Troubleshooting Siemens IM151-3 PN ST Ping Failure on PROFINET

Problem Overview

The ET 200S distributed I/O station built around the IM151-3 PN ST interface module continues to operate correctly on PROFINET — cyclic I/O is exchanged, the controller reports the device as reachable, and no diagnostic alarms are raised. Yet a standard ping issued from an engineering station, HMI panel, or a third-party network scanner returns "Destination host unreachable" or "Request timed out". Other PROFINET nodes on the same subnet (a CPU 315-2 PN/DP, an HMI panel, and Impact67 fieldbus modules) reply normally to the same ping request.

This is one of the most common confusion points in PROFINET commissioning: an operational device that "refuses" ping without actually being offline. Before swapping hardware, the engineer must distinguish between a real L2/L3 fault and a probe-method mismatch on a working device.

Affected Hardware and Order Numbers

The IM 151-3 PN family has several hardware revisions, all of which share the same basic PROFINET-IO device behavior described in this article:

The official device manual — ET 200S IM151-3 PN Interface Module Manual (PDF) — is the primary reference for diagnostics, port pinouts, and LED indicators discussed below.

Root Cause: ICMP vs TCP Port Probe

The ping utility in modern Windows and Linux is implemented as an ICMP Echo Request / Echo Reply exchange (protocol 1, type 8 / type 0). A number of monitoring tools, however, use a "TCP ping" or "TCP port probe" instead — they open a TCP SYN to a target port and treat a SYN-ACK as evidence that the host is alive. The two probes are not equivalent, and an embedded PROFINET device treats them very differently.

• ICMP Echo (RFC 792): The PROFINET stack on the IM151-3 PN answers this on a best-effort basis. In practice every IM151-3 PN (any firmware revision) replies to ICMP echo as long as the IP stack is up. A failed ICMP echo is therefore a real L3 fault.
• TCP port probe (e.g. port 7, 80, 8080, 49152): A closed port returns RST; a filtered or silently dropped port returns nothing. The IM151-3 PN does not run a user-configurable TCP listener on the well-known diagnostic ports by default — only the PROFINET RT/IRT channels (UDP/TCP ports reserved by the PROFINET specification) and, if enabled, the integrated web server (TCP/80) are open.

If a monitoring tool is set to "TCP ping" against port 7 (echo) or any non-listening port, the IM151-3 PN appears unresponsive. Switching the tool to ICMP, or probing a port that the device actually serves (TCP/80 if the web server is enabled, TCP/161 for SNMP if configured), restores the expected response.

PROFINET Communication Stack on the IM151-3 PN

To understand why a TCP probe fails, it helps to know which services the device stack actually exposes. The PROFINET IO device firmware in the IM151-3 PN binds the following sockets by default:

There is no user-accessible toggle to disable ICMP on the IM151-3 PN; PROFINET devices are required by the specification to handle DCP/UDP and ICMP for commissioning. If ICMP fails, the IP stack itself is the problem.

Network Topology and the XB008 Switch

The original installation used a Siemens SCALANCE XB008 unmanaged switch to fan the PROFINET line out to the controller, HMI, Impact67 stations, and the IM151-3 PN. Unmanaged switches do not filter ICMP, do not perform stateful inspection, and do not run a firewall — so the switch itself cannot be the reason ICMP is dropped.

That said, two switch-related issues still produce a "no ping" symptom and should be ruled out:

1. VLAN tagging mismatch. If a managed switch is later inserted in the path and ports are placed in different VLANs, ICMP is filtered at L2 even if the IP is correct. The XB008 has no VLAN concept, so this only applies if the topology changed.
2. Duplex / speed mismatch. PROFINET requires 100 Mbit/s full-duplex with auto-negotiation disabled or forced. A 10 Mbit/s half-duplex link still passes ARP but drops many ICMP frames. The IM151-3 PN ports are hard-wired to 100 Mbit/s FD with auto-crossover — verify with the LINK LED steady green and SF/BF off.

Diagnostic Procedure

Run the following sequence before assuming a hardware defect. Each step isolates one layer of the ISO/OSI model.

1. L1 — Physical link: Verify the RJ45 is seated, the patch cable is ≤ 100 m, and the LINK LED on the IM151-3 PN and on the switch port is steady green. Replace the patch cable if LINK is off.
2. L2 — ARP / MAC learning: From the engineering station, run arp -a and confirm the IM151-3 PN MAC address is listed with its assigned IPv4. The MAC is printed on the front of the module and on the shipping label; format is 00-0E-8C-xx-xx-xx for Siemens PROFINET devices.
3. L3 — ICMP probe: Issue ping -4 <IP> (force IPv4). The IM151-3 PN must reply. A "Request timed out" at this point indicates IP stack failure — usually missing device name.
4. L4 — DCP discovery: In STEP 7 (V5.5 + SPx) or TIA Portal, open Online → Accessible Nodes. The IM151-3 PN must appear in the list. If it does, the PROFINET stack is up; if it does not, the device name is unassigned.
5. L7 — Web server: Open http://<IP> in a browser. The integrated diagnostic page must load if the web server is enabled in the device properties.

