Resolving SNTPR Timeout Error in SIMATIC TDC CP51M1 with SICLOCK

1. Problem Summary

The SNTPR (NTP Request) function block on a SIMATIC TDC system returns the error code Timeout at output T01 when a CP51M1 Ethernet communication processor cannot reach its configured time source (a Siemens SICLOCK) across a routed IP network. The most frequent root cause observed in field installations is a mistyped or incorrectly assigned IP address in the SICLOCK time source (sometimes called a "multitask problem" when the same SICLOCK is configured with the destination of the second CP51M1 instead of the first, or vice versa). The error is also reported when a downstream block such as RTCM (Real Time Clock Messaging) has never successfully synchronized, because SNTPR is supposed to follow an already-synchronized RTCM.

This article documents:

• How the SIMATIC TDC time-synchronization chain (SICLOCK → router → CP51M1 → SNTPR → RTCM → application) is intended to work.
• How to identify the timeout, separate it from a network routing problem, and prove whether the SICLOCK is reachable on UDP/123.
• The IP-routing and subnet errors that the original fault report was eventually traced to.
• Concrete resolution steps, verification procedures, and a fault-diagnosis matrix.

2. SIMATIC TDC Time-Synchronization Architecture

SIMATIC TDC (Technology and Drives Control) is a high-performance multiprocessor control platform used for closed-loop drive, process, and technological control. Time synchronization is a baseline requirement in TDC because drive setpoints, cross-coupling values, and trace records are time-stamped with a 1 ms or finer resolution. The synchronization chain is composed of:

1. SICLOCK time transmitter – a Siemens GPS/DCF77/IRIG radio clock that delivers an absolute, stratum-1 time reference. SICLOCK models referenced in Siemens support materials include the TC 400, TC 100, and the TS 24V GPS1000 PAKET variants. See the SICLOCK product index on Siemens Industry Online Support – SICLOCK search for the full catalog and behavior descriptions.
2. Industrial router / managed switch – a layer-3 device that connects the SICLOCK to one or more SIMATIC TDC racks. In the field case, two Ethernet links run from the router to a single TDC rack (redundancy or load-shared paths).
3. CP51M1 – the SIMATIC TDC Ethernet communication module that receives NTP frames and forwards the time stamps into the TDC backplane.
4. SNTPR FB – the NTP client function block running on the TDC CPU that issues NTP requests through the CP51M1 and validates responses.
5. RTCM FB – the real-time clock messaging block that broadcasts the synchronized time on the TDC backplane to all consumers (drives, I/O, application tasks).

Reference for time-master redundancy behavior (TC 400 / TC100): the application note "SICLOCK TC 400 / TC100 and Redundancy" (Application Note 0108) describes the Redundancy/Redundancy (0.9.00010) parameter, whose timeout value (default 30 minutes) governs when the system switches to the second radio clock. The same application note documents the error-handling philosophy for SICLOCK TM/TS variants, including behavior when SICLOCK has not yet synchronized.

3. CP51M1 Communication Module – Key Facts

The CP51M1 is the Ethernet communication module for the SIMATIC TDC rack. It terminates one or two external Ethernet interfaces and exposes them to the TDC CPU as a logical NTP client target. Configuration is performed in the hardware configurator (HWConfig under STEP 7 / D7-SYS) where the following are defined:

4. SNTPR and RTCM Function Blocks

The SNTPR (S7 NTP Request) FB issues an NTP client request to one or more NTP servers and evaluates the response. Its key I/O and status outputs are summarized below.

The RTCM (Real Time Clock Messaging) FB consumes a synchronized time on the TDC backplane and broadcasts it to drive, I/O, and trace consumers. Its output TS indicates the synchronization state:

• TS = 0 → RTCM is synchronized and the time is being broadcast.
• TS ≠ 0 → RTCM is not synchronized; downstream consumers (including SNTPR-facilitated time stamps) cannot be trusted.

Siemens guidance, reinforced in field practice, is that RTCM must be synchronized first; only then can SNTPR operate reliably. If RTCM is healthy but SNTPR is not, the problem is in the network path or in the SNTPR configuration, not in the backplane clock.

