Resolving IM 151-3 PN ST MAC Discovery Failures in STEP 7 V5.5

Problem Overview: IM 151-3 PN ST Not Found by STEP 7

The IM 151-3 PN ST is the PROFINET interface module of the SIMATIC ET 200S distributed I/O family. It terminates a PROFINET IO line, exposes two switched copper ports (X1P1, X1P2) on the front, and requires both an IP address and a PROFINET device name before it can be addressed by an IO controller. The first commissioning step is therefore almost always the same: open STEP 7 / SIMATIC Manager, navigate to PLC → Ethernet → Edit Ethernet Node, browse the network, and either select the module from the list or enter the MAC address printed on its front label.

On a healthy network the module appears in the browse dialog within a second. On a problem network nothing is returned. The BF LED on the module blinks at 2 Hz, the LINK LEDs on X1P1/X1P2 are off or intermittent, and entering the MAC manually produces the error "The Ethernet node is not accessible". The PLC is reachable, the PC can ping the controller, but the IO device that is physically connected to the same switch is invisible to STEP 7.

This article documents the root causes, the resolution path, and the verification steps that bring the IM 151-3 PN ST (and the related IM 151-3 PN / IM 151-3 PN HIGH FEATURE) back into STEP 7's discovery dialog without replacing hardware.

Affected Hardware and Firmware

Article number	Designation	Firmware	Notes
6ES7151-3AB23-0AB0	IM 151-3 PN ST	V2.0.0 and higher	Standard variant, RT only, two-port switch
6ES7151-3AA23-0AB0	IM 151-3 PN	V2.0.0 and higher	Basic variant described in the standard manual
6ES7151-3BA23-0AB0	IM 151-3 PN HIGH FEATURE	V3.0.0 and higher	Adds IRT, isochronous mode, C-PLUG support, shared device
6ES7151-3CA23-0AB0	IM 151-3 PN FO	V2.0.0 and higher	Fibre-optic variant, two POF ports

The first three modules share the same DCP discovery path and the same front-panel MAC sticker format. Diagnostics described below apply identically to the ST, standard, and HIGH FEATURE variants. Refer to the ET 200S IM 151-3 PN manual (6ES7151-3AA23-0AB0) and the ET 200S IM 151-3 PN HIGH FEATURE manual (6ES7151-3BA23-0AB0) for the official pinout and LED definitions.

Root Cause Analysis

STEP 7 V5.5's Edit Ethernet Node wizard uses the PROFINET Discovery and Configuration Protocol (DCP) over UDP port 34964 with the PROFINET multicast MAC 01-0E-CF-00-00-00 for the Identify request and a unicast Identify Response from the target. DCP is a Layer-2 protocol: it does not traverse routers and is rarely forwarded by managed switches unless PROFINET multicast filtering or LLDP-MED/PTCP-aware features are enabled. When the PC cannot reach the IM 151-3 PN ST by DCP, the failure is almost always caused by one of the conditions below, listed in order of frequency observed in field commissioning.

1. PC interface is on a different subnet or has no link-local address

DCP operates in the local broadcast domain. If the PC's Ethernet adapter has a fixed IP such as 192.168.0.50 / 24 and the IM 151-3 PN ST is on a separate 10.1.22.0 / 24 subnet that the PC is not directly connected to, DCP frames are simply not delivered. The PC must be physically patched into the same switch or directly to the IM and the IP of the PC must be in the same subnet (or the PC must have an additional adapter with a 0.0.0.0/0 route to that segment).

2. No IP address on the PC adapter, or APIPA / link-local only

STEP 7 sends DCP from any local IP, so a missing IP is not strictly fatal, but several NIC drivers and Windows firewall rule sets behave differently when the interface only has an APIPA address in 169.254.0.0/16. Force a static IP in the target subnet before opening Edit Ethernet Node.

3. Windows firewall or third-party endpoint protection blocks UDP 34964

Default Windows Defender rules allow DCP when the network profile is Private and block it on a Public profile. Norton, McAfee, Kaspersky, Trend Micro, and Sophos suites routinely block unsolicited UDP on engineering ports. Disable the third-party suite, switch the network profile to Private, or add an explicit allow rule for UDP 34964 in/out on S7ONLINE / SIMATIC Manager.

4. Multiple active Ethernet adapters

If the PC has a WLAN adapter active with a different subnet and a wired adapter with the target subnet, Windows binds outgoing UDP to the wrong source by mistake under specific routing table states. Open cmd as administrator and run route print; the destination subnet must resolve to the wired adapter. Disable the WLAN adapter temporarily.

