TwinCAT time sources

Name	BIOS motherboard	CPU time	Windows/NT time	TwinCAT/TC time	Distributed clocks/DC time
Description	RTC (RealTimeClock), battery-operated on the motherboard	CPU counter from the controller hardware, not regulated Initialized by RTC	Local system time of the Windows operating system (NT) Initialized by RTC	Running TwinCAT clock Initialized by Windows	The system returns the start time of the current task cycle. Initialized by TwinCAT.
Data		64 bit resolution: 100 ns_{PC Base}	from 1.1.1601 00:00 resolution: 1 ms Scope: structure with year, month, day, hour etc.	from 1.1.1601 00:00 resolution: 100 ns Scope: 64 bit	from 1.1.2000 00:00 resolution: 1 ns Scope: 64 bit
Reference			Local	Local time taking into account the set time zone, i.e. usually UTC	Local time taking into account the set time zone, i.e. usually UTC
PLC format		T_ULARGE_INTEGER	TIMESTRUCT	T_FILETIME	T_DcTime
Call		GetCpuCounter	NT_GetTime()	GetSystemTime()	F_GetActualDcTime() (ab TwinCAT 2.11) F_GetCurDcTickTime() (= GetSystemTime) F_GetCurDcTaskTime() (from TwinCAT 2.11)
Update		With each call, possibly several times within a PLC cycle		with each base tick (System Manager \| BaseTime)	ActualDcTime with each call, possibly several times within a PLC cycle TickTime with each base tick (System Manager \| BaseTime) TaskTime At the start of the Sync task cycle
Sample application	can be used by the PLC block Nt_SetTimeToRtcTime for correcting the NT time	relative time measurements	Logging, time stamping at operating system level	High-precision, relative time-based tasks within one or across several task cycles	- High-precision, relative time-based tasks within the EtherCAT system - Definitive reference to the global time through external synchronization possible
Manipulation options			- It can be changed to the current RTC time by the PLC function block Nt_SetTimeToRtcTime; this also triggers a correction of the RTC time ATTENTION Use of this function in conjunction with EtherCAT distributed clocks systems is not recommended. - Synchronization at network level (SNTP, NTP)		Synchronization with external reference time from TwinCAT 2.11

Table 1: time architecture, required libraries: TcEtherCAT.lib, TcUtilities.lib, TcSystem.lib

Application scenario 1: Local control system without forced synchronization via the network

The local Windows clock is free-running and can be coupled to the RTC via Nt_SetTimeToRtcTime. This option is not recommended when using distributed clocks components!

We recommend using the TC or DC clock if absolute time references are required.

Ideally, the DC time should be coupled to an external reference time via suitable EtherCAT components.

Application scenario 2: Local control system with forced synchronization via the network

The local Windows clock is cyclically synchronized with the world time through a network clock/server/Internet time server.

Coupling of the Windows clock to the RTC through Nt_SetTimeToRtcTime is not recommended.

We recommend using the TC or DC clock. The reference to the absolute time can be established via the application through offset calculation at the NT time.

Ideally, the DC time should be coupled to an external reference time via suitable EtherCAT components.

Application scenario 3: Local control system with external reference time via EtherCAT

A frequency- and phase-synchronized analog time is available through coupling of the DC time to an external time source (GPS, radio clock, PTP/IEEE1588, EtherCAT). The NT and TC time are not required in the application.

	Common time synchronization The usual time synchronization at operating system level works in discrete intervals ranging from several seconds to days. In the synchronization case this leads to an erratic/unsteady change of the subordinate time! The usual network synchronizations (SNTP, NTP and similar) or even Nt_SetTimeToRtcTime are affected by this. The application must be able to anticipate these sudden changes in the "absolute" time. Beckhoff only integrates continuous synchronization techniques in EtherCAT.