ET 200S IM151-8: Fix Communications Server Error in STEP 7 V5.5

Problem Summary

When the Download button is pressed in SIMATIC Manager V5.5 targeting a SIMATIC ET 200S station with an IM 151-8 PN/DP interface module (article number 6ES7151-8AB00-0AB0), the dialog raises "The communications server could not be started". The PC has IP 192.168.0.2/24, the IM 151-8 is configured for 192.168.0.1/24, and the link is a green PROFINET patch cable (Type C, 100 Mbit/s).

The fault is raised in the SIMATIC Manager communication layer - the PLC is not the source of the error message. The S7DOS service (or the SIMATIC NET service pack depending on the STEP 7 V5.5 build) returns a WSAStartup or connect failed condition before any S7 protocol frame reaches the IM 151-8. The IM 151-8 sits on the ET 200S backplane and integrates an S7-300-compatible CPU with PROFINET IO Controller and PROFIBUS DP Master/Slave capability, so download failures can be recovered either through the PROFINET path or via the PROFIBUS path (IM 151-8 PN/DP exposes both).

Affected Hardware and Software

The PROFINET port on the IM 151-8 PN/DP is an RJ45 X3 P1 / P2 pair (with internal 2-port switch). The PROFIBUS interface is a 9-pin D-sub. Either may be used for the initial download if one path is broken.

Root Cause Analysis

The "communications server could not be started" failure is produced by the STEP 7 / SIMATIC NET layer when at least one of the following preconditions is not met. The same root cause analysis applies to STEP 7 V5.4 and V5.5, including SP8 / K5.5.0.42 and later service packs.

1. PG/PC interface not selected: Options > Set PG/PC Interface shows "None" or a non-TCP/IP entry such as PC internal (local).
2. TCP/IP auto-cross disabled: With Auto-MDI/MDX not negotiating, a crossover may be required on older PC NICs and the PROFINET cable is straight-through (PROFINET Type C, 4-wire). Direct PC-to-IM 151-8 connection is allowed by PROFINET cabling rules.
3. IP address conflict / duplicate: A router, a second NIC, or a virtual network adapter holds 192.168.0.2 on the PC side, so the bind() to that address fails.
4. Subnet mismatch: Subnet mask on the PC differs from 255.255.255.0 (e.g. 255.255.0.0) and the IM 151-8's gateway setting is inconsistent.
5. Windows firewall: step7.exe, S7DOS.exe, and SIMATIC NET services are blocked on the public profile. STEP 7 returns the communication server error if no listening socket can be opened.
6. S7DOS service not started: The S7DOS / SIMATIC Communication Server service is set to Disabled or fails to start (often after a Windows update has changed logon credentials).
7. STEP 7 installation corrupt: Missing or unregistered s7wtrapi.dll, s7otbxdx.dll, or s7wtrsx.dll. A second-hand install on Windows 10 / 11 without SP8 may not have updated these COM/DCOM components.
8. MMC card issues: 128 KB card is too small for the compiled S7 project (or empty / not inserted). IM 151-8 returns no PROFINET DCP response if the MMC is missing.
9. PROFINET cable / port: One of the two IM 151-8 ports is configured as PROFINET-disabled in HW Config and a switch with spanning tree blocks the second port.
10. Third-party security software: Antivirus, EDR, or DLP software intercepts SIMATIC NET calls.

Preliminary Verification Checklist

Before opening a STEP 7 project, perform these one-minute checks. Each item maps to one of the root causes above.

1. Verify the green PROFINET cable is a Type C patch cable (2 x 2 wires, 100 Mbit/s) and is fully inserted in IM 151-8 X3 P1 and the PC's RJ45 jack.
2. Confirm the link LED on the IM 151-8 PN port is solid green, not flashing. Steady green = link up; flashing green = activity; off = no link.
3. Open Control Panel > Network and Sharing Center > Ethernet > Properties > Internet Protocol Version 4 and confirm a static IP of 192.168.0.2 with mask 255.255.255.0. Uncheck IPv6 to remove a common DCP source of confusion.
4. Disable any virtual adapters (VMware / VirtualBox / Hyper-V vEthernet, VPN TAP, etc.) and disable all wireless radios.
5. Open a CMD and run arp -d * then ping 192.168.0.1. If the request times out, the IM 151-8 is not reachable - skip to Layer 1 below.
6. Open services.msc and confirm SIMATIC Communication Server (S7DOS) and SIMATIC NET services are Started and the startup type is Automatic.
7. Run Options > Set PG/PC Interface in SIMATIC Manager and ensure the active access point is S7ONLINE set to TCP/IP (Auto) -> <your NIC>.