5. Managed switch with PROFINET-aware filtering disabled, or DCP blocked by ACL

PROFINET uses LLDP for topology detection and DCP for naming. Some managed switches drop LLDP/DCP multicast by default. Enable PROFINET / DCP multicast pass-through or configure a static MAC entry for the IM 151-3 PN ST on the relevant switch port.

6. Module is in delivery state, no IP, no name, BF blinking 2 Hz

Out of the box the IM 151-3 PN ST has no IP and no PROFINET device name, and the BF LED blinks at 2 Hz to indicate that the IO controller has not established a PROFINET relationship. This is the normal starting state and is not in itself a fault — it is a precondition for the very step you are trying to perform.

7. Wrong MAC format in the manual entry field

STEP 7 V5.5's Edit Ethernet Node manual entry accepts the MAC with or without hyphens. With colons (00:0E:8C:xx:xx:xx) it works on V5.5 SP4 and later. Earlier service packs reject the colon form. Use the hyphen form 00-0E-8C-xx-xx-xx to be safe.

8. C-PLUG with a foreign configuration (HIGH FEATURE only)

On the IM 151-3 PN HIGH FEATURE, a C-PLUG containing another project's IP and device name will be loaded on every restart and overrides the in-module configuration. Remove the C-PLUG, power-cycle, and then re-assign. See the HIGH FEATURE manual section on C-PLUG behavior.

BF LED Blink Pattern Reference

BF LED	MAINT / SF LED	LINK LED (X1P1 or X1P2)	Meaning
Off	Off	Green on active port	PROFINET IO connection established, controller in RUN, no fault
2 Hz blink	Off	Green on at least one port	No PROFINET IO relationship, no configuration downloaded, or no device name
Solid on	Off	Off or on	Physical link to controller missing, duplicate IP, or duplicate name
Off	Yellow blink 0.5 Hz	Green	Maintenance demanded, e.g. port error counter exceeded threshold
Off	Solid yellow	Off	Module in firmware update mode, no PROFINET traffic
3 Hz flash	Off	Off	Self-test running, no IO traffic

The 2 Hz blink is the state you are correcting; it disappears the moment STEP 7 successfully writes the IP and device name AND the controller's PROFINET IO connection is established.

Prerequisites

Item	Requirement
STEP 7 / SIMATIC Manager	V5.5 + SP4 or later (HF1 recommended for Win10 / Win11)
Operating system	Windows 7 SP1 / 10 / 11; Windows Server 2016 or later for server SKUs
PC network adapter	Static IPv4 in the same subnet as the target IM, e.g. 10.1.22.100 / 24
Cable	Cat5e or better, max 100 m, no crossover required (Auto-MDI/X on all ET 200S PN ports)
Switch	Unmanaged is sufficient; managed must allow DCP multicast (01-0E-CF-00-00-00) and LLDP
MAC address	From the front sticker of the IM, format 00-0E-8C-xx-xx-xx
Firewall posture	Windows network profile set to Private OR explicit UDP 34964 allow rule
WLAN	Disabled on the engineering PC during commissioning

Step-by-Step Resolution Procedure

1. Capture the MAC address from the front of the IM 151-3 PN ST. On a new module the sticker is on the lower face of the housing. Format is six pairs of hex digits separated by hyphens, for example 00-0E-8C-A1-B2-C3. The same MAC is shown on the cardboard packaging label.
2. Patch the PC directly to port X1P1 of the IM 151-3 PN ST with a patch cable. Avoid going through the controller's PN port for the initial discovery; the integrated switch inside the IM will forward DCP, but a direct link removes one variable.
3. Configure the PC Ethernet adapter with a static IPv4 address in the same subnet you intend to assign to the IM. Example: PC 10.1.22.100 / 255.255.255.0, target IM 10.1.22.50 / 255.255.255.0. Leave gateway empty for direct connections.
4. Disable WLAN, Bluetooth PAN, and any Hyper-V virtual switches that bind to the physical adapter. ncpa.cpl should show only one active adapter during commissioning.
5. Set the active network profile to Private (Windows: Settings → Network & Internet → Properties). On a corporate-managed machine override the group policy if required.
6. Open SIMATIC Manager and load the project that owns the CPU (or open an offline project containing the ET 200S station). Select PLC → Ethernet → Edit Ethernet Node.
7. Click Browse. The IM 151-3 PN ST should appear in the list with its MAC, current IP (blank on first commissioning), and current device name (blank on first commissioning). If the list is empty after 10 s, jump to the troubleshooting matrix below.
8. Select the row corresponding to the MAC and click OK. In the Assign Ethernet Node dialog enter:
   • IP address: e.g. 10.1.22.50
   • Subnet mask: e.g. 255.255.255.0
   • Use router: unchecked for direct connections
   • PROFINET device name: e.g. et200s-im151-3 (must match the name typed in HW Config / NetPro for the IO device)
9. Click Assign IP Configuration, then Assign Name. The status bar at the bottom of the dialog should report "Assignment of IP address and name to the node was successful."
10. Close Edit Ethernet Node. Open the HW Config of the ET 200S station, confirm the IM is plugged at slot 0, and that the PROFINET IO device name on its properties dialog matches what you assigned. Save and compile (Station → Save and Compile).
11. Download the station configuration to the IM: select the IM in HW Config and choose PLC → Download to Target Station. Accept the prompt to assign the device name from HW Config. STEP 7 will re-issue a DCP Set Name request to overwrite whatever you typed manually.
12. Observe the LEDs: BF goes from 2 Hz blink to off, LINK on the active port stays green, and the MAINT / SF LED stays off on a healthy module.