Step-by-Step Resolution

The most frequent fix for an IM151-3 PN that "does not ping" is the device name. PROFINET IO devices do not accept IO AR (IO Application Relationship) requests until they have been assigned a name via DCP. Without that name, the IP stack still runs — but the IO controller's AR is rejected, the BF LED blinks, and many monitoring tools conclude the device is offline.

Assign the device name from STEP 7 / TIA Portal

1. Connect the engineering station to the same PROFINET subnet (physically on the XB008 or via a direct patch cable).
2. Set the network adapter of the PC to a free address in the same subnet (e.g. 192.168.0.100/24 when the device will be 192.168.0.10).
3. In STEP 7 V5.5: PLC → Ethernet Node → Browse. In TIA Portal: Online → Accessible Nodes → [search].
4. Select the IM151-3 PN MAC, then click Assign Name. Use the name configured in the HW Config / Device View (e.g. im151_3_st_01). Name syntax: max 240 characters, no spaces, no leading digit, no .-_ at the end.
5. Restart the interface module by power-cycling the ET 200S backplane or by toggling the 24 V supply. DCP name is non-volatile on modules with a SIMATIC Micro Memory Card inserted; without the card, the name is lost on power-off.

Set a static IP and confirm ICMP

1. Open the device properties → PROFINET interface → Ethernet addresses.
2. Set IP address = 192.168.0.10, Subnet mask = 255.255.255.0, leave Use router unchecked for a flat subnet.
3. Download the configuration to the IO controller (CPU 315-2 PN/DP or equivalent) so the controller pushes the IP via DCP at startup.
4. From the engineering station, run ping 192.168.0.10 -n 10. Expect 10/10 replies. The round-trip time on a 100 Mbit/s copper segment is typically < 1 ms.

Switching the Probe to ICMP

If the ICMP test passes after the above steps, the original "ping problem" is closed. If the user is using a third-party monitoring tool, switch the probe to ICMP explicitly:

For tools that do not expose ICMP, point the TCP probe at a port the device actually serves:

• TCP/80 — integrated web server (verify status with curl -v http://192.168.0.10)
• TCP/161 — SNMP (enable in device properties → SNMP → "Activate SNMP")

Probing TCP/7 (legacy echo) on a PROFINET device is not meaningful; the device does not implement RFC 862 echo service.

Verification and Functional Test

Confirm the fix end-to-end:

1. ICMP loop: 10 consecutive echo replies, 0% loss.
2. DCP discoverable: "Accessible Nodes" lists the IM151-3 PN with correct device name and IP.
3. PROFINET AR established: BF LED off, SF LED off, controller's online diagnostic view shows the device as "OK" and IO data is exchanged (green status on the slot icons).
4. Web server: Browser to http://<IP> returns the IM151-3 PN diagnostic page with module status, port statistics, and firmware version.
5. Cyclic I/O: Force a digital output at the controller; verify the corresponding LED on the ET 200S terminal module toggles within one PROFINET update cycle (typically 1 ms in RT mode).

Extended Diagnostics via the Web Server

The integrated web server (TCP/80) is the most powerful in-band diagnostic tool. It exposes:

• Module identification: order number, firmware version, hardware revision, serial number.
• Port statistics: discarded frames, CRC errors, late collisions — useful to detect a marginal cable.
• I/O diagnostics: per-channel fault state, channel value, and short-circuit / wire-break indicators.
• PROFINET diagnostics: device name, IP, AR state, MRP role, and PROFIenergy status.

If the web server is reachable but the controller still reports an I/O fault, the IM151-3 PN is healthy and the issue lies in the controller's project configuration (slot mapping, device name mismatch, GSD version).

Firmware and Compatibility Notes

Cross-reference the firmware on the device label with the project GSD:

The official manual contains the full list of supported GSD revisions and the recommended firmware for new installations — see the ET 200S IM151-3 PN Manual.

Troubleshooting Matrix

Field-Proven Caveats

• Windows firewall on the engineering station blocks ICMP by default on first boot. Allow inbound ICMPv4 echo replies or temporarily disable the firewall for diagnostics.
• Energy-saving NIC options ("Green Ethernet", "Energy Efficient Ethernet") on the PC can drop 100 Mbit/s PROFINET traffic after a few minutes of low utilization. Disable EEE on the PC NIC.
• DCP flooding from multiple engineering stations racing to assign a name can leave the device in a half-configured state. Run the assignment from a single station with no other PRONETA or TIA Portal sessions active.
• The IM151-3 PN does not support PROFINET Security Class 1 features (signed DCP). If the project enables secure DCP, name assignment will fail silently — uncheck the secure-DCP option in the IO controller's PROFINET interface properties.

FAQ