Layer 1 - Physical and PROFINET Link Verification

The IM 151-8 PN/DP exposes two PROFINET ports (X3 P1 and X3 P2) wired through an internal 2-port switch. The link must come up at 100 Mbit/s full duplex for STEP 7 to discover the device via DCP (Discovery and Configuration Protocol, Ethertype 0x8892).

Step 1 - Cable and port test

1. Remove the green PROFINET cable and reseat it. Listen for the click.
2. Confirm the green Link LED is on. If the LED is off, swap to the second PROFINET port on the IM 151-8.
3. Test the cable with a cable tester or substitute a known-good Type C patch cable. PROFINET Type C uses only pins 1-2 and 3-6 (two pairs); 4-5 and 7-8 must be left unused per IEC 61784-5-3.
4. Check that no managed switch sits between PC and IM 151-8. If a switch is in the path and supports spanning tree, ports may stay in blocking state for 30-50 s. Bypass the switch and connect PC directly to the IM 151-8 to remove this variable.

Step 2 - Link speed verification

Open Device Manager > Network Adapters > [your NIC] > Properties > Advanced > Speed & Duplex. Set to 100 Mbps / Full Duplex (Auto-Negotiation is acceptable on PROFINET-certified NICs but manual is more deterministic for first-time commissioning). Verify the IM 151-8 PN port's negotiated speed in HW Config (Properties > Ethernet > Port 1 / Port 2).

Layer 3 - IP Addressing and PG/PC Interface

STEP 7 V5.5 uses the S7ONLINE access point. The active module of that access point must be a TCP/IP driver bound to the NIC that is physically connected to the IM 151-8.

PG/PC interface configuration

1. Open SIMATIC Manager and select Options > Set PG/PC Interface.
2. In Access Point of the Application, select S7ONLINE (this is the only default that STEP 7 downloads use).
3. In Interface Parameter Assignment Used, choose the row that reads TCP/IP (Auto) -> <your NIC> or simply TCP/IP -> <your NIC>.
4. Click Properties and confirm the NIC's MAC address matches the physical adapter connected to the IM 151-8.
5. Click OK and OK to commit. The PG/PC interface takes effect immediately; you do not need to restart STEP 7.

Address plan for the affected station

Do not assign a gateway to a single-segment commissioning link. The PC's gateway should be left at 0.0.0.0 to prevent Windows from sending a redirect that breaks the PROFINET DCP multicast on link-local 255.255.255.255.

Test with ping and arp

C:\> arp -d *
C:\> ping -n 4 192.168.0.1

Pinging 192.168.0.1 with 32 bytes of data:
Reply from 192.168.0.1: bytes=32 time<1ms TTL=64
Reply from 192.168.0.1: bytes=32 time<1ms TTL=64
Reply from 192.168.0.1: bytes=32 time<1ms TTL=64
Reply from 192.168.0.1: bytes=32 time<1ms TTL=64

C:\> arp -a
Interface: 192.168.0.2 --- 0xb
  Internet Address      Physical Address      Type
  192.168.0.1           00-0e-8c-xx-xx-xx     dynamic

The TTL of 64 confirms an embedded TCP stack (IM 151-8). If the TTL is 128, the responder is a Windows machine - the wrong target. If the request times out, the IM 151-8 is not on the link - go back to Layer 1.

STEP 7 V5.5 Communication Service Diagnosis

The error message text suggests a server start failure. The relevant service is S7DOS (S7 DOS Helper Service) - the helper that exposes ISO-on-TCP, ISO, and TCP transport to STEP 7. If S7DOS is not running, every download attempt fails with the same dialog.

Service verification

1. Open an elevated CMD and run sc query s7dos. State must be RUNNING. If STOPPED, run sc start s7dos.
2. Run sc query "SIMATIC Communication Server" as a synonym. Some Windows builds register the display name only.
3. Confirm the service logon is Local System with Allow service to interact with desktop unchecked.
4. If the service fails to start with Error 1053: The service did not respond to the start or control request in a timely fashion, another application is binding TCP port 102 (S7 default). Run netstat -ano | findstr :102 and identify the PID. Stop the conflicting service.