Port Configuration in HW Config

On the IM 151-3 PN, port X1P1 and port X1P2 are configured inside HW Config. Open the properties of the IM, switch to the Port tab, and configure each port explicitly. The standard manual recommends leaving the ports at their defaults unless the network requires specific behavior.

Parameter	Typical default	When to change
Transmission mode / duplex	Automatic	Lock to 100 Mbit/s full duplex when connecting to a managed switch that does not auto-negotiate correctly
Monitor	Off	Enable on a single ring port for line diagnostics
Port diagnostics	On	Keep on; disabling hides the diagnostic interrupt on cable breaks
Transmission delay	Automatic	Constrain only when building a PROFINET IRT topology with high feature
Topology discovery	LLDP enabled	Required for neighbor detection and IRT slot timing

The IM 151-3 PN manual section "Properties of IM151-3 PN - Port" enumerates every port parameter; values not listed in your project default to the firmware defaults shown in the table.

Verification and Diagnostics

1. Ping the assigned IP from the engineering PC: ping 10.1.22.50 -t. Reply within 1 ms is expected on a directly connected segment.
2. In SIMATIC Manager open PLC → Diagnostics/Settings → Hardware Diagnostics. The IM 151-3 PN ST must appear in the online view with green status. Click the module and read the diagnostic buffer: an entry "PROFINET IO: AR established" confirms that the IO controller and the device are exchanging cyclic data.
3. Open the module information on the IM. Check Diagnostic → Port Statistics for X1P1 and X1P2. On a healthy network the discarded frame count, CRC error count, and late collision count are all zero. Watch the discard counter for 30 s while the system runs; it must not increment.
4. From a web browser navigate to http://10.1.22.50. The IM serves a read-only web page that exposes the device name, order number, firmware version, serial number, and a port-statistics table. Cross-check the firmware against the table above.
5. From the same browser fetch http://10.1.22.50/portstat.xml for machine-readable counters. The XML is suitable for logging into a commissioning report.

Alternative Discovery Methods When STEP 7 Browse Fails

If the Browse button in Edit Ethernet Node returns no nodes after 10 s of waiting, the DCP path is broken at the network layer. Use one of the following tools to localize the fault before changing STEP 7 settings.

Method A: PRONETA (Siemens free tool)

PRONETA is a free-of-charge engineering tool from Siemens that uses raw DCP and does not depend on STEP 7. Install it on the same PC, configure the adapter to the same subnet, and run Network Analysis. PRONETA will list every PROFINET device that responds to the multicast Identify, including the IM 151-3 PN ST, with its name, IP, MAC, and role. From PRONETA you can assign IP and name directly, which is often the fastest recovery path when STEP 7 refuses to.

Method B: Topology Editor inside TIA Portal

If you have TIA Portal V15.1 or later installed on the same PC, open Online → Accessible Nodes. The accessible-nodes scan uses the same DCP multicast as STEP 7 V5.5 but with a fresh socket binding, which sometimes succeeds where STEP 7's S7DOS helper does not. Once the IM is visible in TIA, set its IP and name there and re-open STEP 7.

Method C: Direct PROFINET DCP via Wireshark

Capture on the PC's Ethernet interface with Wireshark using the display filter pn_dcp. The Identify request is a UDP frame from the PC to multicast 01-0E-CF-00-00-00:34964. If no Identify Response is seen from the IM's MAC, the network is dropping DCP. If you see the Identify Response from the IM but STEP 7 still reports no nodes, the S7DOS / setpoint-IPC layer on the PC is the problem; reinstall STEP 7.

