General notes for setting the watchdog

The EtherCAT Terminals are equipped with a safety device (watchdog) which, for example, switches the outputs (if present) to a pre-settable state after a pre-settable time in the event of interrupted process data traffic, depending on the device and setting, e.g. to FALSE (off) or an output value.

The EtherCAT Slave Controller has two watchdogs:

• Sync manager (SM) watchdog (default: 100 ms)
• Process data (PDI) watchdog (default: 100 ms)

Their times are parameterized individually in TwinCAT as follows:

Fig.5: EtherCAT tab -> Advanced Settings -> Behavior --> Watchdog

Remarks:

• The Multiplier register 400h (hexadecimal, i.e. 0x0400) is valid for both watchdogs.
• Each watchdog has its own timer setting 410h or 420h, which together with the multiplier results in a time.
• Important: the multiplier/timer setting is only loaded into the slave at EtherCAT start if the checkbox in front of it is activated.
  If this is not activated, nothing is downloaded and the setting in the ESC remains unchanged.
• The downloaded values can be viewed in the ESC registers 400h, 410h and 420h: ESC Access -> Memory (ESC Access -> Memory).

SM watchdog (SyncManager watchdog)

The SyncManager watchdog is reset with every successful EtherCAT Terminal process data communication. If no EtherCAT process data communication takes place with the terminal for longer than the set and activated SM watchdog time, e.g. in the event of a line interruption, the watchdog is triggered. The status of the terminal (usually OP) remains unaffected. The watchdog is only reset after a successful EtherCAT process data access.

The SyncManager watchdog therefore monitors correct and timely process data communication between the master and ESC, which takes place exclusively at EtherCAT level.

The maximum possible watchdog time depends on the device. For example, it is usually up to 170 seconds for "simple" EtherCAT slaves (without firmware) with watchdog execution in the ESC. For sophisticated EtherCAT slaves (with firmware), the SM watchdog function is usually parameterized via register 400h/420h, but is executed by the microcontroller (µC) and can be significantly lower. In addition, the execution may then be subject to a certain degree of time uncertainty. As the TwinCAT dialog may allow entries up to 65535, a test of the desired watchdog time is recommended.

PDI watchdog (Process Data Watchdog)

If there is no PDI (Process Data Interface) communication with the ESC for longer than the set and activated PDI watchdog time, this watchdog is triggered.
PDI is the internal interface of the ESC, e.g. to local processors in the EtherCAT slave. The PDI watchdog can be used to monitor this communication for failure.

The PDI watchdog monitors correct and timely process data communication with the ESC but now from the application side.

Calculation

Watchdog time = [1/25 MHz * (watchdog multiplier + 2)] * SM/PDI watchdog

Example: Default setting Multiplier'= 2498, SM watchdog = 1000 => 100 ms

The value in “Watchdog multiplier + 2” in the formula above corresponds to the number of 40ns base ticks that represent a watchdog tick.