5. Root Cause Analysis: SICLOCK IP Misconfiguration

The original report was ultimately traced to an incorrect destination IP address in the SICLOCK configuration (referred to in the report as a "multitask problem"). The deployment had two CP51M1 paths through the router, and the SICLOCK had been configured to point its NTP responses to the wrong CP51M1 interface (or to a non-routable address). Because NTP uses UDP/123, the SICLOCK must send its responses to the exact IP of the CP51M1 interface that originated the request. The following sequence captures what failed and what fixed it:

1. CP51M1 issued an NTP request to the SICLOCK's IP on UDP/123.
2. SICLOCK received the request but, due to misconfigured destination/NAT rules, replied to a different IP (the second CP51M1, or an unrelated address).
3. The originating CP51M1 never received a reply, so SNTPR expired its poll window and raised Timeout on T01.
4. RTCM therefore never received a valid time, so RTCM.TS ≠ 0 and the backplane was unsynchronized.
5. After correcting the IP routing table in the SICLOCK, both the NTP reply path and the RTCM synchronization restored on the next poll.

Confirmation in the field: "It is now okay, it was a multitask problem (IP address not correct) in SICLOCK."

6. Network Topology and Routing Considerations

The field topology that produced the fault was:

[ SICLOCK ] --- (1 cable) --- [ ROUTER ] --- (2 cables) --- [ SIMATIC TDC: CP51M1-A, CP51M1-B ]

Critical topology rules:

• Subnet alignment. Each CP51M1 interface must be in a subnet that the router can reach, and the SICLOCK must be in a subnet that the router can reach, on the same router. Verify with ping from a maintenance PC connected to the same router segment.
• UDP/123 must be permitted end-to-end. NTP is a connectionless UDP protocol; routers and managed switches must not drop UDP/123 frames. Stateful firewalls must accept both the request and the reply.
• No asymmetric routing for NTP. If request goes out LAN1 and the response comes back on LAN2, the originating CP51M1 will not see the reply. Either keep the response path on the same interface or configure the router to NAT the reply to the correct source IP.
• No double NTP sources at SNTPR inputs. If CT1 and CT2 are both populated, the FB will arbitrate, but if one source is faulty the FB may still report a timeout on T01. Use CT1 only; set CT2 to '0'.

6.1 Reachability test from the TDC side

From a maintenance terminal on the TDC service network, prove the path to the SICLOCK with a simple ICMP test before investigating the SNTPR block at all:

1. Identify the SICLOCK IP (default on older units 192.168.1.1 or whatever was assigned in the SICLOCK web UI).
2. From a service PC on the same router segment, ping <SICLOCK_IP>. A response confirms L3 reachability; no response confirms the routing problem is below the SNTPR layer.
3. If the ping fails on one router port, repeat from the second port. The original fault report noted that "I receive a ping reply" – meaning L3 was working in one direction, which is exactly the symptom of a mis-routed response rather than an unreachable host.

6.2 Confirm the response path on UDP/123

A plain ping does not prove that UDP/123 is open. Use a tool such as ntpdate -d <SICLOCK_IP> (Linux) or w32tm /monitor /computers:<SICLOCK_IP> (Windows) to verify the application-layer round trip. If the ICMP ping succeeds but UDP/123 times out, the fault is in the router's ACL or NAT configuration, not in the SICLOCK IP table.

7. HWConfig Settings for the CP51M1

When SNTPR is the active time client, the CP51M1's internal NTP client must be disabled. The setting is located under the CP51M1's properties → "Time synchronization" or "NTP" in HWConfig. Field-validated values:

8. Diagnostic Procedure

Use this ordered procedure on every SNTPR Timeout event. Stop at the first step that fails and remediate there before continuing.

1. Verify RTCM synchronization. Read RTCM.TS. If TS ≠ 0, the backplane clock has never been synchronized. Restore RTCM first by re-loading the TDC program, checking RTCM input wiring, or rebooting the SICLOCK.
2. Confirm only one SNTPR time source. Read SNTPR.CT1 and SNTPR.CT2. CT1 must be the SICLOCK IP; CT2 must be '0'.
3. Confirm CP51M1 NTP-client is off in HWConfig. Empty NTP-server list on the CP.
4. Ping the SICLOCK from a PC on the same router segment.
5. Ping the CP51M1 interfaces from the same PC and from the SICLOCK's diagnostic port (if available).
6. Issue a UDP/123 test with ntpdate -d or w32tm to prove the application layer.
7. Inspect the SICLOCK's NTP client / response configuration. Confirm that the destination IP for NTP responses is the exact IP of the CP51M1 interface that originates the request. In multi-CP setups, the SICLOCK must answer on the same path the request came from, or the router must NAT the response back to the originator.
8. Watch the SNTPR error output for one full poll interval. If T01 still reads Timeout after the IP is corrected, increase the SNTPR poll/timeout parameter and re-test.