COM/DCOM re-registration

STEP 7 V5.5 registers several COM components that are essential to the S7DOS bridge. If the install was performed on a different user account, those components are visible to the current user but not registered for SYSTEM. Re-register them from an elevated CMD:

cd "C:\Program Files\Siemens\Automation\SIMATIC Manager\bin"
regsvr32 s7wtrapi.dll
regsvr32 s7otbxdx.dll
regsvr32 s7cmpsbx.dll
regsvr32 s7otbsx.dll
regsvr32 s7wspsx.dll

After re-registration, restart the S7DOS service and retry the download.

MMC Card and Firmware Verification

The IM 151-8 PN/DP does not have internal non-volatile program memory. The MMC (Micro Memory Card) is mandatory; the IM 151-8 will not respond to DCP discovery if the MMC is absent or corrupt.

MMC requirements

128 KB is the absolute minimum for IM 151-8 AB00 (firmware V3.0). The diagnostic buffer message "Card type not supported" or a stop LED pattern of 0.5 Hz on the IM 151-8 indicates an MMC size or format problem.

MMC health check

1. Power off the IM 151-8 (24 V off).
2. Remove the MMC.
3. Inspect the contacts for oxide or contamination. Clean with isopropyl alcohol and a lint-free swab.
4. Insert a known-good MMC with the same capacity (start with a 2 MB card for diagnostic purposes).
5. Reapply 24 V and observe the SF / BF / RUN / STOP LED pattern.

PROFINET Device Discovery via Edit Ethernet Node

If ping 192.168.0.1 fails but the link LED is solid green, the IM 151-8 has no IP yet. Use the Edit Ethernet Node function to assign the IP by MAC.

1. From the Windows Start menu, run SIMATIC > STEP 7 > Edit Ethernet Node (or in SIMATIC Manager, PLC > Ethernet > Edit Ethernet Node).
2. Click Browse. STEP 7 issues a DCP Identification request (Ethertype 0x8892) and lists every PROFINET device on the link.
3. Locate the IM 151-8 by its MAC address (printed on the front of the device, Siemens OUI e.g. 00-0E-8C-xx-xx-xx).
4. Select the device, then in Set IP configuration, tick Use IP parameters and enter 192.168.0.1, subnet 255.255.255.0.
5. Click Assign IP Configuration. The IM 151-8 stores the IP in its non-volatile memory and restarts PROFINET.
6. Close Edit Ethernet Node and verify with ping 192.168.0.1.

PROFIBUS Fallback Procedure

The IM 151-8 PN/DP has a PROFIBUS DP interface (9-pin D-sub, X2) configured as DP Master. If the PROFINET path cannot be recovered, the download can be performed over PROFIBUS using the integrated DP port and a CP 5611 / CP 5613 / CP 5711 PC adapter.

1. Connect the PC to the IM 151-8 X2 port via PROFIBUS cable and a CP 5611 A2 (or CP 5711 USB).
2. In SIMATIC Manager, open Options > Set PG/PC Interface and select the CP 5611 / CP 5711 PROFIBUS entry for the S7ONLINE access point.
3. Set the IM 151-8's PROFIBUS address to 2 (default) in HW Config or via the address switch on the side of the module.
4. Use Accessible Nodes in SIMATIC Manager (PLC > Accessible Nodes) to confirm the IM 151-8 is visible.
5. Download the corrected project. The IM 151-8 will write the PROFINET IP back to the MMC during the download.
6. Switch the PG/PC interface back to TCP/IP -> <your NIC> and retry the PROFINET download.

STEP 7 V5.5 Reinstallation and Repair

If the diagnostic service still does not start after PG/PC, firewall, and S7DOS checks, the STEP 7 installation is corrupt. Repair before a full reinstall.

Repair install

1. Insert the STEP 7 V5.5 DVD or mount the ISO. The SP8 / K5.5.0.42 or later update is required for Windows 10 / 11.
2. Run Setup.exe and choose Modify / Repair.
3. Accept the default component set. Setup will detect and re-register s7otbxdx.dll, s7dos.exe, and the SIMATIC NET communication drivers.
4. Reboot. Verify the S7DOS service starts automatically.

