the Ethernet port of a Siemens operator panel (TP 177, TP 277, OP 277, MP 277, MP 377, Comfort Panels where applicable), and the panel bridges through to the CPU on PROFIBUS.

This pass-through is supported on legacy operator systems such as the COROS OP5/OP15 family and on certain Beijer Electronics HMI devices that expose an integrated S7 routing layer. On current Siemens HMI panels, the request fails.

Root Cause: Siemens TP/OP/MP Panels Do Not Implement S7 Routing

The Siemens S7 Routing function is the only standardized mechanism that lets a PG traverse one physical network (e.g., Ethernet/PROFINET) and reach a CPU on a different physical network (e.g., PROFIBUS or MPI). The function is described in Siemens Support Entry 584459 and in the STEP 7 Online Help under "S7 Routing Functions."

S7 Routing requires the device that sits between the two subnets to act as an active router. It must terminate both subnets, hold routing tables, and forward S7 communication frames (S7 communication, programming, HMI) on behalf of the PG. SIMATIC S7-300 CPUs, S7-400 CPUs, and most CPs (e.g., CP 343-1, CP 443-1) implement this routing function in firmware.

Operator panels of the TP 177, TP 277, OP 277, MP 277, MP 377, and the current Comfort and Unified Comfort families do not implement S7 routing. They terminate the Ethernet network only as an HMI client and do not forward S7 protocol frames from the PG port onward to the PROFIBUS network. When STEP 7 tries to traverse the panel, the routing handshake fails with diagnostic entries such as:

• 0xFFFE — Module does not support the routing function.
• 0x8001 — Timeout: the partner (TP panel) did not respond to the routing request.
• STEP 7 message: "The partner does not support S7 routing."

S7 Routing Architecture: What Is Actually Required

S7 routing is a firmware feature of an S7 CPU or CP. The PG sees only one routing-aware device, but inside the project the engineer must program a "Router" subnet transition.

For routing to function, three conditions must all be true:

1. The router device sits on both subnets and has both interfaces online.
2. The router's project node in STEP 7 / TIA Portal shows both interfaces with valid addresses and a unique MPI/PROFIBUS node on each.
3. STEP 7 is configured with the routing path (e.g., PG → Ethernet (router) → PROFIBUS (target CPU)) using the S7 routing table built from the project.

S7-300 CPUs That Support Routing to PROFIBUS

Whether a CPU 314 family can route from Ethernet to PROFIBUS depends on its hardware variant and the interface modules installed. Use the table below to map the topology in your STEP 7 project.

For a CPU 314-2 DP installation like the one in the field report, the CPU itself has no Ethernet interface. S7 routing must therefore be provided by adding a CP 343-1 family module in the S7-300 rack. The CP handles the Ethernet side and, in cooperation with the host CPU, performs the S7 routing function. References: CP 343-1 Lean manual (Siemens Support), S7-300 Module Data manual.

Alternative Topologies When a TP Panel Is the Only Ethernet Drop

Three working architectures replace the failed "PG → TP panel → CPU" path. Choose based on the physical infrastructure already in place.

Option A — Add an Ethernet CP to the S7-300 Rack

1. Install a CP 343-1 Lean or CP 343-1 in the S7-300 rack (slot 4–11 depending on the existing configuration).
2. Connect the CP Ethernet port to the same physical Ethernet network the PG and the TP panel already use.
3. Configure the CP in HW Config (STEP 7) or in the device configuration (TIA Portal) with an IP address in the same subnet.
4. Enable the S7 routing function on the CP (default is ON in firmware ≥ V2.0).
5. Assign a unique MPI/PROFIBUS address on the DP side of the CPU. The CPU itself remains the PROFIBUS master; the CP does not consume an additional PROFIBUS node.

The PG then selects the S7 routing path PG → CP 343-1 (Ethernet) → CPU 314 (PROFIBUS). STEP 7 builds this routing table from the project; the engineer only specifies the target CPU and the "online → accessible nodes" view shows the CPU across the router.

Option B — Use the PG on PROFIBUS / MPI Directly

If the operator station is near the cabinet, the most reliable solution is a direct MPI/PROFIBUS connection between the PG and the CPU using a PC adapter (e.g., Siemens PC Adapter USB A2, 6GK1571-0BA00-0AA0, or PC Adapter USB A2 with order number 6ES7972-0CB20-0XA0 for the MPI cable variant). Pros: zero network change, fully supported. Cons: the PC must be physically near the panel or you must install a remote MPI/PROFIBUS tap.

Option C — Route via a PN/DP CPU on the Same Ethernet Segment

If the line has a CPU 315-2 PN/DP, CPU 317-2 PN/DP, or CPU 319-3 PN/DP on the same Ethernet segment, use that CPU as the router. The CPU's integrated PROFINET interface acts as the Ethernet side and its integrated DP interface acts as the PROFIBUS side. No additional hardware is needed. Configure the routing function in HW Config: Properties of the PN interface → "Use this module as a router" — the checkbox is enabled by default.