9. Resolution Steps (Field-Validated)

1. Open the SICLOCK web configuration (typically on HTTPS port 443 of the SICLOCK's management IP) or the SICLOCK display menu.
2. Navigate to the NTP client / server mapping. Identify which logical interface is the active CP51M1 path (use the one matching SNTPR.CT1).
3. Set the NTP response destination IP to the exact IPv4 address of the CP51M1 interface that originates the request. Remove stale entries pointing to the second CP51M1 (or to a non-existent destination).
4. If the SICLOCK is configured in redundant mode, re-check the Redundancy/Redundancy (0.9.00010) parameter per Application Note 0108 – the default 30-minute timeout determines when the system switches to the second radio clock.
5. Save the SICLOCK configuration and reboot the SICLOCK if the menu prompts for it.
6. On the TDC side, set SNTPR.CT1 to the SICLOCK's IP and SNTPR.CT2 to '0'.
7. Empty the NTP server list on the CP51M1 in HWConfig and re-download the hardware configuration to the rack.
8. Cold-start the TDC CPU to force RTCM and SNTPR to re-initialize from a clean state.
9. Watch the online view: RTCM.TS should fall to 0 within a few poll intervals, and SNTPR.T01 should leave the Timeout state on the next successful NTP exchange.

10. Verification

After resolution, confirm synchronization with the following checks:

• Online view of SNTPR. T01 = 0 (no error), TV01 = TRUE (time valid), TS01 populated with the current time-of-day.
• Online view of RTCM. TS = 0, indicating backplane clock is healthy.
• Trace timestamp sanity. Open a TDC trace, add a digital event, and verify the trace's time stamps match the SICLOCK's wall clock to within ±1 ms.
• Network-layer confirmation. ping to the SICLOCK responds; ntpdate -d returns a non-zero offset without an error.
• Redundancy test. If two CP51M1 paths exist, manually disconnect the active path. The second CP should take over within the redundancy timeout defined in Application Note 0108 (default 30 min for SICLOCK TC 400 / TC100).

11. Fault Diagnosis Matrix

12. Preventive Measures

• Document the SICLOCK → router → CP51M1 → TDC path in the plant's network register. Every node's IP, mask, and gateway must be recorded and reviewed at every change.
• Enforce the "one NTP source at SNTPR input" rule. CT1 only; CT2 = '0'.
• Keep CP51M1 HWConfig NTP server list empty whenever the SNTPR FB is in service. Reject any project that violates this rule in code review.
• Use NTP-aware monitoring. Add a heartbeat to SCADA that raises an alarm when RTCM.TS ≠ 0 for more than 5 minutes, so a SICLOCK or router fault is caught at the operator level, not at the trace level.
• Apply SICLOCK firmware updates per Siemens release notes; check the SICLOCK product index for behavior changes that affect NTP response handling.
• Validate redundancy parameter Redundancy/Redundancy (0.9.00010) against the application note's recommended value for the specific radio-clock model in use (TC 400, TC 100, TS 24V GPS1000 PAKET, etc.).
• Confirm the SICLOCK has synchronized at least once before declaring the time-synchronization chain healthy. Per the SICLOCK TM/TS behavior notes on Siemens Support, an error state is reported specifically when the unit has never completed a first-time synchronization.

13. Common Pitfalls and Field Notes

• Ping success is necessary but not sufficient. ICMP works even when UDP/123 is dropped. A passing ping in the original fault report was the strongest hint that the failure was in the NTP response path, not in basic L3 reachability.
• Two SICLOCKs vs. one SICLOCK with two destinations. If you have only one SICLOCK, do not configure two NTP destinations in the SICLOCK and two destinations in SNTPR. Choose the active CP51M1 and stick to it; reserve the second CP51M1 for redundancy or hot-standby.
• Don't fight the FB. If the SNTPR FB is timing out despite a healthy network, do not increase the timeout to "make it go away". The correct response is to fix the IP routing. A larger timeout simply hides the problem.
• Watch DST transitions. SICLOCK delivers UTC. If the TDC expects local time without a configured offset, the time will jump by one hour at DST boundaries, which can look like a "synchronization loss" in trace records even though RTCM.TS = 0.
• Cold-restart the CPU after a SICLOCK config change. A warm restart often does not re-initialize the SNTPR poll table; a cold restart does.

14. Related References and Standards

This troubleshooting procedure is independent of any single NTP RFC, but it assumes standard NTP behavior on UDP/123 per IETF RFC 5905. Plant-side documentation should reference the following Siemens official materials:

• SICLOCK TC 400 / TC100 and Redundancy – Application Note 0108 for the redundancy timeout semantics (parameter 0.9.00010, default 30 min) and error-behavior description for SICLOCK TM/TS variants.
• Siemens Industry Online Support – SICLOCK product search for firmware notes, manuals, and behavior-in-case-of-error documentation across the SICLOCK family.

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