Resolving NTP Server Configuration and Subnet Gateway Errors on WinLC RTX / PCS 7 Systems

In SIMATIC PCS 7 and WinLC RTX configurations, compilation errors occurring after specifying an NTP server IP address on an Industrial Ethernet interface (IE General or CP 1623) are caused by an unroutable IP subnet configuration. In HW Config, if the configured NTP server IP address resides on a different logical IP subnet than the network card's IP address, the compiler requires a valid Default Gateway (Router). If the 'Use router' checkbox is unselected or the router IP field is left blank, HW Config compilation aborts with a subnet routing error. Entering the correct gateway IP or assigning an NTP server within the local subnet resolves the compiler fault.

Subnet and Routing Requirements for NTP in SIMATIC HW Config

In SIMATIC PCS 7 systems incorporating software controllers (such as SIMATIC WinAC RTX / WinLC RTX) or standard AS controllers, time synchronization across Industrial Ethernet can be achieved via Network Time Protocol (NTP). The NTP client engine runs inside the controller communication stack and cyclically polls designated NTP time servers over UDP port 123.

To establish valid network communication, the operating system must understand how to forward NTP telegrams:

Network Topology ScenarioNetwork Interface IPNTP Server IPRouter / Gateway Required?Compiler Action if Gateway MissingLocal Subnet Sync192.168.1.10 /24192.168.1.1 /24No (Direct Layer 2 ARP broadcast)Compiles cleanly without errorsCross-Subnet Sync192.168.1.10 /2410.10.5.50 /24Yes (Mandatory Layer 3 Gateway)Compiler Abort / Syntax Error

Because HW Config enforces strict network consistency validation during compilation, entering an off-subnet NTP server address without parameterizing a gateway triggers an immediate compiler diagnostic abort.

Why HW Config Compiler Flags NTP Server Address Mismatches

When an engineer double-clicks an IE General card or CPU PROFINET interface in HW Config and opens Time-of-Day Synchronization, entering an external corporate NTP server address (e.g., 10.160.20.1) without configuring router properties produces a compilation failure.

The STEP 7 compiler performs a logical bitwise comparison between the network interface's IP address and subnet mask against the NTP server address:

$$\text{Target Subnet} = \text{IP} \;\&\; \text{Subnet Mask}$$

If the resulting subnet ID does not match the local interface subnet ID, the compiler checks the IP Protocol tab for the parameter Use router. If Use router is unchecked or set to 0.0.0.0, the compiler determines that the controller possesses no route to reach the time server, generating error text such as 'The NTP server address cannot be reached with the current subnet configuration'.

Step-by-Step Configuration Workflow on IE General and WinLC RTX

To resolve the compilation fault and establish valid NTP synchronization, execute the following technical workflow:

Audit IP Addressing Scheme: Determine the IP address, subnet mask, and default industrial gateway (e.g. SCALANCE S or SCALANCE X router IP) for the PC Station / WinLC RTX interface.
Configure Router in HW Config: Open HW Config for the PC Station containing WinLC RTX and IE General.
- Double-click the IE General communication card.
- On the General tab, click Properties under the Ethernet interface.
- In the IP Protocol section, check the box 'Use router'.
- Enter the valid gateway IP address (e.g. 192.168.1.254).
Configure NTP Server Settings:
- Switch to the Time-of-Day Synchronization tab.
- Check 'Enable NTP time-of-day synchronization'.
- Enter the target NTP server IP address (e.g. 10.10.5.50).
- Set the synchronization interval (recommended: 10 seconds).
Compile HW Config: Click Save and Compile. Verify that compilation finishes with 0 Errors.
Update NetPro and Download: Open NetPro, execute Save and Compile (Check Routing), and download the station configuration to the PC Station via the Station Configuration Editor.

Verification

Confirm that NTP time synchronization operates properly through these diagnostic checks:

HW Config Compiler Log: Verify that the compilation window completes cleanly without warning or error dialogs regarding unreachable NTP addresses.
Station Configuration Editor Inspection: Open Station Configuration Editor on the target PC. Confirm that IE General and WinLC RTX run in green active status without communication errors.
Time Synchronization Log: In WinAC / WinLC RTX diagnostic buffer, confirm periodic confirmation events indicating 'NTP time telegram received and time updated'.

Limitations and Operational Precautions

When configuring time synchronization across industrial subnets, observe these operational safeguards:

Firewall Rule Verification: Industrial firewalls (e.g. SCALANCE S615) between the control network and corporate NTP servers must explicitly permit inbound and outbound UDP port 123 traffic.
Plant Bus Isolation Standard: Standard PCS 7 architecture recommends keeping the Plant Bus strictly isolated from corporate enterprise networks. Rather than routing AS controllers directly to external corporate NTP servers, use a redundant OS Server pair or dedicated SICLOCK TC 400 on the Plant Bus to act as an isolated local time master.
Avoid Excessive Polling Frequencies: Do not set NTP update intervals below 5 seconds; rapid polling creates unnecessary network interrupts on software controllers without improving time resolution.

Engineer FAQ

Why can I not just enter any IP address for the NTP server?

Siemens HW Config compiles hardware system data blocks (SDBs) that configure the low-level IP stack. If an address is off-subnet without a gateway, the operating system cannot resolve the MAC address via ARP, causing communication aborts.

Can I configure multiple NTP servers on IE General?

Yes. The Time-of-Day Synchronization tab allows configuring up to four separate NTP server IP addresses for redundancy.

Does WinLC RTX support SIMATIC Mode time synchronization?

WinLC RTX running with standard commercial Ethernet cards (IE General) supports NTP mode. SIMATIC Mode requires a dedicated CP 1613 or CP 1623 communications processor.