Resolving Siemens 840Dsl X127 Online Connection Failures ("Partner No Response")

On a SINUMERIK 840D sl (NCU 710.3 PN, NCU 720.3 PN, NCU 730.3 PN or NX10/NX15), engineers and service technicians commonly use the X127 service interface on the front of the NCU to load PLC, NCK and HMI projects from TIA Portal (or the legacy STEP 7 / SinuTrain commissioning tool) into the integrated CP 840D sl communications processor. A frequent field fault is that the PC can upload the project from the controller but cannot go online or download, with TIA Portal returning a 0x0000134F-class dialog or simply "Partner no response / The TCP connection to (IP) could not be established". The root cause is almost always an out-of-range CP IP address or a wrong PC subnet configuration on the X127 service port.

1. Problem Description and Symptoms

Typical field scenario on a 840D sl NCU:

• A commissioning PC is connected directly to the X127 port using a crossed or 1:1 patch cable (X127 is Auto-MDIX, so a straight patch is acceptable on most NCU revisions).
• The hardware configuration and the S7 PLC program can be uploaded from the controller to the PC (read direction works).
• Selecting Online → Go online in TIA Portal V17/V18/V19/V20 returns one of the following:
  • "Partner no response within the configured time"
  • "The TCP/IP connection could not be established"
  • "Online: There is no connection to the selected target system."
• Downloading the PLC project (S7-1500 software controller inside the NCU) is impossible; PG functions such as monitor/modify, online diagnostics, and trace fail.

The first thing to verify is the CP 840D sl IP address as configured in the STEP 7 / TIA Portal hardware configuration (HWCN) inside the project, and the actual live IP that the NCU is using after Power-On / NCK reset.

2. Root Cause Analysis

There are three root causes that recur in 840D sl installations:

2.1 CP IP address outside the X127 service subnet

Siemens ships the X127 service interface with a fixed, non-routable address so the service PC and the NCU are guaranteed to be in the same broadcast domain. The default is:

When the CP address is changed to something like 192.168.0.1/24, 192.168.30.1/24, or any other /24 network, the PC's default Windows TCP/IP stack will still sit on its old /24 subnet (e.g., 192.168.214.0/24 from a previous session on X120). The result is a subnet mismatch: ARP requests go unanswered, and TIA Portal waits for the S7 handshake (port 102 / ISO-on-TCP) until the configured PG timeout elapses. The diagnostic is not "wrong protocol" but "wrong Layer-3 reachability".

2.2 PC IP on the wrong subnet or with an unexpected static entry

If the PC was previously connected to X120 (X120 default = 192.168.214.1/24) or X130 (192.168.214.1/24 PROFINET side), it will still hold a 192.168.214.x/24 lease or static entry. Plugging that same PC into X127 with the NCU at 192.168.215.1/27 yields no route between the two.

2.3 Half-loaded CP configuration (PLC stops)

Field reports show that changing the CP IP from 192.168.214.1 (X120 plant network) to 192.168.0.1 and downloading the new HWCN to the NCU has caused the internal S7-1500 software controller to stop. The PLC stop is triggered by the inconsistency between the new CP IP and the internally configured PROFINET device names / IP suites of the integrated NCK. The safe rule is: keep X127 on 192.168.215.1/27 and X120/X130 on 192.168.214.1/24 unless you have a documented reason to change them.

3. The X127, X120 and X130 Interface Map

The 840D sl NCU exposes three (or four) Ethernet ports. Knowing which is which is essential before troubleshooting online access.

On most current NCUs the CP 840D sl is the single PROFINET device with two PROFINET interfaces: port 1 = X120, port 2 = X130 (PN interface). X127 is a separate Ethernet port bridged to the same CP for service traffic only. Per the Softing commissioning guide for uaGate 840D, X127 is intended strictly for service access and "cannot be used for connection purposes" when establishing a third-party S7 or OPC UA gateway to the plant network.

4. Step-by-Step Solution

The following procedure restores online access via X127 and is the recommended baseline for every 840D sl installation.

4.1 Prerequisites

1. PC with TIA Portal V16 or later (V17/V18/V19/V20 supported), installed on Windows 10 / Windows 11.
2. Direct Ethernet cable between the PC NIC and the NCU X127 port. Auto-MDIX handles crossover on most NCU variants (NCU 720.3 PN, NCU 730.3 PN, NX15).
3. Project archive or offline TIA Portal project that contains the 840D sl device with a configured CP 840D sl module.
4. Administrator rights on the PC (required to change the NIC and to add a Siemens PG/PC interface).

4.2 Restore the CP 840D sl address to factory defaults

1. Open the project in TIA Portal.
2. Open the Devices & Networks editor and double-click the CP 840D sl module.
3. In Properties → Ethernet addresses (or PROFINET interface → Ethernet addresses for the X120/X130 side), set:
   • IP address: 192.168.215.1
   • Subnet mask: 255.255.255.224
4. Open the X127 service interface entry and verify it shows the same address / subnet; if the editor exposes the field, leave the X127 assignment checked.
5. Compile the hardware (HW) configuration: Project tree → 840D sl device → Compile → Hardware (rebuild all).

4.3 Configure the PC service interface

1. Open Control Panel → Network and Internet → Network and Sharing Center → Change adapter settings on the PC.
2. Select the NIC connected to the NCU and open Properties → Internet Protocol Version 4 (TCP/IPv4).
3. Recommended setting (Siemens default): select Obtain an IP address automatically and Obtain DNS server address automatically. The X127 service interface is configured as a DHCP server on SINUMERIK 840D sl from firmware 4.5 onward and will hand out an address in the 192.168.215.0/27 range, e.g. 192.168.215.2.
4. Manual alternative (only if DHCP is disabled on the NCU side or the PC is on a locked-down network): assign 192.168.215.2 with subnet 255.255.255.224 and leave the gateway empty.