Configuration Procedure in STEP 7 (Classic) for the CP 343-1 Routing Path

1. Open the S7 project in STEP 7 V5.5 (or compatible).
2. In HW Config, drag a CP 343-1 Lean (6GK7343-1CX10-0XE0) into the S7-300 rack at an empty slot.
3. Double-click the CP and assign an IP address, subnet mask, and (if used) router address on the "Properties" → "Parameters" tab. Set "Use this module as a router" to checked.
4. On the PROFIBUS side, the CP requires no PROFIBUS node — it uses the MPI/DP of the host CPU. Ensure the CPU's MPI/DP interface is configured (e.g., address 2, 1.5 Mbps).
5. Save and compile (Station → Save and Compile). Download the HW configuration to the CPU and the CP.
6. Set the PG/PC interface to TCP/IP → Intel(R) Ethernet Connection ... (or the actual NIC) using "Set PG/PC Interface" in the Windows Start menu or in SIMATIC Manager under Options → Set PG/PC Interface.
7. From SIMATIC Manager, select the CPU target and click PLC → Online → Accessible Nodes. STEP 7 traverses the routing table and lists the CPU at its PROFIBUS address even though the PG is on Ethernet.

Configuration Procedure in TIA Portal for the PN/DP Routing Path

1. Open the project in TIA Portal V15.1 or later.
2. Select the S7-300 CPU with PROFINET interface (e.g., CPU 315-2 PN/DP) in Project Tree → Devices.
3. Open Properties → PROFINET interface → Ethernet addresses. Enter the IP address, subnet mask, and the connected subnet.
4. Open Properties → PROFINET interface → Operating mode. Confirm "IO controller" and ensure the option "Use PROFINET interface as router" is enabled (default).
5. Compile and download the device configuration to the CPU.
6. In the project, navigate to Online → Accessible devices. TIA Portal discovers both the PROFINET and the PROFIBUS sides of the CPU; selecting the PROFIBUS-side CPU opens the online view as if the PG were directly connected.

Verifying the Routing Path After Commissioning

After hardware and configuration are complete, verify end-to-end reachability:

1. From the PG, ping the CP 343-1 (or PN/DP CPU) Ethernet address. Expect replies.
2. In STEP 7 or TIA Portal, open Online → Accessible Nodes/Devices. Confirm that the routed CPU appears with its PROFIBUS address.
3. Open the online view of the routed CPU (PLC → Online → Monitor/Modify) and confirm DB values update.
4. Trigger a project download to the routed CPU and verify a successful transfer. If a download fails with diagnostic buffer entry "Communication error, partner not reachable", re-check the routing path and the firewall on the PG.
5. Check the diagnostic buffer of the router CPU/CP: a successful routing session logs entries of type "Communication established" with the PG as the partner.

Troubleshooting Matrix

Field-Proven Caveats and Edge Cases

• S7 routing across multiple hops: STEP 7 supports up to four routing hops in a single path. Multi-hop topologies are rare on S7-300 but possible when using a S7-400 as an intermediate router.
• Routing through firewalls: Industrial firewalls (e.g., SCALANCE S615, RUGGEDCOM ROX) must allow the S7 protocol (TCP port 102) and the S7-Routing protocol. Most do by default; verify the rule set.
• Routing with PROFINET IO devices: If a PROFINET IO device sits between the PG and the CPU, the device itself is not a router — only the CPU/CP is. The IO device is transparent to S7 routing at the network layer.
• Routing and VPN: Routing through a VPN tunnel works only if the VPN preserves the Ethernet frame (layer-2) or the IP routing supports broadcast/multicast for S7 discovery. Site-to-site IPSec with route-based VPN is preferred over policy-based VPN for STEP 7 routing.
• Online via cellular/WAN: S7 routing over cellular routers (SCALANCE M874, M876) is feasible but slow. Plan for 30–120 s delay on project downloads of large programs.
• Routing through a panel accidentally: If the project still contains the TP panel node, STEP 7 may attempt the path PG → TP → CPU first. Remove the panel from the routing path by explicitly selecting the CP 343-1 as the gateway when defining the online connection.

Safety and Operational Notes

Routing access to a running S7-300 CPU does not interrupt the process unless a block download triggers a STOP. To minimize risk during commissioning:

• Set the CPU to Process Mode (no STOP on download) before any online activity that might otherwise stop the PLC.
• Lock the CPU with a password (Properties → Protection → CPU → Write-protection password) to prevent unauthorized block downloads via the routing path.
• Document the routing path in the system manual so the next engineer does not waste hours on the TP panel pass-through attempt that this article addresses.

Summary

The pass-through path PG → TP 277 → S7-314 (PROFIBUS) cannot be implemented because Siemens operator panels of the TP/OP/MP families do not include the S7 routing function. The only standardized, supported mechanism for Ethernet-to-PROFIBUS access on an S7-300 station is to route through an S7-300 CPU that has an integrated PROFINET interface (CPU 31x PN/DP) or through a CP 343-1 family module. Add the CP, configure the IP parameters, mark the module as a router, and STEP 7 will discover the CPU across the PROFIBUS side of the project. The TP panel can remain connected in parallel — it is no longer in the routing path, but it continues its HMI role.