Notifications, polling and timestamps

For EPICS input records the driver sets up ADS notifications by default: the PLC samples the variable every TS_MS milliseconds and pushes a telegram whenever the value changed, buffering for at most T_DLY_MS. Adding POLL_RATE=1.0/ to the link switches the symbol to the cyclic bulk read instead.

When to use which

Notifications work well for	Notifications work badly for
Values that change seldom	Noisy sensors, e.g. temperature readings
Values that need a PLC timestamp (technically the TwinCAT time)	Very many sensors
	Output records with readback

The last point matters: if the PLC silently ignores or clips a written value, no change notification is generated, so an output record with info(asyn:READBACK,"1") would keep showing the rejected value. Polling always re-reads the actual PLC value.

Note

POLL_RATE currently only selects bulk-read mode. All polled symbols share one loop that runs at 1 Hz (or at the port default sample time if that is larger than 1000 ms). Symbols larger than 1 MiB cannot be polled.

Choosing parameters

Goal	Suggested settings
Slow process value, archive only	`POLL_RATE=1.0/` or `TS_MS=1000/`
Output readback for slow control	`POLL_RATE=1.0/` + `info(asyn:READBACK,"1")`
Fast transient capture with PLC time	`TIMEBASE=PLC/TS_MS=1/T_DLY_MS=500/`
Noisy analogue sensor	poll, or filter/deadband in the PLC

A measured example

Two longin records read the same encoder counter through notifications, with different sample times:

ADSPORT=852/TIMEBASE=PLC/T_DLY_MS=500/TS_MS=200/GVL_PILS.stEL5101CounterValue.nvalue?
ADSPORT=852/TIMEBASE=PLC/T_DLY_MS=500/TS_MS=1/GVL_PILS.stEL5101CounterValue.nvalue?

Running camonitor while moving the motor by 1 mm gives:

# First "callback" when the system starts
EncoderRawP1ms   2026-03-17 10:58:56.732280 4787
EncoderRawP200ms 2026-03-17 10:58:56.732286 4787
# Move the motor
EncoderRawP1ms   2026-03-17 10:59:23.448157 4785
EncoderRawP1ms   2026-03-17 10:59:23.488586 4783
EncoderRawP1ms   2026-03-17 10:59:23.518551 4781
EncoderRawP1ms   2026-03-17 10:59:23.528207 4779
EncoderRawP200ms 2026-03-17 10:59:23.557945 4778
EncoderRawP1ms   2026-03-17 10:59:23.558489 4777
EncoderRawP1ms   2026-03-17 10:59:23.598498 4775
EncoderRawP1ms   2026-03-17 10:59:23.628552 4773
EncoderRawP1ms   2026-03-17 10:59:23.638531 4771
EncoderRawP1ms   2026-03-17 10:59:23.669819 4769
EncoderRawP1ms   2026-03-17 10:59:23.699587 4767
EncoderRawP1ms   2026-03-17 10:59:23.729942 4765
EncoderRawP200ms 2026-03-17 10:59:23.758619 4764
EncoderRawP1ms   2026-03-17 10:59:23.777669 4763
EncoderRawP1ms   2026-03-17 10:59:23.807956 4761
EncoderRawP1ms   2026-03-17 10:59:23.837835 4759
EncoderRawP1ms   2026-03-17 10:59:23.858510 4757
EncoderRawP1ms   2026-03-17 10:59:23.897407 4755
EncoderRawP200ms 2026-03-17 10:59:24.028100 4754

The 1 ms sampled PV produces a dense stream of callbacks for a single 1 mm move, while the 200 ms sampled PV combines the motion into a handful of updates. On a system with many such signals, the 1 ms variant multiplies network traffic, archiver load and CA/PVA client load for very little extra information.

Timestamps

Each value delivered by the driver carries a timestamp. Which clock it comes from is selected by the TIMEBASE option, falling back to argument 11 of adsAsynPortDriverConfigure.

Timebase	Enum value	Meaning
`PLC`	`ADS_TIME_BASE_PLC = 0`	The TwinCAT timestamp of the sample, converted from the Windows FILETIME epoch to the EPICS epoch.
`EPICS`	`ADS_TIME_BASE_EPICS = 1`	The IOC time at which the value was received/processed.

The record must ask asyn for the timestamp with field(TSE, "-2"). Without it, the record timestamps itself when it processes and the TIMEBASE option has no visible effect.

PLC time is the right choice when:

the value is sampled fast in the PLC and buffered (T_DLY_MS), so that several samples arrive in one telegram and only the PLC timestamp tells them apart;
EPICS data has to be correlated with PLC-internal logs or other ADS clients.

For PLC time to be meaningful, the PLC clock must be synchronised (NTP or the EtherCAT distributed clock). An unsynchronised PLC produces timestamps that drift against the rest of the facility.

Some values never have a PLC timestamp and always use the EPICS timebase regardless of the option — in particular the .AMSPORTSTATE. pseudo-symbol, which is generated inside the driver.