Firmware and STEP 7 Version Compatibility

IM firmware	STEP 7 V5.4 SPx	STEP 7 V5.5 + SPx	STEP 7 V5.6	TIA Portal
V1.0.0 (delivery)	SP3 required	All SPs	Supported	V13 SP1 and later
V2.0.0	SP4 required	All SPs	Supported	V14 and later
V3.0.0 (HIGH FEATURE only)	SP4 with HSP0180 / HSP0183	SP4 with HSP0183 / HSP0185	Supported	V15.1 and later
V3.1.0 (HIGH FEATURE only)	Not supported	SP4 with HSP0260	Supported	V16 and later

STEP 7 V5.5 SP3 and earlier cannot load the GSD file for V3.x IM 151-3 PN HIGH FEATURE. The fix for an environment that runs V5.5 SP3 is to install the latest hardware support package from the Siemens Industry Online Support portal, or fall back to V2.0.0 firmware on the module by reinstalling the older FW from the HSP archive.

Troubleshooting Matrix

Symptom	Likely root cause	Action
Browse returns empty, BF blinks 2 Hz, LINK off	No link, cable or port fault	Swap cable, try X1P2, verify switch port is up
Browse returns empty, BF blinks 2 Hz, LINK green on both ports	PC on different subnet or firewall	Set PC IP in target subnet, set profile to Private, disable third-party AV
Browse returns empty, BF solid on, LINK green	Duplicate IP or duplicate name on the segment	Disconnect other devices, retry, then trace nmap
Manual MAC entry returns "node not accessible"	MAC format wrong, DCP blocked, or PC in wrong subnet	Use hyphen form, switch subnet, check switch ACL
IP assignment succeeds, but BF keeps blinking 2 Hz	Device name not assigned, or name in HW Config does not match	Reopen Edit Ethernet Node, assign name, then download HW Config
IP and name assigned, hardware download fails with "device not found"	Controller in wrong subnet, or PROFINET interface not in same project	Recheck controller IP and PROFINET subnet settings in NetPro
Module accessible, ping works, AR fails to establish	Slot configuration mismatch or GSD version mismatch	Update GSD, recompile station, redownload
After restart the IM loses its name and reverts to MAC-only	C-PLUG with foreign config (HIGH FEATURE)	Remove C-PLUG, power-cycle, reassign
LED pattern is 3 Hz flash, no link on any port	Firmware update in progress or failed	Wait 2 min, then re-evaluate; if persistent, reflash via SIMATIC Manager

Field Commissioning Checklist

• MAC sticker recorded in the commissioning report with port assignment and patch-panel number
• PC adapter static IP set, gateway empty, network profile Private
• WLAN disabled, third-party AV temporarily disabled or UDP 34964 allow rule added
• DCP multicast 01-0E-CF-00-00-00 confirmed in switch ACL if a managed switch is in use
• IP, subnet mask, and PROFINET device name assigned via Edit Ethernet Node
• Device name in HW Config matches the assigned name character-for-character (PROFINET names are case-sensitive)
• Station configuration downloaded to the IM with PLC → Download to Target Station
• BF LED off, MAINT / SF off, LINK green on the active port
• Web server http://<IP> reachable, firmware version logged
• Port statistics captured for the commissioning report, error counters zero

Sample Diagnostic Buffer (Post-Resolution)

After a successful first commissioning the diagnostic buffer of the IM 151-3 PN ST reads, top-down, with the most recent entry at the top:

0x001E  PROFINET IO: AR established (AR #1, slot 0, subslot 1)
0x0001  Module parameterized successfully
0x0002  Module ready for operation
0x0045  PROFINET IO: Device name "et200s-im151-3" assigned
0x0044  PROFINET IO: IP address 10.1.22.50 assigned via DCP
0x0001  Restart after power up

Entry 0x001E confirms the IO controller has established an AR (Application Relationship) with the device. The AR is the cyclic PROFINET connection; until it appears, no IO data is exchanged regardless of how the IP and name are set.

DCP and PROFINET Naming Notes

PROFINET device names are limited to 240 characters, must be DNS-label compatible (a-z, 0-9, hyphen, dot), and are case-insensitive on the wire but case-sensitive in STEP 7's display. The IM 151-3 PN ST stores the name in non-volatile memory and uses it as part of the LLDP chassis ID. Replacing the module without re-assigning the name breaks the AR until the name is reissued from HW Config / NetPro.

For a CPU 314C-2 PN/DP or CPU 315-2 PN/DP acting as IO controller, the controller's PROFINET interface must be in the same IP subnet and on the same IO subnet. The subnet in NetPro is shown as PN-IO-1 by default; if a second PROFINET interface is added the subnet names diverge. Always check that the IM 151-3 PN ST and the CPU's PROFINET port are both members of the same IO subnet or no AR will be established even if the IP layer is healthy.

FAQ