Configuring NTP Time Synchronization for S7-400 CPU 416-2 DP via CP 443-1 in SIMATIC PCS 7

To synchronize the system clock of an S7-400 CPU 416-2 DP from an external NTP time server via an Industrial Ethernet CP 443-1 communications processor in SIMATIC PCS 7, you must configure clock synchronization on both the CP and the CPU in HW Config. In the CP 443-1 properties under 'Time-of-Day Synchronization', activate the NTP procedure, define NTP server IP addresses, set the update interval, and check 'Forward time of day to station'. In the CPU 416-2 DP properties under 'Diagnostics/Clock', set the synchronization type 'In the AS' to 'As slave'. This allows the CP to forward decoded UTC time frames across the S7-400 backplane bus to the CPU clock.

Time Synchronization Architecture in S7-400 Controller Stations

In SIMATIC PCS 7 distributed control systems, chronological accuracy is vital for sequence of events (SOE) recording, alarm logging, and cross-controller interlocking. The S7-400 CPU 416-2 DP controller lacks an onboard Industrial Ethernet interface, relying instead on rack-mounted CP 443-1 communications processors to interface with the plant bus.

To establish plant-wide time synchronization without specialized pulse generators, the Network Time Protocol (NTP) is utilized. The CP 443-1 queries designated NTP servers (such as a central SICLOCK TC 400 or network GPS clock), decodes the high-precision timestamp, and forwards the time frame across the S7-400 backplane bus (communication bus / K-bus) to the CPU. For the CPU to ingest this backplane broadcast, its internal hardware clock must be explicitly designated as a slave.

Configuring NTP Synchronization on Industrial Ethernet CP 443-1

The communications processor acts as the NTP client on the Industrial Ethernet network:

Open HW Config in SIMATIC Manager and double-click the CP 443-1 module in the S7-400 rack.
Navigate to the Time-of-Day Synchronization tab.
Check the box Activate NTP procedure.
Enter the IP addresses of up to four NTP time servers available on the plant network.
Define the Synchronization interval (recommended: 10 to 60 seconds for industrial networks).
Check the critical checkbox: Forward time of day to station.

Without checking 'Forward time of day to station', the CP updates its own internal clock but never broadcasts the time synchronization telegrams over the backplane bus to the host CPU.

Configuring CPU 416-2 DP Clock Synchronization as Slave

The controller CPU must be configured to listen to backplane time telegrams:

In the same HW Config rack view, double-click the CPU 416-2 DP.
Select the Diagnostics/Clock tab.
Under Synchronization type, observe the two separate synchronization scopes:
- In the AS: Governs communication across the internal backplane bus. Select As slave.
- On MPI: Governs subnetwork time distribution via the CPU's onboard MPI/DP port. If the CPU is not broadcasting time to downstream operator panels or remote racks via MPI, leave this set to None (or configure as master if acting as an MPI time source).
Save and compile HW Config, then download the hardware configuration to the AS controller during an authorized maintenance window.

UTC Time Base, Local Time Offsets, and Daylight Saving Transitions

A foundational design principle of SIMATIC PCS 7 is that controller system clocks always operate in Coordinated Universal Time (UTC):

Monotonic Timestamps: UTC guarantees continuous, monotonically increasing timestamps without seasonal backward steps that corrupt process archive databases.
Local Time Conversion: The conversion from UTC to local time (and daylight saving time adjustments) is performed exclusively at the HMI presentation layer on WinCC OS Servers and Clients.
NTP Telegrams: NTP servers transmit timestamps in UTC by definition. When the CP 443-1 forwards NTP time to the CPU, the CPU clock is synchronized to pure UTC. Never configure the CPU clock with local seasonal offsets in PCS 7 projects.

Hardware Configuration and Parameter Mapping Matrix

The following table summarizes the required hardware settings across modules:

ModuleHW Config TabConfiguration ParameterMandatory SettingTechnical PurposeCP 443-1Time-of-Day SynchronizationNTP ProcedureEnabledQueries external NTP time servers via UDP 123CP 443-1Time-of-Day SynchronizationNTP Server IP ListValid IP addressesPrimary and redundant time sourcesCP 443-1Time-of-Day SynchronizationForward time to stationEnabledBroadcasts time telegrams across backplane K-busCPU 416-2 DPDiagnostics/ClockSynchronization in ASAs slaveIngests backplane time telegrams into CPU clockCPU 416-2 DPDiagnostics/ClockSynchronization on MPINone (or Master)Prevents conflicting subnetwork broadcastsCPU 416-2 DPDiagnostics/ClockTime Correction Factor0 msDefault drift compensation

Step-by-Step Commissioning and Diagnostic Verification

After downloading the hardware configuration to the S7-400 station:

In SIMATIC Manager, select the CPU and open PLC -> Diagnostics/Hardware -> Module Information.
Open the Time System tab. Verify that the current module time displays UTC matching the master NTP server within a few milliseconds.
Select the CP 443-1 and open its Module Information. Navigate to the Time-of-day tab and verify that NTP status reports 'Synchronized' with the active server IP highlighted.
Check the Diagnostic Buffer of both the CPU and CP 443-1 to ensure no time synchronization fault telegrams (e.g. event ID 16#4940) are recorded.

Verification

Verify end-to-end time synchronization in the running automation environment:

In CFC test mode, insert a monitoring block or check the timestamp of a newly triggered alarm in the WinCC Alarm Logging list.
Confirm that the alarm timestamp matches the external reference clock down to the second.
Temporarily disconnect the Ethernet cable from the CP 443-1; verify that the CPU continues operating on internal quartz drift without jumping.
Reconnect the cable; verify that the CP re-establishes NTP lock and smoothly re-aligns the CPU clock.

Limitations and Operational Precautions

Adhere to the following operational precautions:

Never Configure Both CPU and CP as Time Masters: If both the CPU and CP are set to broadcast time on the backplane, telegram collisions occur, causing cyclic time jumping alarms (16#4940) in the diagnostic buffer.
Firewall Port UDP 123: Ensure network switches and plant firewalls allow outbound and inbound UDP port 123 traffic between CP 443-1 and the designated NTP servers.
Hardware Download Requirement: Changes to the CPU Diagnostics/Clock tab require downloading the system data blocks (SDBs) to the CPU, which may require a controller STOP on non-redundant S7-400 CPUs.

Engineer FAQ

What is the difference between 'In the AS' and 'On MPI' synchronization in CPU properties?

'In the AS' governs time telegram exchange across the internal S7-400 backplane rack between the CPU and communication modules (CP 443-1). 'On MPI' governs time broadcasting over the external MPI subnetwork port to other connected stations.

Can multiple CP 443-1 modules in the same rack synchronize the CPU?

Only one CP 443-1 should have 'Forward time of day to station' enabled. Enabling time forwarding on two CPs simultaneously causes conflicting time frames on the backplane bus.

Does downloading time synchronization settings require stopping the CPU 416-2 DP?

Yes. Modifying system clock parameters in HW Config alters System Data Blocks (SDBs), which requires a CPU STOP/RUN transition on standard non-H S7-400 controllers.