4.4 Verify the layer-3 path

1. Open an elevated command prompt on the PC and run:

   ipconfig /all
   ping 192.168.215.1
   

   The ping should reply in < 1 ms. If it times out, re-check the cable, the X127 LED state, and any Windows firewall rule that may block ICMPv4 Echo.
2. Run arp -a and confirm that the entry for 192.168.215.1 resolves to a MAC starting with 28-63-36 (Siemens) or the documented NCU MAC. A static ARP entry here usually indicates a previous session left an old binding; clear with arp -d *.
3. Optionally, run a TCP port test against the S7 port using PowerShell to confirm ISO-on-TCP/TCP port 102 is open:

   Test-NetConnection -ComputerName 192.168.215.1 -Port 102
   

   A TcpTestSucceeded : True confirms the application layer is reachable.

4.5 Establish the online connection in TIA Portal

1. In TIA Portal, open the project for the 840D sl device.
2. From the menu bar, select Online → Go online (or press Ctrl+O) as described in the TIA Portal online documentation.
3. In the Go online dialog, select the PG/PC interface type PN/IE and the NIC that is wired to X127. The dialog auto-populates the target IP if a device with the same IP exists in the offline project.
4. Click Go online. TIA Portal establishes an ISO-on-TCP connection to port 102 on 192.168.215.1 and logs in. The status line will show Online with: 840D sl [CP 840D sl].

5. Why 192.168.215.1 / 255.255.255.224 Is the Right Answer

The address 192.168.215.1 with mask 255.255.255.224 defines a /27 subnet. The host range is 192.168.215.1 through 192.168.215.30 (broadcast is .31), giving 30 usable hosts — enough for a service PC plus the NCU without ever leaking into a customer /24. Because the mask is /27, the PC's auto-assigned address from the NCU's DHCP server falls inside this same network, so the Layer-3 path is consistent. The /24 default of 255.255.255.0 would widen the broadcast domain and is the wrong mask for the service interface.

By contrast, 192.168.214.0/24 is reserved for the plant network on X120 and X130. If X127 is set to 192.168.214.1, you essentially make X127 indistinguishable from the plant port. This sometimes "works" for engineering on a single-port test bench, but in a real cabinet the plant network is bridged and the PC will see the wrong devices. Siemens intentionally chose a non-overlapping /27 service subnet to keep the two domains separated.

6. Verification Matrix

7. Related Failure Modes and Edge Cases

7.1 Multiple active PG/PC interface assignments on the PC

Windows can store more than one PG/PC interface (PN/IE1, PN/IE2) with different MAC/IP bindings. If TIA Portal picks the wrong one, the connection will fail. In Set PG/PC interface (applet or Options → Set PG/PC interface in TIA Portal) make sure the active interface points to the physical NIC wired to X127, not to a virtual TAP or Hyper-V virtual switch.

7.2 CP IP set inside the offline project but never downloaded

Some engineers edit the CP IP in the project "to prepare for a future plant rollout" and forget to remove the change when re-using the project for service on X127. The TIA Portal online view shows the configured (offline) IP, not the live one, and the user assumes a mismatch. Always look at the online accessible devices list (Online → Accessible devices) for the true live address.

7.3 PLC stops after CP IP change

If a previous change to 192.168.0.1 (or any non-192.168.215.x / non-192.168.214.x address) caused the integrated S7-1500 to STOP, do the following to recover:

1. Reconnect the service PC to X127 with the NCU running.
2. In TIA Portal, go Online → Accessible devices and read the device IP currently in use. The NCU will keep its last successfully loaded IP until the next download or restart with default config.
3. Reset the CP address to 192.168.215.1/27 inside the offline HWCN and perform Download to device → Hardware and software. Select Reset to factory settings if TIA Portal offers it; this forces the CP to apply the new IP even when the PLC is STOP.
4. Run the PLC back to RUN from Online → Online & diagnostics → Operating mode → Start (warm restart).

7.4 X127 and corporate networks

Do not connect X127 to a corporate LAN, even temporarily. The X127 port runs an internal DHCP server and a fixed IP. Bridging it to a corporate /24 will either disrupt both networks or expose the NCU service menu to unauthorized users. If a long-distance connection is required, use a small dedicated engineering switch.

7.5 Third-party gateways (uaGate 840D, dataFEED, OPC UA servers)

Some third-party tools such as the Softing uaGate 840D are designed to communicate with the 840D sl only via X120/X130, not via X127. If you are trying to commission such a gateway, ensure the gateway is wired to the plant network and that the X127 port is reserved for service PG access.

8. Firmware / Software Compatibility

9. Quick Reference: Default IP Assignments

10. Summary of the Fix

1. Set the CP 840D sl address back to 192.168.215.1 with subnet 255.255.255.224 in the TIA Portal HWCN.
2. Configure the PC to obtain an IP address automatically on the NIC wired to X127.
3. Verify the Layer-3 path with ping 192.168.215.1 and Test-NetConnection -Port 102.
4. Use Online → Go online in TIA Portal to connect.
5. Re-download the hardware configuration to the NCU so the new (or restored) IP becomes live.

Following this procedure resolves the "partner no response" error on X127 and prevents the related failure where the integrated S7-1500 software controller stops due to an inconsistent CP IP. The address scheme 192.168.215.1/27 is the Siemens default and should be preserved unless a specific plant design document mandates a different value.