Clean reinstall

1. Open Control Panel > Programs and Features and uninstall SIMATIC Manager V5.5, SIMATIC NET, and S7-PCT if installed.
2. Delete the residual folders: C:\Program Files\Siemens\Automation, C:\Program Files\Common Files\Siemens, and C:\ProgramData\Siemens.
3. Remove leftover services: sc delete S7DOS, sc delete "SIMATIC Communication Server" (these will be re-created on install).
4. Reboot, then install STEP 7 V5.5 base, then the latest service pack (SP8 / K5.5.0.42 or newer).
5. Reinstall SIMATIC NET 2008 HF2 or newer to provide the TCP/IP -> <NIC> PG/PC option.
6. Reboot and re-apply the PG/PC interface selection.

Windows Firewall and Network Configuration

The Windows firewall frequently blocks the SIMATIC NET services. Apply the following rules to the active network profile (typically Domain for managed PCs or Private for engineering laptops).

Firewall rules (PowerShell, elevated)

New-NetFirewallRule -DisplayName "STEP7 S7DOS" -Direction Inbound -Program "C:\Program Files\Siemens\Automation\SIMATIC Manager\bin\S7DOS.exe" -Action Allow -Profile Any
New-NetFirewallRule -DisplayName "STEP7 Manager" -Direction Inbound -Program "C:\Program Files\Siemens\Automation\SIMATIC Manager\bin\step7.exe" -Action Allow -Profile Any
New-NetFirewallRule -DisplayName "S7 DOS Helper TCP 102" -Direction Inbound -LocalPort 102 -Protocol TCP -Action Allow -Profile Any
New-NetFirewallRule -DisplayName "PROFINET DCP 34964" -Direction Inbound -LocalPort 34964 -Protocol UDP -Action Allow -Profile Any
New-NetFirewallRule -DisplayName "PROFINET IO 0x8892" -Direction Inbound -Protocol 256 -Action Allow -Profile Any

Protocol 256 is the IP protocol number assigned to PROFINET IO (Ethertype 0x8892) per the PROFINET specification. After the rules are applied, run netsh advfirewall reset only if needed - the rules above are additive.

Disable IPv6 and other NICs

To remove confusion in DCP discovery, disable IPv6 on the PROFINET NIC. Use Network Connections > NIC > Properties > Uncheck "Internet Protocol Version 6". Repeat for any virtual NICs and disable Bluetooth PAN / Hyper-V vEthernet / VMware adapters before commissioning.

Verification and Validation Matrix

After applying fixes, run the following checks in sequence. Each step produces a pass/fail that the engineer can record on a commissioning sheet.

Field-Proven Tips and Escalation Path

After running the verification matrix, the IM 151-8 is either accepting the download or is still failing. The next escalation steps are drawn from maintenance shop-floor experience.

Tip 1 - Reset the IM 151-8 to factory IP

Hold the MRES button on the IM 151-8 for 5 s with the MMC removed. The PROFINET IP is cleared. Reinsert the MMC and assign a new IP via Edit Ethernet Node.

Tip 2 - Match the PG/PC profile to the project's PC station

The STEP 7 project contains a PC Station configuration that includes the S7ONLINE access point. If the active profile has the wrong NIC index, STEP 7 silently uses the wrong interface and the IM 151-8 appears offline. Open the PC Station in HW Config and verify the index matches the actual NIC.

Tip 3 - Re-flash firmware as a last resort

If the IM 151-8 AB00 has firmware < V3.0, certain MMC card types and certain STEP 7 service packs cannot download. Use SIMATIC Manager > PLC > Update Firmware to flash V3.0 or later. Firmware files (FW_UPDATE) for IM 151-8 are available on the Siemens Industry Online Support portal under entry ID 109750389.

Tip 4 - Test with a known-good PC

If the path still fails, swap the PC. A different NIC chipset, a different Windows build, or a different PG/PC profile often isolates the failure in minutes. The IM 151-8 is rarely the root cause when the link LED is on and the IM 151-8 responds to DCP browse.

Tip 5 - Escalate to Siemens support

If the IM 151-8 does not respond to Edit Ethernet Node browse, capture a Wireshark trace filtered to pn_dcp and attach it to the support request. The relevant support entry is PROFINET IO commissioning, entry ID 109750389 on the Siemens support portal.

Frequently Asked Questions